surface pretreatment by phosphate conversion coatings – a review

130 Rev.Adv.Mater.Sci. 9 (2005) 130-177 

T.S.N. Sankara Narayanan 

SURFACE PRETREATMENT BY PHOSPHATE 

CONVERSION COATINGS – A REVIEW 

Corresponding author: T.S.N. Sankara Narayanan, e-mail: tsnsn@rediffmail.com 

© 2005 Advanced Study Center Co. Ltd. 

T.S.N. Sankara Narayanan 

National Metallurgical Laboratory, Madras Centre CSIR, Complex, Taramani, Chennai-600 113, India 

Received: April 22, 2005 

Abstract. Phosphating is the most widely used metal pretreatment process for the surface 

treatment and finishing of ferrous and non-ferrous metals. Due to its economy, speed of operation 

and ability to afford excellent corrosion resistance, wear resistance, adhesion and lubricative 

properties, it plays a significant role in the automobile, process and appliance industries. Though 

the process was initially developed as a simple method of preventing corrosion, the changing 

end uses of phosphated articles have forced the modification of the existing processes and 

development of innovative methods to substitute the conventional ones. To keep pace with the 

rapid changing need of the finishing systems, numerous modifications have been put forth in 

their development - both in the processing sequence as well as in the phosphating formulations. 

This review addresses the various aspects of phosphating in detail. In spite of the numerous 

modifications put forth on the deposition technologies to achieve different types of coatings and 

desirable properties such as improved corrosion resistance, wear resistance, etc., phosphate 

conversion coating still plays a vital part in the automobile, process and appliance industries. 

Contents 

1. Introduction 

2. Chemical conversion coatings 

3. Phosphating 

3.1. History and development of the 

phosphating process 

3.2. Chemistry of phosphating 

3.3. Acceleration of the phosphating 

process 

3.3.1. Chemical acceleration 

3.3.2. Mechanical acceleration 

3.3.3. Electrochemical acceleration 

3.4. Kinetics of the phosphating process 

3.5. Process details 

3.5.1. Cleaning 

3.5.2. Rinsing 

3.5.3. Phosphating 

3.5.4. Rinsing after phosphating 

3.5.5. Chromic acid sealing 

3.5.6. Drying 

3.6. Coating characteristics 

3.6.1. Structure and composition 

3.6.2. Coating thickness and coating weight 

3.6.3. Coating porosity 

3.6.4. Stability of the phosphate coating 

3.7. Influence of various factors on coating 

properties 

3.7.1. Nature of the substrate 

3.7.1.1. Composition of the metal 

3.7.1.2. Structure of the metal surface 

3.7.1.3. Surface preparation 

3.7.1.4 Surface activation

Surface pretreatment by phosphate conversion coatings – a review 

3.7.1.5. Thermal treatments and machining 

3.7.2. Phosphating parameters 

3.8. Processing problems and remedial 

measures 

3.9. Defects in coatings and remedies 

3.10. Characterization of phosphate 

coatings 

3.11. Testing the quality of phosphate 


3.11.1. Evaluation of physical 

characteristics 

3.11.2. Evaluation of corrosion performance 

3.12. Applications 

3.13. Environmental impact 

4. Summary 

References 

1. INTRODUCTION 

Metals have been the backbone of civilization. Efforts 

have been spared to find alternatives and replacements 

for metals but these still play a major 

role in the manufacture and construction and are 

likely to do so for many more years. This is due to 

the combination of several useful properties like 

strength, workability, low-cost and ability to be recycled, 

that the metals possess. However, metals 

which are extracted from their ores by chemical or 

electrochemical means show a strong tendency to 

revert to their oxide form at the first available opportunity, 

i.e., they tend to corrode [1-4] and as a result 

they create a tremendous economic loss besides 

posing a serious threat to the national resources 

of a country. 

The methods of corrosion prevention are many 

and varied. These methods may be generally classified 

[3] as: 

• Modification of the metal by alloying and/or surface 

modification; 

• Modification of the environment by the use of inhibitors; 

and 

• Change of metal/environment potential by cathodic 

or anodic protection. 

The most commonly used method of corrosion 

protection involves bulk alloying or surface modification. 

Surface modification is however, far more 

economical than bulk alloying and is more widely 

practiced. The methods generally used for surface 

modification involve the formation of a physical barrier 

to protect the metal against its corrosive environment 

[5]. This can be achieved by relatively more 

modern methods such as: (i) physical vapour deposition 

(PVD); (ii) chemical vapour deposition (CVD); 

131 

(iii) ion implantation; (iv) laser treatment; (v) deposition 

by thermal spray, plasma spray and arc methods; 

(vi) nitriding; (vii) carbiding; etc., or through more 

conventional techniques such as: (i) painting; (ii) 

anodizing; and (iii) chemical conversion coatings. 

While the former methods are usually less economic 

as they involve the use of sophisticated application 

techniques and are meant for specialized applications, 

the latter methods are more cost-effective and 

have a wider spectrum of end applications. 

2. CHEMICAL CONVERSION 

COATINGS 

Chemical conversion coatings are adherent, insoluble, 

inorganic crystalline or amorphous surface 

films, formed as an integral part of the metal surface 

by means of a non-electrolytic chemical reaction 

between the metal surface and the dipped in 

solution [6]. In such coatings, a portion of the base 

metal is converted into one of the components of 

the resultant protective film, which is much less reactive 

to subsequent corrosion than the original metal 

surface. This film imparts an equal potential to the 

metal surface, neutralizing the potential of the local 

anodic and cathodic galvanic corrosion sites [7]. 

They also serve as absorptive bases for improving 

the adhesion to paints and other organic finishes. 

Chemical conversion coatings are preferred because 

of their adherent nature and high speed of coating 

formation besides being economical. Further these 

can be formed using simple equipment and without 

the application of any external potential. Chemical 

conversion coating processes are classified as phosphating, 

chromating and oxalating according to their 

essential constituents viz., phosphates, chromates, 

and oxalates respectively [8]. The present review 

focuses on phosphate conversion coatings with a 

special emphasize on zinc phosphate coatings on 

mild steel. 

3. PHOSPHATING 

Phosphating process can be defined as the treatment 

of a metal surface so as to give a reasonably 

hard, electrically non-conducting surface coating of 

insoluble phosphate which is contiguous and highly 

adherent to the underlying metal and is considerably 

more absorptive than the metal [9]. The coating 

is formed as a result of a topochemical reaction, 

which causes the surface of the base metal to 

integrate itself as a part of the corrosion resistant 

film.

132 T.S.N. Sankara Narayanan 

Table 1. Historical development of the phosphating process. 

Sl. Year/ Advancement made/process developed References 

No. Period 

1. 1906 Phosphating of iron and steel using phosphoric acid and iron filings 11 

2. 1908 Treatment of phosphate coatings with oxidizing agents to reduce 12 

process time 

3. 1909 Regeneration of the bath and formulation of zinc phosphate baths 13,14 

requiring high temperature – process time of one hour 

4. 1911 Formulation of manganese phosphate bath requiring high temperature - 15 

process time of 2-2.5 hours 

5. 1914 Parkerising process with maintenance of total acid to free acid ratio 16,17 

6. 1928 Recognition of phosphate coating as paint base 18,19 

7. 1929 Bonderizing process with the addition of Copper accelerator-coating 20 

time: 10 minutes to 1 hour 

8. 1933 Use of oxidizing agents like nitrate for acceleration – coating time: 21 

5 minutes 

9. 1934 Use of phosphate coating for cold working operations for metals 22 

10. 1937 Spray phosphating – phospating time: 60-90 seconds 23 

11. 1940 Development of non-coating phosphate process based on sodium 24 

or ammonium phosphates 

12. 1940 Development of cold phosphating methods 25 

13. 1941 Phosphating of aluminium surfaces using zinc phosphate and fluorides 26 

14. 1943 Use of disodium phosphate containing titanium as pre-dip before 27 

phosphating 

15. 1950’s Large scale application of manganese phosphate coatings as an oil 28 

retaining medium – for use on bearing or sliding surfaces etc. 

16. 1960’s Use of special additives to control coating weight 29 

17. 1960’s Spray process at operating temperature of 25-30 °C 18,19 

18. 1970’s Improvement in coating quality, use of spray cleaners based on 18,19 

surfactant technology 

3.1. History and development of the 

phosphating process 

The use of phosphate coatings for protecting steel 

surfaces has been known since the turn of the century 

and during this period the greater part of the 

World’s production of cars, refrigerators and furniture 

were treated this way. The first reliable record 

of phosphate coatings applied to prevent rusting of 

iron and steel is a British patent of 1869 granted to 

Ross [10]. In the method used by him, red hot iron 

articles were plunged into the phosphoric acid to 

prevent them from rusting. Since then numerous 

developments have taken place, of which the major 

developments are listed in Table 1. 

During the last 30 years, work has been concentrated 

mainly on improvements in quality, particularly 

to keep in pace with the recent changing 

needs of the organic finishing systems. Prominent 

among these are: (i) use of low temperature phosphating 

baths to overcome the energy crisis [30- 

32]; (ii) use of low zinc technology [18,19]; (iii) use 

of special additives in the phosphating bath [33-42]; 

(iv) use of more than one heavy metal ions in existing 

composition-particularly tri-cation phosphating 

[43]; etc. New types of phosphate coatings such 

as tin, nickel and lead phosphate coatings have been 

introduced [44,45] besides the development of compositions 

for simultaneous phosphating of multiple 

metal substrates [46,47]. There has been a grow-


Table 2. Special additives used in phosphating baths. 

Sl. Additive used Purpose Impact References 

No. 

1. α-hydroxy carboxylic acids To reduce the coating Improve bath life through 49-54 

like tartaric and citric acids, weight lesser consumption of 

tripolyphosphate, sodium, chemicals 

potassium tartrate, 

nitrobenzene sulphonate 

2. Chelants such as EDTA, NTA, To increase the coating Improved corrosion 42, 43 

DTPA gluconic acids and weight protection, shorter 55-58 

polycarboxy o-amino acids. processing times. 

3. Quaternary ammonium Grain refinement Better adhesion of 59-63 

compounds, N and P subsequent finishes; 

compound containing better corrosion 

amido or amino group. protection. 

Calcium, Formic acid ester, 

chelate of an acidic organic 

phosphate 

4. Nickel (II) To improve surface Better adhesion of 64, 65 

texture subsequent finishes; 

better corrosion 

protection. 

5. Lead compounds, tungstate To accelerate the Reduction in 66-69 

ions, gaseous nitrogen phosphating process processing time 

peroxide, hydroxylamine 

sulphate, hexamine 

6. Persulphate and To prevent concentration Better utilization of 70 

permonosulphuric acid of ferro-nitroso complex nitrite and reduction 

in nitrite accelerated in the evolution of 

zinc phosphating bath. toxic vapours 

7. Cyclic trimetaphosphate To reduce the operating Thinner, smoother and 71 

temperature improved corrosion 

To increase the resistant coating 

tolerance to dissolved 

iron 

8. Lauric, Palmitic, and To improve lubricant Improved workability 72 

Stearic acids with fatty properties of the metal 

amines and ethoxylated 

amines 

9. Carbohydrates, dialkyl- To decrease scaling Improve the service 73-75 

triaminepentakis life of heating coils 

methylene phosphonic and provide uniform 

acid and its salts and heating of the bath 

fluoborate or fluosilicate 

10. Phosphonic acid ester, To prevent the build up Improved service life 76, 77 

Magnesium or zinc nitrate of sludge on tank walls of the equipment 

11. Amines, Tin (IV), Arsenic As inhibitors Improves corrosion 78, 79 

compounds, zinc salt of resistance 

an organic N-compound 

12. Zinc carbonate To reduce the acidity Consistent performance 80 

of the bath and to of the bath 

maintain the equilibrium 

133


Table 2. Continued. 

13. Thiourea Stabilizer in non- Improved stability of 81 

aqueous phosphating the bath 

solutions, as inhibitors 

14 Sodium lignosulphonate To modify the physical Reduce the tendency 82, 83 

form of sludge to form a crust on heat 

transfer surfaces 

Improves process 

efficiency 

15 Methylaminoethoxysilane To prevent rehydration Impart hydrophobic and 84 

of phosphate dihydrate anti-corrosion properties 

16 Hexametaphosphate To decrease the surface Enables deep drawing 85 

roughness and increase of tubes (~50% length) 

the extent of absorption with reduction in wall 

of sodium sterate thickness 

ing use of substitutes to conventional Cr(VI) postrinses 

[48] to suit the regulations imposed by the 

pollution control authorities on the use of Cr(VI) compounds. 

The special additives used in phosphating 

baths is complied in Table 2 and the alternatives to 

Cr(VI) post-rinse treatment is given in Table 3. 

3.2. Chemistry of phosphating 

All conventional phosphating solutions are dilute 

phosphoric acid based solutions of one or more alkali 

metal/heavy metal ions, which essentially contain 

free phosphoric acid and primary phosphates 

of the metal ions contained in the bath 

[18,19,24,127,128]. When a steel panel is introduced 

into the phosphating solution a topochemical 

reaction takes place in which the iron dissolution is 

initiated at the microanodes present on the substrate 

by the free phosphoric acid present in the 

bath. Hydrogen evolution occurs at the 

microcathodic sites. 

Fe + 2H 3 PO 4 → Fe(H 2 PO 4 ) 2 + H 2 ↑ . (1) 

The formation of soluble primary ferrous phosphate 

leads to a concurrent local depletion of free acid 

concentration in the solution resulting in a rise in 

pH at the metal/solution interface. This change in 

pH alters the hydrolytic equilibrium which exists 

between the soluble primary phosphates and the 

insoluble tertiary phosphates of the heavy metal ions 

present in the phosphating solution, resulting in the 

rapid conversion and deposition of insoluble heavy 

metal tertiary phosphates [18,19,24,127, 128]. In a 

zinc phosphating bath these equilibria may be represented 

as: 

Zn(H 2 PO 4 ) 2 ⇔ ZnHPO 4 + H 3 PO 4 , (2) 

3ZnHPO 4 ⇔ Zn 3 (PO 4 ) 2 + H 3 PO 4 . (3) 

A certain amount of free phosphoric acid must 

be present to repress the hydrolysis and to keep 

the bath stable for effective deposition of phosphate 

at the microcathodic sites. Another factor affecting 

the shift in the primary to tertiary phosphate equilibria 

is the temperature of the bath. Higher temperatures 

favour easy precipitation of the tertiary phosphates 

in a shorter time. Hence, more amount of 

phosphoric acid is needed for the baths operating 

at higher temperatures. In contrast, in the case of 

phosphating baths operated at room temperature, 

the possibility of the increase in acidity during continuous 

operation is more likely [129,130] and is 

normally neutralised by the addition of the carbonate 

of the metal which forms the coating (Zn(CO 3 ) 2 

in zinc phosphating bath). Hence, depending upon 

the working temperatures and the concentrations 

of the constituents in the bath, the free phosphoric 

acid content must be chosen to maintain the equilibrium 

condition. Too much of phosphoric acid not 

only delays the formation of the coating, but also 

leads to excessive metal loss. 

3.3. Acceleration of the phosphating 

process 

In practice, phosphating reaction tends to be slow 

owing to the polarization caused by the hydrogen 

evolved in the cathodic reaction. In order to achieve 

coating formation in a practicable time, some mode 

of acceleration must be employed. The importance 

of the acceleration of the phosphating process was


Table 3. Alternatives to Cr(VI) post treatment. 

Sl.No. Type of compounds Compound used Reference 

1. Chromium containing a. 0.001% Cr(III) as Cr- chromate complex 86, 87 

substitutes b.CrO solution reduced by HCHO 3 

2- c. Cr O as complex of divalent metal 2 7 

d. Chromate and colloidal silicic acid 

88 

89 

90 

e. Aqeuous solution of Aluminium chromate polymer 91, 92 

f. Trivalent chromium and ferricyanide 93 

2. Phosphonic acid a. Alkane phosphonic acids 94, 95 

derivatives b. Alkene phosphonic acids 96, 97 

c. Polyvinyl phosphonic acids 98 

3. Ammonium phosphate a. Dilute solution of ammonium primary phosphate 99, 100 

derivatives and/or trietha nolamine ydrogen phosphate 

4. Carboxylic acids a. Citric, glutaric, maleic, succinic and phthalic acids 101 

b. Citrate in combination with nitrite 102, 103 

5. Oxidizing agents a. Peroxides and persulphates 104 

b. Potassium permanganate 105 

6. Polymers a. Modified hydroxystyrene 106 

b. Melamine-formaldehyde 107 

7. Tannins a. Tannin + thiourea compound 108 

b. Tannin + melamine – formaldehyde 109 

8. Tin salts Aqueous stannous salt solution containing Mn, Pb, 

Cd, Co and Ni 

110-113 

9. Zirconium and Titanium a. Ti (III) in acid solution 114, 115 

compounds b. Zr or Ti with an inositol phosphate ester 116 

c. Zr + myo-inositol phosphate and/or its salt 117 

10. Molybdenum type 

rinses 

a. Dilute aqueous solution of MoO in HNO 2 3 118 

11. Hypophosphorus acid a. Hypophosphorus acid + Hydrofluorosilicic acid 119 

and hypophosphite b. Sodium hypophosphite + Hydrofluorosilicic acid 

12. Mixture of organo- a. Carboxyethylene phosphonic acid esters of butyl 120 

phophates or 

phosphonates and 

fluoride 

diglycidyl ether and bisphenol + fluorosilicic acid 

13 Mixture of Zr, V, F a. Hydrofluorozirconic acid + fluoroboric acid + 121 

and phosphate ions Phosphoric acid + amm. metavanadate 

14 Mixture of Li, Cu and 

Ag ions 

a. Nitrate (or) carboxylate (or) acetates of Li, Cu and Ag 122, 123 

15 Mixture of Ti, V, Mo, a. As nitrates, sulphates (or) chloride 124 

Ag, Sn, Sb + cathodic 

treatment at 0.1 to 

10 A/dm 

b. Salt of hydroxycarboxylic acid 

2 

16 Tetravalent Ti (or) a. Titanyl acetylacetonate 125 

Divalent Mn, Co, Ni b. Divalent manganese/cobalt/nickel/copper/ethanote 

or Cu c. Divalent 

17 Ni + Co + Sn + Pb a. As acetates + Pb sheets + stannous chloride 126 

135


felt way back in the 19th century and its development 

has gained a rapid momentum with the advent 

of the Bonderite process in 1929. Recently, 

Sankara Narayanan et al. [131] have made an overview 

on the acceleration of the phosphating process 

and justified its role on the hunting demand to reduce 

process time. The different means of accelerating 

the formation of phosphate coatings can be 

broadly classified as: (i) Chemical acceleration; (ii) 

Mechanical acceleration; and (iii) Electrochemical 

acceleration. 

3.3.1. Chemical acceleration 

Oxidizing substances [132,133] and metals more 

noble than iron such as, Cu, Ni, [19] etc., constitute 

the most important class of chemical accelerators. 

They accelerate the deposition process 

through different mechanisms. Oxidizing agents 

depolarize the cathode half-cell reaction by preventing 

the accumulation of hydrogen at the cathodic 

sites, whereas noble metal ions promote metal dissolution 

by providing low over-potential cathode sites 

by their deposition [134]. Since acceleration through 

depolarization is preferred to mere promotion of 

metal dissolution, oxidizing agents have found widespread 

use than metals. Moreover, they prevent the 

excessive build up of iron in the bath, which can be 

detrimental to good coating formation [25]. The most 

commonly employed oxidizing accelerators are nitrites, 

chlorates, nitrates, peroxides and organic nitro 

compounds either alone or in various combinations. 

Common combinations are nitrite-nitrate, nitrite-chlorate-nitrate 

and chlorate-nitrobenzene sulphonic 

acid. The characteristics of some of the commonly 

used oxidizing accelerators are given in Table 4. 

Some reducing agents such as alkali metal sulphites 

[135], hypophosphites [136], phosphites [137], formaldehyde, 

benzaldehyde, hydroxylamine [138], acetaldehyde 

oxime [139], Pyridine N-oxime [140], 

morpholine N-oxime [140], quinones [141], etc., have 

also been tried as accelerators but have not been 

as successful as oxidizing accelerators from the 

industrial point of view. 

3.3.2. Mechanical acceleration 

When a phosphating solution is sprayed forcibly on 

to a metal surface, coatings are formed more readily 

than would be by immersion in the same solution, 

since the former process eliminates the delay due 

to the diffusion of the constituents in the solution to 

the metal surface. The comparative kinetics of spray 

and dip phosphating was determined by Laukonis 

[142]. The resultant coatings are thin, fine-crystalline 

and perfectly suitable as a paint base. Other 

means of physical acceleration are the action of 

brushes and rollers [143] on the surface during processing. 

3.3.3. Electrochemical acceleration 

Several electrochemical methods of acceleration, 

both anodic, cathodic and pulse method have been 

described in literature [144-161]. Coslett recognized 

acceleration of the phosphating process by cathodic 

treatment as early as 1909 by Coslett [144]. Subsequent 

studies reveal that anodic methods are 

more appropriate and advantageous than the cathodic 

methods, as they promote metal dissolution 

as well as passivity. Zantout and Gabe [148] claim 

to have achieved higher coating weights of low porosity 

in a shorter treatment time by the application 

of a small current. Ravichandran et al., [157-159] 

established the utility of galvanic coupling of steel 

substrate with metals which are more noble than 

steel for accelerating phosphating processes. This 

methodology employs the galvanic corrosion principle 

for accelerating the metal dissolution reaction, 

which enables a quicker consumption of free phosphoric 

acid and an earlier attainment of the point of 

incipient precipitation, resulting in higher amount of 

phosphate coating formation. The application of a 

cathodic current to form phosphate coating on stainless 

steel using a calcium modified zinc phosphating 

bath was patented by Bjerrum et al. [160]. Sinha 

and Feser [161] have also studied the formation of 

phosphate coating on steel and stainless steel substrates 

by this method. 

These three methods of acceleration described 

above are widely practiced in industries; but 

each of them has its own merits and demerits. 

Though chemical accelerators (Primarily those of 

the oxidizing type) accelerate the phosphating process 

by their mere addition, their concentration in 

the bath is very critical to yield the desired results. 

Acceleration by mechanical means is limited to 

spray processes, which are capable of providing 

fresh bath solution constantly. Electrochemical 

methods of acceleration though capable of yielding 

higher deposition rate, the practical difficulty in adding 

‘electrics’ to the processing stage makes it less 

popular.


Table 4. Different types of accelerators used in phosphating. 

Type of Example Effective Advantages Disadvantages 

accelerator concentration 

- NO NaNO 1-3% Lower sludge Reduction of FePO increases 

3 

3 4 

Zn(NO ) the iron content of the coating 

3 2 

Ni(NO ) 3 2 

NO 2 

ClO 3 

- NaNO 2 0.1-0.2 g/l Affords rapid processing Corrosive fumes. 

137 

even at low temperatures Highly unstable at high bath 

temperatures. 

Frequent addition is required. 

- Zn(ClO 3 ) 2 0.5 - 1% Stable in liquid Corrosive nature of chlorate and 

concentrates. Can be its reduction products. 

used for bath make-up High concentrations poison 

and replenishment. the bath. Removal of gelati 

Overcomes the white nous precipitate from the 

staining problem. resultant phosphate coatings is 

difficult. 

H 2 O 2 H 2 O 2 0.05 g/l Low coating weight. Bath control tends to be 

(liquid) No harmful products. critical. Heavy sludge 

Free from staining formation. Limited stability. 

Continuous addition is required. 

Perborate Sodium – No separate neutraliser Continuous addition is 

perborate is required. required. Voluminous sludge. 

Good corrosion 

resistance 

Nitroguanidine Nitroguanidine – Neither the accelerator Slightly soluble. 

nor its reduction No control for build-up 

products are corrosive. of ferrous iron in the bath. 

Highly expensive. 

Oxime Acetaldehyde 1-5 g/l Stable in acidic – 

oxime environment 

Environmentally 

acceptable 

Organic Pyridine 0.3-2 g/l – Need to use highly 

N-oxide N-oxide concentrated activating 

Morpholine solution before 

N-oxide phosphating 

Hydroxylamine Hydroxylamine 0.5-5 g/l Spherical and/or Decomposes in presence of Cu 

Hydroxylamine columnar phosphate or H 2 O 2 in the bath 

sulphate crystal 

Does not decompose 

on its own 

Quinones Benzoquinone – Solubility of Quinone High concentration causes a 

Chloranil wavy pattern. Lower concentra 

tion not effective in acceleration 

Amido Benzoicacid 0.1-6 g/l Immediate operation Effectiveness is impaired in 

sulphonic acid sulphamide of the bath presence of Ca ions in the 

and their Benzenesulphoanilide Fine grained coating bath 

N-substituted N-cyclohexyl Improved lacquer 

derivates sulphamic acid adhesion and corrosion 

resistance. Sludge 

formation is suppressed 

Alkali metal Sodium bromate 0.8-1.1 + Fine grained coating – 

bromate + +nitrocompound 0.25-0.5 Improved adhesion and 

Aromatic m-nitro benzene- g/l corrosion resistance 

sulphonate


3.4. Kinetics of the phosphating 

process 

Kinetics of the phosphating process reveals the 

steps involved in the course of phosphating and their 

rates. Three different methods have been used so 

far for investigating the kinetics of formation of phosphate 

coatings namely: (i) the gravimetric method, 

by the quantitative determination of the quantity of 

phosphate deposited per unit time; (ii) the electrochemical 

method based on the determination of free, 

reactive uncoated areas through electrochemical 

passivation; and (iii) the radiographic method based 

on the determination of the intensity of the characteristic 

X-ray of the resulting compound. 

All the three methods gave a similar picture of 

the phosphating process that showed the coating 

formation did not take place in a linear fashion, rather 

it was initially very fast, after which the rate slowly 

decreased with time. 

Studies on the kinetics of phosphating indicate 

that there are four distinct stages in coating formation 

(Fig. 1), namely, the induction stage (α), the 

commencement of film growth (β), the main exponential 

growth stage (γ) and the stage of linear increase 

in film growth (δ). During the induction period, 

the oxide film remaining on the surface even 

after cleaning is removed. When film growth commences, 

the first nuclei are formed and the rate of 

nucleation increases rapidly with time. This, however, 

depends considerably on the conditions of the 

surface, the pretreatment procedures adopted and 

the oxidizing agents present in the phosphating bath. 

Growth occurs in the main exponential growth stage. 

Addition of accelerators reduces the induction period 

and extends the stage of linear growth. 

In the opinion of Gebhardt [162], the rate of phosphating 

depends on the rate of diffusion of Fe 2+ ions 

from the structural lattice to the coating/solution 

interface through the coating formed. Machu [25] 

has found that the rate of the phosphating reaction 

is a function of microanodes on the surface. 

-dF A /dt = K . F A dt 

where dt – change in time; F A – surface area of anodes 

in microcells; and K – reaction rate constant. 

The rate of formation of the phosphate coating 

depends primarily upon the metal viz., the ratio of 

anode surface which was initially present, F Ao , to 

the anode surface at any given moment, F A . The 

influence of various other factors controlling the rate 

of reaction, e.g., temperature, surface condition etc., 

is reflected in the value of the rate constant K, which 

is different for different processes. 

Fig. 1. The various stages in the growth of phosphate 

coating (in presence of accelerator). 

The overall coating growth process can be followed 

by potential-time curves (Fig. 2), which indicate 

the different stages of coating besides indicating 

when the effective phosphating has ceased. 

Correlation of potential-time relationships with film 

properties leads to the conclusion that coating formation 

proceeds through the following stages: 

(a) electrochemical attack of steel; 

(b) amorphous precipitation; 

(c) dissolution of the base metal; 

(d) crystallization and growth; and 

(e) crystal reorganization. 

But, in actual practice, it is difficult to identify the ‘b’ 

and ‘c’ stages, and the curve mainly exhibits the 

process of metal dissolution, coating formation and 

coating completion. The use of potential-time measurements 

for monitoring the phosphating process 

was first described by Machu [25]. It has also been 

used by several workers to reveal the nature of the 

Fig. 2. Potential-time curve showing the various 

steps of phosphating.


process taking place during phosphating [145] and 

the properties of the coating formed [163]. Sankara 

Narayanan et al. [164,165] have discussed the usefulness 

of the potential-time curves in predicting the 

kinetics of the phosphating process. The utility of 

potential-time measurement for effective on-line 

monitoring of the phosphating process is also established 

[166,167]. The correlation between coating 

weight and potential time measurements is established 

by Sankara Narayanan [168-170], which 

enables calculation of crystallization kinetics from 

potential time measurement. 

3.5. Process details 

In general, phosphating sequence comprises of seven 

operations, as indicated in the flow chart. However, 

depending upon the surface conditions of the base 

metal, some of these operations may be omitted or 

additional operations may be incorporated into the 

system. A typical seven-stage sequence of phosphate 

pretreatment process is given in Fig. 3. 

3.5.1. Cleaning 

Perhaps, the most important requisite for proper 

coating formation is a clean substrate, free from 

contaminants such as oils, greases, waxes, corrosion 

products and other soils. Many coating failures 

can be attributed to the poor metal surface 

preparation [171]. An ideal cleaning agent is the 

one, which is capable of removing all the contaminants 

from the metal surface, and prevents their redeposition 

or the formation of other detrimental reaction 

products [172]. A variety of methods such 

as sand blasting, solvent degreasing, vapour 

degreasing, alkaline cleaning and pickling have been 

used to achieve this end. 

Sand blasting is an effective method of mechanical 

cleaning. However, it is highly expensive and its 

use is justified as a field procedure where chemical 

treatments cannot be used and it is necessary to 

remove the loosened mill scale as well as paint [173]. 

Organic solvents are widely used to remove organic 

contaminants from the metal substrates. But 

they are of toxic and flammable nature and need to 

be used in large quantities, which is uneconomical. 

This has led to its replacement by the vapour 

degreasing technique. The unique advantage of the 

latter technique over solvent degreasing is that, continuous 

cleaning with small quantities of solvent is 

possible [174]. 

Alkaline cleaning provides an economical and 

effective alternative to the use of organic solvents to 

Degreasing 

pickling 

Rinsing 

Phosphating 

Rinsing 

Chromic acid sealing 

Drying 

139 

Fig. 3. Flow chart depicting the operating sequence 

involved in phosphating process. 

remove greases, oils and waxes. They are also used 

in conjunction with surface active (wetting) agents 

and emulsified hydrocarbon solvents [174]. Alkaline 

cleaners are particularly efficient when used hot 

(approx. 79 °C). While alkaline cleaning is free from 

the fire and toxicity hazards associated with organic 

solvent cleaning (unless emulsified solvents have 

been incorporated), the corrosive effects of alkaline 

materials on the skin and on ordinary clothing must 

be guarded against. Caustic soda in particular can 

cause serious burns to the skin and eyes and is 

extremely irritating to the nasal and bronchial membranes 

if inhaled. 

Acid cleaning or pickling using acids such as 

HCl, H 2 SO 4 , and H 3 PO 4 is a very effective method


for the removal of rust and mill scale [175]. Dilute 

solutions (5-10% by weight) of H 2 SO 4 and HCl are 

used in presence of inhibitors to remove the inorganic 

contaminants by converting them into their 

ferrous salts. Pickling in H 2 SO 4 is usually performed 

at high temperatures (about 60 °C). H 3 PO 4 is an 

excellent time-tested cleaning agent which not only 

removes organic and inorganic solids present on 

the metal but also causes chemical etching of the 

surface by reacting with it to produce a mechanically 

and chemically receptive surface for subsequent 

coating formation [176]. 

Electrolytic pickling is an alternative to chemical 

pickling, which provides better and rapid cleaning 

through an increased hydrogen evolution, resulting 

in greater agitation and blasting action [174]. 

3.5.2. Rinsing 

The rinsing step followed by cleaning plays a vital 

role in the phosphating sequence [172]. Rinsing 

prevents the dragout of chemicals used in the earlier 

cleaning that may contaminate the subsequent 

stages. 

3.5.3. Phosphating 

Suitably cleaned surfaces are next subjected to 

phosphating, which causes the formation of an insoluble, 

corrosion resistant phosphate layer on the 

substrate surface. A wide variety of phosphating 

compositions are available. However, the right choice 

of the components and the operating conditions of 

the phosphating bath are made based on the nature 

of the material to be treated and its end use. 

All the phosphating compositions are essentially 

dilute phosphoric acid based solutions containing 

alkali metal/heavy metal ions in them besides suitable 

accelerators [18,19,24,127,128]. Based on the 

nature of the metal ion constituting the major component 

of the phosphating solution, these compositions 

are classified as zinc, manganese and iron 

phosphating baths. The characteristics of the coatings 

obtained using these baths are presented in 

Table 5. 

Phosphating can be effectively performed on both 

ferrous and non-ferrous metals. Among the ferrous 

metals, mild steels are most frequently used although 

maraging steels, galvanized steels and stainless 

steels can also be coated [177-180]. Non-ferrous 

metals that can be phosphated include zinc, 

aluminium, magnesium and cadmium [181-183]. 

Physical properties like hardness, tensile strength 

and workability of the original metal are retained af- 

ter phosphating [21]. The dimensional change 

caused by phosphate coatings on the metal surface 

is of the order of 10 -3 mm. 

Phosphate deposition can be achieved through 

the use of both spray and immersion processes and 

the choice of the appropriate method depends upon 

the size and shape of the substrate to be coated 

and based on the end use for which the coating is 

made. Spray process is preferred where shorter processing 

times are required. This method, however, 

requires more factory floor space and special equipment 

for their application. Immersion process though 

slower, produce uniform coatings and they require 

less factory floor space as the process tanks can 

be arranged in a compact manner. The benefits of 

phosphating by total immersion were considered by 

Wyvill [184]. But, immersion processes are more 

susceptible to contamination during continuous 

operation than are spray processes. Smaller parts 

can be effectively and economically phosphated by 

immersion process whereas spray process is more 

suitable for larger work pieces. Nowadays, a combination 

of both spray and immersion process has 

been successfully used particularly in automobile 

industries [185]. 

Phosphating may be carried out at temperatures 

ranging from 30-99 °C and processing time can be 

varied from a few seconds to several minutes. Suitable 

choice of these parameters is determined by 

factors such as nature of the metal to be coated, 

thickness and weight of the coating required and 

bath composition. The process of phosphating involves 

a consistent depletion of bath constituents 

and in order to obtain a satisfactory phosphate coating, 

the bath parameters such as: (i) the free acid 

value (FA) which refers to the free H + ions present in 

the phosphating solution; (ii) total acid value (TA) 

which represents the total phosphate content of the 

phosphating solution; (iii) the ratio of FA to TA, expressed 

as the acid coefficient; (iv) accelerator content; 

(v) iron content; and (vi) other metallic and nonmetallic 

constituents present, have to be strictly 

controlled within the optimum limits. 

3.5.4. Rinsing after phosphating 

The surface that has been subjected to phosphating 

should be thoroughly rinsed with deionized water 

to remove any acid residue, soluble salts and 

non-adherent particles present on it which would 

otherwise promote blistering of paint films used for 

finishing. Generally overflow rinsing and spray rinsing 

are preferred [186].


Table 5. Characteristics of phosphate coatings. 

Characteristic Type of coating 

Iron Phosphate Zinc Phosphate Heavy Phosphate 

Coating weight 0.16-0.80 g/m 2 1.4 - 4.0 g/m 2 7.5-30 g/m 2 

Types Cleaner/coater Standard Manganese phosphate 

Standard Nickel-modified Zinc phosphate 

Organic phosphate Low-zinc Ferrous phosphate 

Calcium-modified 

Manganese-modified 

Common accelerators Nitrite/nitrate Nitrite/nitrate None 

Chlorate Chlorate Chlorate 

Molybdate Nitrobenzene Nitrate 

Sulphonic acid Nitroguanidine 

Operating temp. Room – 70 o C Room-70 °C 60-100 o C 

Free acid (points) -2.0 to 2.0 0.5 - 3.0 3.6 - 9.0 

Total acid (points) 5-10 10-25 20-40+ 

Prephosphate None Titanium phosphate Manganese Phosphate 

conditioners None Titanium phosphate 

None 

Primary use Paint base for low- Paint base for high- Unpainted applications 

corrosion environments corrosion environments 

Limitations Low painted corrosion Poor unpainted Expensive, long processing 

resistance; low corrosion resistance times 

unpainted corrosion 

resistance 

Materials needed Low-carbon steel Low-carbon steel, Stainless steel or low-carbon 

for tanks stainless steel or steel 

plastic-lined steel 

Application method Spray and immersion Spray and immersion Immersion only 

3.5.5. Chromic acid sealing 

The phosphate coatings are usually porous in nature. 

The porous nature will have a detrimental influence 

on the corrosion resistance of the phosphate 

coating unless they are sealed. A hot (70-80 °C) 

dilute chromic acid rinse is usually used for this 

purpose. This treatment reduces the porosity by 

about 50%. It improves the corrosion resistance by 

the deposition of insoluble chromates on the bare 

areas of the coating [25]. In addition to the benefits 

derived from precipitating insoluble salts and passivating 

any metal that might be exposed, the dilute 

chromic acid treatment is advantageous in that it 

helps to dissolve protruding crystals of the phosphate 

coating [187]. Besides, chromic acid posttreatment 

leaves a residue that is slightly acidic, 

which most paints can tolerate. Usually a concen- 

141 

tration range of 0.0125-0.050% is used. A gross 

excess of chromic acid causes blistering, poor adhesion 

and irregular yellowing of paint [188]. A mixture 

of chromic acid and phosphoric acid has also 

been used [188]. Acidic solutions of chromic acid 

of pH 2-5 are preferred, the free acid being controlled 

between 0.2 and 0.8 points and the total acid below 

5 points. 

The role of chromates in improving the passivity 

of phosphated steel and in improving the paint film 

adhesion has been the subject of many papers. 

According to Cheever [189], chromium atoms are 

distributed over the surface and not just between 

the phosphate crystals. Wenz and Claus [190] have 

shown that chromic acid final rinsing completely 

removes any adsorbed calcium from tap water rinsing 

after phosphating, deposits some chromium and


alters the Zn/Fe ratio. Maeda and Yamamoto [187] 

have studied the nature of chromate-treated phosphated 

surface using XPS. The Cr 2P spectra 

3/2 

suggest that the coating mainly consisted of trivalent 

chromium oxide with a small amount of a 

hexavalent chromium compound (Fig. 4). 

Deconvolution of the trivalent chromium peak results 

in two more peaks which can be attributed to Cr O 2 3 

(major part) and Cr(OH) . 1.5H2O (minor part), sug- 

3 

gesting an intermediate compound (polymerized 

state) with -ol (Cr–OH) and -oxo (Cr–O–) bonds. 

From the calculation based on the ratio of the two 

components, the chemical composition of the chromium 

oxide(III) was formulated to be CrOOH . 0.2H O. 2 

The CrOOH . 0.2H O contributes to increased paint 

2 

adhesion due to hydrogen bonding with resin components. 

Though the chromic acid sealing improves the 

corrosion resistance, the need for regular disposal 

of Cr(VI) effluents is a matter of concern because 

Cr(VI) causes serious occupational and health hazards. 

Several alternatives for Cr(VI) treatment were 

proposed (Table 3) [191]. In spite of the development 

of various alternative treatments, there still 

exists a strong belief that the extent of corrosion 

protection provided by them is not as good as that 

from Cr(VI) treatment and from the point of view of 

use on an industrial scale, none of these alternative 

post-treatments have been proved to be a completely 

acceptable replacement to Cr(VI). 

3.5.6. Drying 

After chromic acid rinsing the parts must be dried 

before finishing, the conventional methods used 

being simple evaporation, forced drying by blowing 

air or by heating [88]. Where evaporation conditions 

are good, warm air circulating fans and compressed 

air blow offs are the most economical methods. After 

drying the phosphated panels are ready for application 

of further finishes such as paints, oils, varnishes, 

etc. 

3.6. Coating characteristics 

3.6.1. Structure and composition 

Phosphate coatings produced on steel, zinc, zinccoated 

steel, aluminium and other similar metals 

show a crystalline structure with crystals ranging 

from a few to about 100 micrometer in size. Various 

workers [24,192,193] have reported a large number 

of different constituents of phosphate coatings. 

Machu [25] listed 30 such phosphate compounds 

identified in a phosphate coating. Neuhaus and 

Fig. 4. Cr2p 3/2 XPS spectra of phosphate coating 

post-treated with chrome rinse (Parlen 60) (Reprinted 

from Progress in Organic Coatings, Vol. 33, 

S. Maeda and M. Yamamoto, The role of chromate 

treatment after phosphating in paint adhesion, pp. 

83-89 (1989) with permission from Elsevier Science). 

Gebhardt [192] have tabulated the main phases in 

phosphate coatings formed on metals from baths of 

various phosphates (Table 6). The composition of 

phosphate coatings is influenced by a number of 

factors such as the method of application (spray or 

dip), the degree of agitation of the bath, bath chemistry, 

the type and quantity of accelerator and the 

presence of other metal ions. Chamberlain and Eisler 

[194] have found using radioactive tracers that the 

base layer was formed initially from the metal being 

attacked during the first few seconds of contact with 

the phosphating bath producing a very thin film. The 

film contains oxides and phosphates of the metal 

being treated. Ferrous phosphate is most likely to 

be present in the case of steel. The growth of phosphate 

coating is initiated by the formation of a subcrystalline 

layer on which crystalline layer of phosphates 

build up rapidly. The number of crystals on 

which growth has occurred is essentially constant 

with time because nucleation and growth takes 

place only at a limited constant number of areas


Table 6. Phase constituents of phosphate coatings on Fe, Zn and Al. 

Metal in the bath Substrate 

Fe Zn Al 

Alkali 

Fe 

Mn 

Fe (PO ) . 8H2O Zn (PO ) . 4H2O 3 4 2 

3 4 2 

Fe H (PO ) . 4H2O Zn (PO ) . 4H2O 3 2 4 4 

3 4 2 

(Fe hureaulite) Zn Fe(PO ) . 4H2O 2 4 2 

FePO . 2H2O (strengite) Fe H (PO ) . 4H2O 4 

5 2 4 4 

(Mn.Fe) H (PO ) . 4H2O Zn (PO ) . 4H2O 5 2 4 4 

3 4 2 

Mn H (PO ) . 4H2O 5 2 4 4 

(Mn hureaulite) 

AlPO4 Mn H (PO ) . 4H2O 5 2 4 4 

Zn Zn Fe(PO ) . 4H2O 2 4 2 

(phosphophyllite) 

Zn (PO ) . 4H2O 3 4 2 

Zn (PO ) . 4H2O 3 4 2 

(hopeite) 

ZnCa Zn Fe(PO ) . 4H2O 2 4 2 

Zn Ca(PO ) . 2H2O 2 4 2 

(scholzite) 

Zn (PO ) . 4H2O 3 4 2 

Zn (PO ) . 4H2O 3 4 2 

Zn Ca(PO ) . 2H2O 2 4 2 

Zn Ca(PO ) . 2H2O 2 2 2 

Zn (PO ) . 4H2O 3 4 2 

[195]. It is believed that the formation of phosphate 

coating follows an active site mechanism i.e., only 

a small percentage of the surface, participates in 

the nucleation of the growth sites. Machu has postulated 

that these active sites were the growth areas 

located predominantly at the grain boundaries 

of the steel [196]. 

Light and electron microscopic studies made on 

zinc phosphated steel, have shown that the formation 

and growth process occur in three stages. 

1st stage: Depending on the zinc phosphating process, 

10 4 -10 6 /cm 2 platelet crystals are formed 

on the surface. These are randomly oriented to 

the steel substrate; some are parallel, some 

vertical and others are inclined at an angle. Those 

platelets, which are not, attached parallel to the 

surface are needle-like in appearance. These first 

stage crystals grow primarily laterally over the 

substrate. 

2nd stage: It consists of nucleation and growth of 

several crystals on the upper surface of the original 

crystals that are attached parallel to the surface. 

This growth process is usually vertical to 

the site and gives the appearance of additional 

needle-like crystals. 

3rd stage: Finally, a thin layer of zinc phosphate 

spreads from the base of the original crystals. 

For paint-base zinc phosphate coatings, the 

143 

growth is primarily lateral. For coatings used for 

oil retention and as bases for cold forming lubricants, 

considerable vertical growth also occurs. 

The crystal habit and size depends on many factors 

like the bath composition, temperature, method 

of surface preparation etc. Crystals may take the 

form of plates, needles and grains having dimension 

from a few to tens of micrometers. 

Many instrumental methods are available for the 

examination of the constituents of phosphate coatings. 

These include energy dispersive spectroscopy 

(EDS), electron spectroscopy for chemical analysis 

(ESCA) and X-ray diffraction (XRD). Neuhaus et 

al. [193] have reported from XRD analysis that 

phosphophyllite, Zn Fe(PO ) . 4H2O and hopeite, 

2 4 2 

Zn (PO ) . 4H2O are the essential constituents of 

3 4 2 

zinc phosphate coatings on ferrous substrate, substantiated 

by others [197, 198] also. 

Phosphophyllite is formed essentially at the surface 

of contact with the basis metal. The quantitative 

ratio of these phases is variable and depends 

on the total iron content of the solution. Iron 

Hureaulite, Fe H (PO ) . 4H2O is formed when the 

5 2 4 4 

Fe(II) content in the solution is high. This is not 

stable under atmospheric conditions and has a detrimental 

influence proportional to its content in the 

coating.


Miyawaki et al. [199] have introduced the concept 

of ‘P ratio’ to express quantitatively the proportion 

of these phases in phosphate coatings. It is 

defined as: 

Phosphophyllite 

‘P ratio’ = Phosphophyllite + Hopeite 

Several authors [199,200] have correlated the ‘P 

ratio’ and corrosion resistance of the phosphate 

coating. But according to Richardson et al. [200] 

the ratio itself is not sufficient to predict the corrosion 

resistance. 

X-ray and electron diffraction studies have shown 

that hopeite and phosphophyllite are oriented perpendicular 

to the plane of the support besides 

exhibiting that there is an excellent orientation between 

the substrate and the zinc phosphate coating 

that follows an epitaxial growth relationship. 

(010) - hopeite and (100) - phosphophyllite || (100)α-Fe 

(100) - hopeite and (001) - phosphophyllite || A o -α- 

Fe and 

(001) - hopeite and (010) - phosphophyllite || A o -α- 

Fe 

The formation of primary valency bonds between 

the coating and the polarized elements of the α-iron 

lattices with the production of a two dimensional 

contact layer of the wustite-type accounts for the 

excellent adhesion of the coating to the substrate. 

3.6.2. Coating thickness and coating 

weight 

One of the principal factors involved in the choice of 

a phosphating bath is in fact the thickness of the 

deposit that it will provide. Neglecting intercrystalline 

voids and surface irregularities and considering the 

phosphate coating as completely homogeneous, the 

thickness can be measured [201]. Phosphate coatings 

range in thickness from 1 to 50 microns but for 

practical purposes the thickness is usually quantified 

in terms of weight per unit area (usually as g/m 2 

or mg/ft 2 ) and commonly referred to as coating 

weight. The reason for the adoption of coating weight 

rather than coating thickness as the usual measure 

of coatings is the difficulty in measuring the 

latter, compounded by the uneven nature of the substrate 

and of the coating. 

According to Lorin [127] the ratio between coating 

weight (g/m 2 ) and coating thickness (μm) varies 

between 1.5 and 3.5 for the majority of industrial 

phosphate coatings. For light and medium weight 

coatings 1 μm can be regarded as equivalent to 1.5- 

2 g/m 2 . 

The determination of coating weight is a destructive 

test, which involves weighing a standard test 

panel before and after stripping the coating in a 

medium, which dissolves the coating and not the 

substrate. Usually methods such as stripping the 

coating in concentrated hydrochloric acid containing 

antimony trioxide (20 g/l) as an inhibitor or high 

concentration of chromic acid solution (5%) or sodium 

hydroxide (15%) is used. A non-destructive 

method based on specular reflectance infrared absorption 

(SRIRA) for the determination of zinc phosphate 

coating weight has been developed by Cheever 

[202, 203]. Tony Mansour [204] has shown a good 

agreement of the results obtained by this method 

with X-ray fluorescence (XRF) data as well as the 

gravimetric measurements. Yap et al. [205] suggest 

that XRF is a nondestructive and accurate technique 

for measuring the thickness of thin phosphate coatings. 

The phosphating industry generally uses coating 

weight as a method of quality control; but it is 

widely agreed that except at extreme values, coating 

weight does not directly relate to corrosion performance. 

Hence coating weight alone is of little 

value in assessing the quality of coatings and must 

be considered in relation to other characteristics of 

the coatings, viz., thickness, structure homogeneity, 

etc. 

3.6.3. Coating porosity 

The layer of phosphate coating consists of numerous 

crystals of very different sizes, which have 

spread from centers of nucleation to join and finally 

cover the surface. A constitution of this type inherently 

implies the existence of fissures and channels 

through to the basis metal at inter-crystalline 

zones. Porosity is generally fairly low, of the order 

of 0.5-1.5% of the phosphated surface [25]. It is 

generally believed that the porosity decreases with 

increasing thickness of the phosphate coating. Porosity 

depends upon the type of phosphate solution, 

the treatment time, the iron content of the bath 

and, the chemical composition of the coating [206]. 

In recent years, much attention has been focused 

on the porosity of phosphate coatings due to the 

presence of tightly bound carbonaceous residues 

formed on steel during steel making [207]. Since 

these cannot be removed by alkaline cleaning process, 

they interfere with the effective deposition of 

phosphate coatings resulting in the formation of 

porous coatings with inferior performance. 

Several chemical and electrochemical methods 

have been developed to determine the porosity of


Fig. 5. FTIR absorption-reflection spectra of zinc 

phosphate coating formed on steel 

(a) as-prepared; (b) partially dehydrated at 150 °C 

for 1 hour; (c) partially dehydrated and rehydrated 

at 95% RH for 70 hours. (Reprinted from Journal of 

Molecular Structure, Vol. 511-512, A. Stoch, Cz. 

Paluszkiewicz and E. Dlugon, An effect of 

methylaminoethoxysilane on zinc phosphate rehydration, 

pp. 295-299 (1999) with permission from 

Elsevier Science). 

phosphate coatings. The time taken for the deposition 

of metallic copper from a copper sulphate-sodium 

chloride solution and the electron microprobe 

determination of the amount of copper deposited in 

the bare areas of the coating are the measures of 

coating porosity [163]. The number of Prussian blue 

spots formed per unit area of a filter paper soaked 

in potassium ferricyanide-sodium chloride-gelatin 

mixture placed on a phosphated sample is yet another 

method of determination of the porosity of 

these coatings [208]. 

Machu’s [25] method of determination of porosity 

of coating by electrochemical means is based 

on the anodic passivation of uncoated areas in sodium 

sulphate solution. A more recent electrochemical 

polarization method is based on the magnitude 

of current generated by the reduction of oxygen at 

the cathodically active sites on the porous coating 

[209,210]. 

Although porosity of the coating has a detrimental 

effect on its corrosion performance, it has some 

advantages too. The pores in the phosphate coatings 

act as large reservoirs into which organic ma- 

145 

terials can collect [195]. This attribute is made use 

of in the protection of coated articles by impregnation 

of oil in the pores. Thus, a homogeneous finecrystalline 

coating is desirable with respect to adhesion 

of a paint film while a coarse-crystalline coating 

is preferred for protection by oils, varnishes etc. 

3.6.4. Stability of the phosphate 

coating 

The stability of the phosphate coating is an important 

characteristic property. Since phosphate coatings 

serve as an effective pretreatment for subsequent 

paint finishes, it is imperative that they must 

be compatible with the applied paint systems. In 

recent years, numerous developments were made 

in the paint finishes. Among them, the cathodic 

electrophoretic deposition has received a widespread 

acceptance. In order to be compatible with 

this deposition technology, the phosphate coating 

must process an excellent thermal and alkaline 

stability. The importance of stability of phosphate 

coating to suit the modern day finishing systems 

was discussed by several authors 

[18,19,24,127,211-218]. 

During cathodic electrophoretic deposition, the 

decomposition of water produces hydroxyl ions, the 

creation of which is considered to be critical as they 

can cause dissolution of the phosphate coating [140, 

141]. Several researchers have confirmed the dissolution 

of phosphate coating in high-pH environments 

[142-147]. It was found that approximately 

30-40% of the phosphate coating gets dissolved 

during cathodic electrophoretic deposition. This has 

lead to a greater porosity of the phosphate coating. 

Moreover, the occlusion of the dissolved ions, which 

are subsequently, concentrated during paint baking 

affects the corrosion resistance. After cathodic 

electrocoating the coated panels are usually cured 

at a temperature of 180 °C for 20 minutes. According 

to Kojima et al. [148] and Sugaya and Kondo 

[149], under such curing conditions the phosphate 

coating will undergo a definite weight loss associated 

with a structural change in the constituent crystals. 

It is generally advised that the loss in weight 

should be restricted to less than 15%, which would 

otherwise cause detoriation of the phosphate coating 

and a loss in corrosion resistance. The other 

important factor is the ability of the dehydrated phosphate 

crystals to revert back to its original hydrated 

form when subjected to humid service conditions. 

The dehydration-rehydration phenomenon is further 

confirmed by X-ray diffraction. During heating at 150 

°C for 1 hour the lines at 9.65 and at 19° 2θ dimin-


Fig. 6. X-ray diffraction pattern of zinc phosphate coating formed on steel (a) as-prepared; (b) partially 

dehydrated at 150 °C for 1 hour; (c) partially dehydrated and rehydrated at 95% RH for 70 hours. (Reprinted 

from Journal of Molecular Structure, Vol. 511-512, A. Stoch, Cz. Paluszkiewicz and E. Dlugon, An effect of 

methylaminoethoxysilane on zinc phosphate rehydration, pp. 295-299 (1999) with permission from Elsevier 

Science). 

ished or disappeared and new lines were observed 

at 10.90, 11.20 and 22° 2θ. During rehydration the 

line at 11.20° 2θ diminished while that at 9.65° 2θ 

grew (Fig. 6). It has been observed that the rehydration 

of the phosphate crystals induces residual 

stresses and reduce the phosphate-paint film adhesiveness 

[150]. 

Numerous research reports are available to date, 

which elaborate the behavior of phosphate coatings 

during chemical and thermal aggressions. The dissolution 

of zinc phosphate coating in high-pH environments 

was first reported by Wiggle et al. [142]. 

Later, using sodium hydroxide as a simulated medium, 

Roberts et al. [143] demonstrated the selective 

leaching of phosphorous from the coating. Van 

Ooij and de Vries [144] interpreted X-ray photoelectron 

spectroscopic studies to show that hydroxyl 

ions exchange phosphate ions in the first few layers 

of the phosphate coating. Servais et al. [140] 

determined the chemical stability of zinc phosphate 

coatings by subjecting them to immersion treatment 

in 0.8 g/l sodium hydroxide solution (pH 12.3) at 40 

°C for 3 minutes. Similarly, the effects of high-pH 

environments on zinc phosphate coatings were determined 

by Sommer and Leidheiser Jr. [141], using 

0.01, 0.1 and 1.0 M sodium hydroxide solutions. 

Kwiatkowski et al. [151] recommended a borate 

buffer solution containing 0.01 M ethylenediaminetetraacetic 

acid (EDTA) as the medium to test the 

alkaline stability of the coating and suggested its 

usefulness in predicting the same at a shorter time 

interval. All these studies uniformly agree that coatings 

richer in phosphophyllite possess greater alkaline 

solubility and can serve as effective bases 

for cathodic electrophoretic painting. 

Phosphate coatings undergo marked changes 

when subjected to variation of temperature. 

Thermogravimetric technique is usually used to 

monitor these changes [152]. The changes occurring 

in zinc phosphate coating on steel upon heating 

was studied by Kojima et al. [148] and Sugaya 

and Kondo [149]. When heated above room temperature, 

a gradual loss in weight occurs. This was 

attributed to a nonstructural weight loss. However, 

with subsequent increase in temperature dehydration 

of the constituent phases of the phosphate crystals, 

occurs. The dehydration of hopeite 

[Zn (PO ) . 4H2O] and phosphophyllite 

3 4 2 

[Zn Fe(PO ) . 4H2O] commences respectively at 80 

2 4 2 

and 110 °C; however, a more pronounced weight 

loss occurs at approximately 150 °C, where hopeite


loses two molecules of water of hydration. When 

heated above 150°C, both phosphophyllite and 

hopeite are transformed into their dehydrated forms. 

The hopeite phase is completely dehydrated at a 

temperature of approximately 240 °C. The weight 

loss considerably increased between 250 and 600 

°C. Above 600 °C sublimation of zinc and phosphorous 

occurs, which results in complete breakdown 

of the coating. The change in appearance, colour 

and morphology of phosphate coatings remain practically 

unaltered up to 200 °C. However, above this 

temperature, the grey crystalline phosphate coating 

changes to a silver grey form and appears dusty. 

Above 500 °C the colour changes to brown and above 

600 °C complete breakdown of coating occurs [148]. 

Bonara and his co-workers [152,153] have accounted 

for the observed difference in temperature 

at which both the hopeite and phosphophyllite crystals 

lose their water of crystallization. According to 

them, hopeite crystals seem to possess only one 

type of crystallization water species, which may be 

readily released during a progressive heating. In 

contrast, the phosphophyllite have three different 

crystallization water types, each with different bond 

types, thus causing a differential release during progressive 

heating. 

Based on the above facts on thermal stability of 

phosphate coatings and the normal baking conditions 

used (180 °C for 20 minutes) for curing cathodic 

electrocoating, it is quite obvious that both 

hopeite and phosphophyllite will exist as bihydrated 

crystals. The rehydration of these bihydrated hopeite 

and phosphophyllite was a subject of considerable 

importance as it determines the wet adhesion property. 

It has now been established that when placed 

under a rehydration condition, like immersion in an 

aqueous solution or exposure to high humid atmospheres, 

the hopeite bihydrated crystals undergo 

rehydration to the tetrahydrated phase along with 

the formation of zinc oxide. The formation of these 

products induces stresses and bond relaxation at 

the paint-phosphate interface and ultimately affects 

the adhesiveness. In contrast to this behavior, the 

phosphophyllite bihydrated crystals are found to 

resist the rehydration phenomenon. Liebau [233] 

has accounted for the difference in rehydration 

behaviour of hopeite and phosphophyllite bihydrate 

crystals. According to him, in hopeite it is the flexibility 

of the Zn 2+ ion co-ordination state, which can 

exist in either octahedral or tetrahedral coordination, 

thus allowing the dehydration process to occur. 

However, in phosphophyllite, the Fe 2+ ions occupy 

the octahedral co-ordination sites, which are 

the unsaturated co-ordination states. Hence an ir- 

147 

reversible structural modification has resulted during 

the dehydration process. 

Hence it is evident that a phosphate coating richer 

in phosphophyllite phase is best suited to withstand 

the environments that occur during cathodic electrophoretic 

painting, paint curing and under service 

conditions. This has necessitated modification of 

the phosphating formulations to produce phosphate 

coatings richer in phosphophyllite phase. The influence 

of the phase constituents on the stability of 

phosphate coatings studied by Sankara Narayanan 

[213] also confirms this generalization. 

The introduction of pre-coated steels such as 

zinc, zinc alloy coated steels, etc., for automotive 

body panels present new challenges regarding the 

stability of phosphate coatings [234]. When coated 

using a zinc phosphating bath, the zinc and zinc 

alloy coated steels will produce phosphate coatings, 

which comprise only the hopeite phase. As 

stated earlier, phosphate coatings richer in hopeite 

is not desirable for subsequent cathodic 

electrocoating finishes. However, these pre-coated 

steels show good performance with regard to formability 

and weldability. The extent of perforation and 

cosmetic corrosion is also less with these materials 

and they also posses a good surface appearance. 

Hence to effectively make use of the zinc and 

zinc alloy coated steels, the phosphating formulation 

has to be modified to produce coatings that are 

not richer in hopeite. It is now realized that although 

low hopeite content is the principle requirement for 

cathodic electrocoating, it is not essential that the 

coating should be richer in phosphophyllite as long 

as the coating has substantial amorphous material. 

The introduction of nickel and manganese modified 

low-zinc phosphating formulations are the most 

significant modification proposed to cope up with 

the cathodic electocoating, particularly in the automobile 

industry. The presence of Ni 2+ and/or Mn 2+ 

ions in these phosphating baths has yielded several 

advantages. Although there is a decrease in 

coating weight, crystal formation is observed at an 

early stage and the crystal size is fine for zinc phosphate 

crystals obtained from solutions containing 

Ni 2+ and Mn 2+ ions. It is confirmed that these heavy 

metal ions participate in forming the crystal and 

brings about the crystal refinement and such an effect 

is found to increase with an increase in their 

concentration [235,236]. It is also reported that Mn 2+ 

ions are more effective in causing the nucleation of 

the crystal when compared to the effect of Ni 2+ ions 

[236]. The usefulness of the addition of Mn 2+ ion 

has been advocated based on several other factors.


Mn 2+ ions demonstratably improve the corrosion 

resistance of coatings obtained from a low zinc phosphating 

process. The presence of Mn 2+ ions in low 

zinc phosphating bath increases the rate of formation 

of phosphate coatings. Hence, it is possible to 

decrease the bath temperature, which in turn allows 

a considerable amount of savings in the heating 

costs. Moreover, the addition of Mn 2+ ions in the 

phosphating bath helps to increase the working width 

of the phosphating bath. Also, the presence of manganese 

in the zinc phosphate coating resists the 

formation of white spots on galvanized steel. However, 

it is also cautioned that if the manganese content 

exceeds a certain level, a decrease in corrosion 

resistance may occur. Advanced analytical 

techniques have shown that the manganese tends 

to be distributed throughout the coating whereas 

nickel is concentrated at the interface and the presence 

of both contributes to maximum performance. 

Based on the results of the electron spin resonance 

(ESR) technique, Sato et al. [237] confirmed that 

both nickel and manganese are present in the zinc 

phosphate films respectively, as Ni(II) and Mn(II). 

According to them, the nickel and manganese doped 

hopeite are formed by the following mechanisms: 

Zn(H PO ) → ZnHPO + H PO , (4) 

2 4 4 3 4 

3Zn(H PO ) → Zn Me (PO ) . 4H2O + H PO . (5) 

2 4 3-x x 4 2 

3 4 

The chemical structure of the modified hopeite 

crystal in general was considered as 

Zn Me (PO ) . 4H2O, where Me = Ni or Mn. 

3-x x 4 2 

Accordingly, the chemical structure of the doped 

hopeite when nickel and manganese coexist is 

suggested as Zn (Ni Mn )(PO ) . 4H2O. The 

3-x-z x Z 4 2 

possibility of existence of such a structure was 

confirmed by various analytical techniques such as 

electron spin resonance (ESR), extended X-ray 

absorption fine structure (EXAFS), laser Raman 

spectroscopy, etc. [237-243]. 

Besides the modification of the phosphating bath 

by Ni2+ and/or Mn2+ ions, developments were also 

made in modifying the surface of the steel sheets. 

The formation of a Fe-P coating of about 2 g/m2 containing a phosphorous content of 0.5% or below, 

on Zn-Ni or Zn-Fe steel sheets is another notable 

development [234, 244]. These materials are 

commonly known as double-layered Fe-P/Zn-Ni and 

Fe-P/Zn-Fe alloy plated steel sheets. The 

phosphatability of these steel sheets are highly 

comparable to that of cold-rolled steel sheets. It is 

the presence of micro quantity of phosphorous, uniformly 

dispersed in the iron coatings, which activates 

the surface and enhances the reaction with 

the phosphating bath, resulting in the production of 

closely packed phosphophyllite rich coating, having 

a crystal size of 3-5 mm. The formation of such 

a coating improves the wet adhesion property. Hence 

the modifications made in the phosphating formulations 

to produce coatings richer in phosphophyllite, 

to serve as an effective base for electrocoat finishing 

systems [225], are rightly justified. 

3.7. Influence of various factors on 

coating properties 

3.7.1. Nature of the substrate 

3.7.1.1. Composition of the metal. The presence 

of alloying elements and their chemical nature cause 

distinct difference in the extent of phosphatability of 

the substrate. It is generally believed that steels 

with small amounts of more noble metals such as 

chromium, nickel, molybdenum and vanadium can 

be phosphated without much difficulty. However, if 

the concentration of these elements exceeds certain 

limit then the steel will encounter with a decrease 

in the intensity of acid attack resulting in a 

poor coating formation and among the noble metals 

in causing such an effect, chromium is considered 

to be the most detrimental one [245]. The 

phosphatability of cold rolled steel containing varying 

amounts of chromium, nickel and copper is given 

in Table 7. It is evident from Table 7 that the 

phosphatability is not affected when the total concentration 

of chromium, nickel and copper is in the 

range of 200-300 ppm and gets drastically affected 

when the total concentration of these metals exceeds 

800 ppm. The amount of carbon, phosphorus, 

sulphur, manganese and silicon can also influence 

the phosphatability of the steel to a great extent. 

Low carbon steels undergo phosphating easily 

and produce superior quality coatings. With increasing 

carbon content, the rate of phosphating 

becomes slower and the resultant crystals are 

larger. In fact, when dealing with the carbon concentration 

in steel, it is the distribution and the form 

in which it is present is rather an important criterion 

in predicting its probable effect on sensitization, 

nucleation and crystallization. The presence of ferrite 

crystals improve metal attack while on the other 

hand increasing concentrations of pearlite leads to 

coarsening of the phosphate crystals as the crystal 

nuclei develop only to a small extent on the islands 

of pearlite [127,246]. The deleterious effect of surface 

carbon content has been the subject of many 

investigations and based on the amount of surface 

carbon, steels have been classified as ‘bad’ or ‘good’ 

with reference to their suitability to phosphating


Table 7. Phosphatability of cold rolled steel containing varying amounts of chromium, nickel and copper 

(after Kim [245]). 

Substrate Cu + Ni + Cr Surface roughness Phosphat- Coating weight (g/m 2 ) 

designation (ppm) (R a ) (mm) ability* In the centre In the edge 

CRS-A 300 1.2 1 4.04 5.39 

CRS-C 800 1.2 4 3.54 3.59 

* Based on visual appearance. Rating: 1 – Good; 5 – Poor. 

[247,248]. Grossman [248] has found up to 30 times 

as much carbon on “bad” steels as on the “good” 

steel samples. A direct correlation between surface 

carbon content and the porosity of the phosphate 

coatings on the one hand and their salt spray resistance 

when painted on the other hand was found by 

Hospadaruk et al. [207,249]. Wojkowiak and Bender 

[250] using a multiple regression analysis have confirmed 

the existence of a positive correlation between 

the extent of underfilm attack and the surface carbon 

content of the steel. 

Takao et al. [251,252] have investigated the effect 

of phosphorus on the phosphatability of ultra 

low-carbon steel sheets. According to them, the 

addition of phosphorus was beneficial in refining the 

grain size and increasing the surface coverage. 

Moreover, they have proved that the phosphorus 

addition helps to improve the perforation corrosion 

resistance after painting. In contrast, Kargol and 

Jordan [253] have claimed that phosphorus alloy 

addition inhibits the hydrogen recombination during 

the initial stages of phosphating resulting in increased 

porosity and inferior corrosion performance. The rate 

of attack of the metal in phosphating solution is also 

affected by the concentration of sulphur. The 

phosphatability of the dual-phase (Si-Mn) steel is 

primarily controlled by the balance of silicon and 

manganese [254]. 

Although it is clearly evident that every common 

constituent element of steel has its own influence 

in determining its phosphatability either to a greater 

or smaller extent, the most unfavorable from the 

phosphating point of view are those which are alloyed 

with carbide forming elements such as chromium 

and tungsten, and the surface carbon content. 

As a result, attempts have been made to eliminate 

such troublesome factors. Accordingly, the 

surface carbon content of the steel chosen for phos- 

149 

phating has to be within the specified limits. Fujino 

et al. [255] have concluded that a contamination of 

greater than 8 mg/m 2 deteriorates the phosphate 

coating and resistance to corrosion after painting. 

Blumel et al. [256] and Balboni [257] have predicted 

an upper limit of 7 mg/m 2 . But Coduti [258] has concluded 

that this amount has to be a maximum of 

4.3 g/m 2 for good performance since only an average 

performance has resulted when the concentration 

lies between 4.3 and 6.4 mg/m 2 . 

3.7.1.2. Structure of the metal surface. The structure 

of the metal surface has also its own influence 

and considered to be equally as important as that 

of the composition. Ghali [259] has made a thorough 

study of the surface structure in relation to 

phosphate treatment. It is generally believed that 

greater the surface roughness, the higher the weight 

of the coating deposited per unit of the apparent 

surface area and the shorter the time of treatment 

required. Moreover, increasing surface roughness 

gives a corresponding increase in the fineness of 

the coating structure whereas polished surfaces 

respond poorly to phosphating. Beauvais and Bary 

[260] have reported that if the metal surface possesses 

a greater number of surface concavities and 

fissures, there will be an increased acid attack during 

phosphating resulting in good anchorage of the 

coating. According to Kim [245], phosphatability can 

be improved by increasing the surface roughness. 

Balboni [257] has considered that a surface roughness 

of 0.76-1.77 μm is acceptable for most of the 

cases. 

The growth of the phosphate coating is considered 

to be an epitaxial growth phenomena and literature 

reports have convincingly established the 

following epitaxial relationships in the case of a zinc 

phosphate coating on mild steel constituting the 

phosphophyllite (FeZn (PO ) . 4H2O) and the hopeite 

2 4 2


(Zn 3 (PO 4 ) 2 .4H 2 O), by means of X-ray diffraction. According 

to Ursini [261, 262] the orientation relationship 

(412) phosphophyllite /(111) Fe α role is very 

important for epitaxial growth on cold rolled steel 

and for an increasing adhesive bond. Laukonis [142] 

has studied the role of the oxide films on phosphating 

of steel and concluded that it is easier to deposit 

zinc phosphate coatings on ferric oxides than 

on ferrous oxides. Hence it is evident that it is not 

only the surface roughness but also the orientation 

and the presence of the oxide film will influence the 

phosphating process to a considerable extent. 

3.7.1.3. Surface preparation. The cleaning methods 

adopted can also influence the phosphating of 

steel sheets. It is essential to remove any greasy 

contaminants and corrosion products from the surface 

to obtain a good phosphate finish. Degreasing 

in organic solvents usually promotes the formation 

of fine-grained coatings while strong alkaline solutions 

and pickling in mineral acids yield coarse coatings. 

Though, the prevention of the excessive metal 

attack during pickling is effected by potent inhibitors, 

it is generally observed that surfaces that have 

been pickled with an effective pickling inhibitor are 

difficult to phosphate unless a strong alkaline cleaning 

operation is employed to remove the inhibitor 

residues. Moreover, it is believed that the smut 

formed during pickling operation can also influence 

the amount and crystal size of the phosphate coating 

[188]. However, there are reports available, which 

are successfully employing potent inhibitors in the 

pickling bath, which eliminates the usual detrimental 

effects on phosphating. It is claimed that the 

inhibitors adsorbed onto the metal surface reduces 

the amount of hydrogen in the surface layer of the 

phosphated metal and the phosphate coating produced 

in such a way are satisfactory for lowering 

friction, facilitating cold working and as undercoats 

for paint [263]. 

3.7.1.4. Surface activation. The activating effect 

of colloidal titanium phosphate was discovered by 

Jernstedt [264] and latter explored by several others 

[265]. The mechanism of the activation has been 

established by Tegehall [266]. The colloids in the 

aqueous dispersion are disc shaped particles which 

have the composition of Na TiO(PO ) . 0-7H2O. These 

4 4 2 

particles are physically adsorbed on the metal surface 

during the application of colloidal dispersion. 

When the activated substrate comes in contact to 

a zinc phosphating bath, an ion exchange between 

the sodium ions on the surface of the titanium phosphate 

particles and the zinc ions of the phosphating 

solution takes place [267]. The ion-exchanged 

particles act as nucleation agents for the zinc phosphate 

crystals because they have nearly the same 

stoichiometry and offer a crystallographic plane for 

an epitaxial growth. 

The rate of nucleation of the phosphate coating 

may be markedly increased by imposing an additional 

activation of the cleaned metal surface. A process 

for activating steel surface prior to phosphating 

was patented by Yamamato et al. [268] and 

Donofrio [269]. Hamilton [270] has proposed a combination 

of acid cleaning and a phosphating compound 

for surface activation, while Hamilton and 

Schneider [271] and Morrison and Deilter [272] have 

proposed a highly alkaline titanated cleaner prior to 

phosphating. Use of 1-2% disodium phosphate solution 

containing 0.01% of titanium compounds have 

been extensively used in industries [273]. Other 

compounds like dilute solutions of cupric or nickel 

sulphates, oxalic acid and polyphosphonates help 

in increasing the number of initial nuclei formed during 

phosphating and their subsequent growth, to 

yield thin and compact coatings of fine-grained nature. 

3.7.1.5. Thermal treatments and machining. 

Thermal or thermo-chemical treatments cause the 

formation of heterogeneous phases, which usually 

alter the grain size. Since the initial metal attack 

during phosphating occurs mainly at the grain boundaries, 

the size of the grains becomes an important 

factor in influencing phosphating and dictates the 

effect of thermal or thermo-chemical treatments. 

Moreover, upon thermal treatment, the distribution 

of the constituent elements of heterogeneous alloys 

may vary and depending upon which the cathodic 

and anodic sites on the surface will result 

and decides the phosphatability. When studying the 

effects of annealing of a 0.3Mn-0.2Ti modified steel 

at 280 °C in H 2 -N 2 atmosphere, Usuki et al. [274] 

has concluded that such a treatment leads to the 

formation of manganese oxide and titanium oxide. 

Since the surface concentration of manganese is 

3-4 times as large as that of titanium, the treatment 

enhances the phosphatability of the modified steel 

by promoting the formation of a predominant proportion 

of manganese oxide on the surface. Like 

wise, Hada et al. [275] have studied the effect of 

thermal treatment on the phosphatability of the uncoated 

side of one-side painted steel. When heated 

for 2 min. at 280 °C, the crystal size and the coating 

weight were increased and the ‘P’ ratio was declined 

causing an inferior paint adhesion. On the 

other hand when heated for a longer period there 

results the diffusion of manganese from the bulk to 

the surface thus improving the phosphatability.


The influence of cold-rolling and the conditions 

of annealing on the formation of phosphate coatings 

were studied by several authors [276-278] According 

to Shimada and Maeda [278], increased deformation 

during cold-rolling and a high-treatment 

temperature favour the phosphate coating formation 

with a good surface coverage whereas a rapid rise 

in temperature in the heat treatment yields an 

unfavourable condition. Cavanagh and Ruble [279] 

have showed that tempering treatments following 

surface hardening processes has a significant effect 

on the response of steel to phosphating. Machined 

surfaces accept phosphate coating very easily 

and often experience a very high rate of deposition. 

3.7.2. Phosphating parameters 

The composition of the phosphating solution and 

the concentration of its constituents determine the 

nature of the coatings formed. Higher concentrations 

of heavy metal ions in accelerated phosphating 

solutions yield coatings of better protective value 

[280]. Concerning the acidity of the bath, the free 

acid value (FA), total acid value (TA) and their ratio 

(FA:TA) should be maintained at the required optimum 

level to obtain coatings of improved quality. 

Increase in total acid pointage generally produce 

coatings of higher coating weight and the free acid 

value is considered to be critical since the initial 

etching attack of the free phosphoric acid present 

in the bath forms the basis of phosphate coating 

formation. As the conversion of soluble primary phosphate 

into insoluble heavy metal tertiary phosphate 

takes place with the regeneration of phosphoric acid, 

it is believed that a certain amount of free phosphoric 

acid must be present to repress the hydrolysis 

and to keep the bath stable for effective deposition 

of phosphate coating. Higher temperatures favour 

easy precipitation of tertiary phosphates in a shorter 

time. Hence, more amount of phosphoric acid is 

needed for the baths operating at higher temperatures. 

In contrast, in the case of phosphating baths 

operated at room temperature, the possibility of the 

increase in acidity during continuous operation is 

more likely [129,130] and is normally neutralized 

by the addition of the carbonate of the metal, which 

forms the coating (Zinc carbonate in zinc phosphating 

bath). Hence, depending upon the working temperatures 

and the concentration of the constituents 

in the bath, the free phosphoric acid content must 

be chosen to maintain the equilibrium conditions. 

Too much of phosphoric acid not only delays the 

151 

formation of the coating, but also leads to excessive 

metal loss. 

Literature reports have revealed the effects of 

insufficient metal dissolution as well as the high 

acidity of the phosphating bath [281,282]. According 

to Kanamaru et al. [281] when the dissolution of 

iron from the steel during phosphating is insufficient, 

the hopeite epitaxial growth plane conforms to the 

phosphophyllite epitaxial growth plane (100) and 

mixed crystals with a high Zn/P ratio grow along 

the steel surface whereas when the dissolution is 

sufficient, the iron concentration in the solution increases, 

the intrinsically free precipitation of 

phosphophyllite without the restraint of epitaxy becomes 

predominant and phosphate crystals with a 

lower Zn/P ratio grow. However, when the acid concentration 

is very high, the phosphated material was 

found to be susceptible to hydrogen embrittlement. 

The effect is compounded if the exposure time is 

also increased and it is generally believed that even 

the relief treatments that are normally practiced are 

not effective when the acid concentration is abnormally 

high [282]. Hence, it is clearly evident that 

the acid coefficient must be maintained properly to 

produce good quality coatings. Usually, baths of 

higher acid coefficients form more fine-grained coatings. 

The conversion ratio, defined as the ratio of 

amount of iron dissolved during phosphating and the 

corresponding coating weight [31] is considered to 

be important as they can predict the efficiency of 

the phosphating process. However, the inability to 

generalize its definition for many phosphating baths 

limits its acceptance as a control parameter in phosphating 

operations [283]. 

El-Mallah et al. [284] have considered the pH of 

the bath as initial pH and the pH at which the consumption 

of all the free phosphoric acid completes 

as the final pH and correlates the difference between 

them with the extent of phosphatability. According 

to them, reducing the difference between the initial 

and final pH leads to the acceleration of the phosphating 

process and in causing such an effect the 

heavy metals and reducing agents such as hydrazine, 

potassium borohydride were evaluated. Several 

additives have been attempted to maintain the 

pH of the phosphating baths. In this respect, organic 

acids and their salts proved their ability to 

buffer the pH of an iron phosphating bath made up 

with hard water [285]. Similarly, the decrease of the 

total and free acidity by means of some buffer additives 

to prevent any significant dissolution of zinc 

coating during the phosphatization of zinc coated 

steels.


The concentration of the accelerators is also very 

important. Though increase in accelerator concentration 

favours better coating formation, too high concentration 

may cause passivation of metal surface 

and inhibit the growth. When phosphating bath is 

modified by the addition of surface active agents, 

the concentration of accelerator should be optimized 

in accordance with the influence of these additives 

[286]. The iron content of the bath is also very critical. 

Although, a small quantity of iron salts favour 

phosphate precipitation (Break-in of the bath), the 

corrosion performance is largely affected as the fraction 

of the ferrous salts in the coating is increased 

[287]. The unfavourable effect of Fe(II) is usually 

decreased by the addition of complexing agents like 

Trilon B [288]. A method for controlling the iron content 

of zinc phosphating bath was proposed by Hill 

[289]. In the case of an iron phosphating bath, the 

ratio of ferrous to ferric iron in solution has been 

related to the tendency for flash rusting. The frequency 

and intensity of flash rust increases as the 

ratio of ferrous to ferric iron in solution decreased 

[290]. 

Addition of specific compounds to the phosphating 

baths has also their own influence on phosphating. 

The incorporation of calcium ions in a zinc phosphating 

bath has resulted in a considerable change 

in the crystal structure, grain-size and corrosion 

resistance of the phosphate coatings. In fact, the 

structure of the zinc phosphate coating changes 

from the phosphophyllite-hopeite to schlozite-hopeite 

[291]. The reduction in grain-size (25 μm to 4 μm) 

and the improvement in the compactness of the 

coating and corrosion resistance, makes this kind 

of modification of zinc phosphating baths as an important 

type of phosphating and it has been classified 

as calcium-modified zinc phosphating [172]. 

Similarly, the inclusion of manganese and nickel 

ions in the zinc phosphating bath proves to be useful 

in refining the crystal size and improving the corrosion 

resistance of the resultant phosphate coatings. 

It has been established that manganese and 

nickel modified zinc phosphate deposits on steel 

have an ordered structure and possess a high corrosion 

resistance [292]. The immense use of manganese 

and nickel ions in modifying the hopeite 

deposits obtained on galvanized steel in such a way 

that the modified hopeite deposit becomes equivalent 

to phosphophyllite to withstand the thermal and 

chemical aggressions, was very well established 

[293] and as a result of such a pronounced influence 

of the nickel and manganese ions, they have 

also been classified as nickel and manganese modi- 

fied zinc phosphating [172]. Moreover, such modifications 

have lead to the development of tri-cation 

phosphating baths consisting of zinc, manganese 

and nickel ions which explored its potential utility 

to phosphate aluminum, steel and galvanized steel 

using the same formulation [294-296]. Besides 

these major types of modifications, phosphating 

baths have experienced a variety of additives incorporated 

in the bath intended for a specific purpose. 

Each additive has its own influence on the 

phosphatability of steel depending upon the operating 

conditions [297-301]. Sankara Narayanan et al. 

[41] have classified the type of special additives used 

in phosphating and the purpose of their use. The 

favourable condition and the precautions in using 

these additives were recommended in the respective 

documents. 

Every phosphating bath reported in literature has 

a specific operating temperature and the baths were 

formulated in such a way that an equilibrium condition 

of the conversion of soluble primary phosphate 

to insoluble tertiary phosphate exists at that temperature. 

Insufficient reach of temperature does not 

favour the precipitation of tertiary phosphate resulted 

from the conversion. However, when the baths were 

overheated above the recommended operating temperature, 

it causes an early conversion of the primary 

phosphate to tertiary phosphate before the 

metal has been treated and as a result increases 

the free acidity of the bath, which consequently 

delays the precipitation of the phosphate coating. 

Hence, in an operating line, which has operating 

time, this effect leads to the production of phosphate 

coatings with a poor coating weight and inferior 

corrosion performance. Sankara Narayanan and 

Subbaiyan [302] have discussed the decisive role 

of overheating the phosphating baths and outlines 

the possible way of eliminating the difficulties encountered 

due to this problem. 

Likewise, every formulation has been assigned 

a fixed operating time based on the kinetics of the 

phosphating process using the particular bath followed 

by coating weight measurements or potential 

measurements with respect to the treatment 

time. It has been established by many workers that 

increasing the treatment time beyond the saturation 

point do not have any influence on the performance 

of the coating. But it has been warned that 

any attempt of reducing the treatment time to decrease 

the amount of phosphate deposition will be 

disasters. 

The method of deposition of phosphate coating 

or the phosphating methodology can influence the


coating formation to a great extent. The most commonly 

used methods of deposition namely the dip 

and spray process, although have received considerable 

attention, it is believed that the dip process 

is most likely to produce an equiaxed structure 

which is consider to be beneficial from the corrosion 

protection point of view. The influence of permanent 

magnetic field during phosphating is studied 

by Bikulcius et al. [303]. According to them, 

when exposed to perpendicular magnetic field, the 

zinc phosphate coating results in larger crystallites 

with lower corrosion resistance whereas the effect 

of parallel magnetic field is insignificant. However, 

the application of a magnetic field either in perpendicular 

or parallel mode results in a fine-grained, 

uniform and more compact zinc calcium phosphate 

coating. 

Electrochemical method of phosphating always 

yields a heavy coating weight [148,304]. In recent 

years, studies have been attempted in electrochemical 

phosphating to produce phosphate coatings 

richer in a particular phase by selecting an appropriate 

applied potential [147]. The success of this 

kind of phosphating methodology has been restricted 

by the difficulties encounter in adding 

‘electrics’ to the existing process plants. Mechanical 

vibration of steel during phosphating is considered 

as a method for obtaining a fine grained phosphate 

coating. However, the amount of coating 

formed is found to be inversely proportional to the 

frequency of vibration [305]. Ultrasonically induced 

cavitation produces a great number of active centres, 

which results in a high rate of nucleation. This 

leads to a uniform fine-grained phosphate coating 

with low porosity [307,308]. 

3.8. Processing problems and 

remedial measures 

Maintenance of the bath parameters and operating 

conditions at an optimum level is a major and complicated 

process, especially in continuous and largescale 

phosphating. In actual practice, the finishers 

have come across several situations that the bath 

is not working properly. These problems viz., overheating 

of the bath, creation of an excess acidity at 

the metal/solution interface especially in the case 

of cold phosphating, change in acidity of the bath 

due to carryover of the alkaline solution used for 

degreasing, the local increase in acidity due to the 

scaling of the heating coils and so on, were discussed 

elsewhere [129, 302]. Moreover, baths that 

have been used for a long time are rendered ineffective 

due to the formation of sludge of ferric phos- 

153 

phate. Sludging occurs to a greater extent in 

unaccelerated and highly acidic baths and has to 

be removed periodically to assure proper bath operation 

[18,19]. In order to maintain bath parameters 

and to enhance bath life, make-up solutions and 

solids are usually required. These additives are so 

formulated that they can be handled easily and are 

inexpensive. 

3.9. Defects in coatings and remedies 

The most commonly encountered defects in phosphated 

parts are low corrosion resistance, stained 

coating with variable corrosion resistance and coatings 

covered with a loose white powdery deposit. 

The possible cause for these defects are insufficient 

care in degreasing and cleaning, incorrect acid coefficient, 

incorrect solution composition, use of incorrect 

operating conditions, incorrect maintenance 

of chemicals, excessive sludge formation and faults 

in after-treatment. These causes, either singly or in 

combination, lead to these defects. 

Low corrosion resistance of coatings may be a 

result of incorrect acid coefficient, incorrect bath 

parameters and operating conditions, presence of 

certain metallic impurities like aluminium, antimony, 

tin and lead compounds and the presence of chloride 

ions in the bath. Improper sealing and abnormally 

thin coatings may also lead to inferior corrosion 

resistance. Stained coatings may be formed 

due to improper cleaning and degreasing. Incorrect 

distribution of articles in the phosphating bath and 

wrong ratio of work surface to solution volume may 

be other causes. Over heating of the phosphating 

bath, make-up during processing, heavy sludging 

and sludge suspended in the bath leads to the formation 

of coatings with a loose powdery deposit. 

Proper cleaning and degreasing, correct maintenance 

of work surface to solution volume ratio, 

control of bath parameters and operating conditions 

within the strict limits, avoidance of overheating and 

excessive sludge formation will yield coatings of consistent 

good quality and corrosion resistance. 

3.10. Characterization of phosphate 


A variety of instrumental methods, which include, 

scanning electron microscopy (SEM), electron probe 

microanalysis (EPMA), X-ray diffraction (XRD), Auger 

electron spectroscopy (AES), X-ray photoelectron 

spectroscopy (XPS), electron spin resonance 

(ESR), X-ray fluorescence (XRF), extended X-ray 

absorption fine structure (EXFAS), Fourier transform


infra red spectroscopy (FTIR), Raman spectroscopy, 

differential thermal analysis (DTA), differential scanning 

calorimetry (DSC), secondary ion mass spectrometry 

(SIMS), atomic force microscopy (AFM), 

glow discharge optical emission spectrometry 

(GDOES), quartz crystal impedance system (QCIS), 

conversion electron Mossbauer spectrometry 

(CEMS), acoustic emission (AE) testing, etc. were 

used to characterize phosphate coatings [308-340]. 

SEM is the most commonly and widely used 

technique for characterizing phosphate coatings. It 

is used to determine the morphology and crystal 

size of phosphate coatings. SEM serves as an effective 

tool to study the nucleation and growth of 

the phosphate coatings and makes evident of the 

fact that the initial growth of the phosphate coating 

is kinetically controlled and at a latter stage it tends 

towards mass transport control [322]. SEM also 

substantiates the fact that preconditioning the substrate 

before phosphating enables the formation of 

a fine-grained and adherent phosphate coating. 

EPMA is used in the determination of the porosity 

of phosphate coatings in which the number of 

copper spots deposited in the pores following immersion 

in copper sulphate solution (pH 5.0) [323]. 

XRD is primarily used to detect the phase constituents 

present in phosphate coatings – the 

phosphophyllite and hopeite phases in zinc phosphate 

coating. Though the manganese and nickel 

modification of zinc phosphate coating reveals only 

the presence of hopeite phase, there observed to 

be significant variations in the orientations of the 

hopeite crystals as evidenced by the relative intensities 

of the H(311), H(241) and H(220), H (040) 

peaks. XRD is also used to characterize phosphate 

coatings in terms their ‘P’ ratio. 

Van Ooij et al. [324] applied high resolution Auger 

electron spectroscopy (AES) in combination with 

energy dispersive X-ray spectrometry (EDX) to study 

the nature of chromium post passivation treatment. 

According to them, Cr(III) was not detected at the 

metal/phosphate boundry but on the surface of phosphate 

crystals and that the Fe/Zn ratio was increased 

in the surface and subsurface layers. 

XPS is used to detect the nature of various species 

in phosphate coating. The presence of fatty 

acid-like contaminants on the metal substrates can 

be identified from the C 1s spectra. Similarly, the 

Zn 2p 3/2 and P 2p spectra enable identification of 

Zn 3 (PO 4 ) 2 . Besides, the formation of ZnO or Zn(OH) 2 

and NaHPO 4 could be identified from the Zn 2p 3/2 

and P 2p spectra, respectively. The Fe 2p 3/2 will give 

an idea about the presence of Fe 2 O 3 and FePO 4 in 

the coating [187]. Cu 2p spectra obtained from the 

sample phosphated using a solution containing 1 

ppm of Cu 2+ ions show two components at binding 

energies 933.6 eV and 932.2 eV [325]. The former 

signifies Cu 2+ , although the latter could arise from 

either Cu + or Cu metal. All evidence supports the 

Cu deposition occurring during the phosphating process. 

ESR is used to confirm the modified structure of 

hopeite films formed on the surface of pre-treated 

steel sheets. The zinc phosphate film formed from 

the bath that does not contain manganese or nickel 

ions exhibits no ESR signal. This is because ESR 

could only detect paramagnetic transition metal ions 

with an unpaired electron whereas in zinc phosphate 

coating the ten electron spins of zinc (II) metal ions 

(3d 10 ) in the ‘d’ orbitals are paired with one another. 

However, ESR detects the manganese and nickel 

components in the modified zinc phosphate coating 

and proves that manganese and nickel exist as 

Mn(II) and Ni(II) in these coatings [316]. 

XRF is used to determine the nature of nickel in 

nickel modified zinc phosphate coating. The XRF 

spectrum obtained for metallic nickel is compared 

with that of the modified zinc phosphate coating. 

The Ni L α peak of metallic nickel occurs at 34.18° 

whereas the Ni L α peak of the nickel component of 

modified phosphate coatings occurs at 34.06°, indicating 

that the nickel component in the films is 

not in the metallic state [316]. 

EXFAS was used to assess the crystal structure 

of manganese modified zinc phosphate coating 

[318]. The radial distributions of the first 

neighbouring atoms appeared at a distance of 0.146 

nm for unmodified hopeite and at 0.144 nm for manganese 

modified hopeite. This decrease is due to 

the disorderness of modified zinc phosphate coating 

following the introduction of manganese. EXFAS 

study also confirms that the manganese component 

is substituted for zinc component in the octahedral 

structure. 

SIMS was used to detect the presence of titanium, 

which are adsorbed on to the metal surface 

and act as sites for crystal nucleation [313]. 

The crystal structure of hopeite and phosphophyllite 

are hydrated. So once heated, dehydration 

reactions are expected to occur. Differential thermal 

analysis (DTA) performed on phosphate coatings 

shows that there is a clear disparity in the dehydration 

process of these two phases as evidenced 

by a 50 °C difference in temperature in the endothermic 

peaks corresponding to hopeite and 

phosphophyllite phases. In the case of nickel and


Fig. 7. Thermal behaviour of zinc phosphate crystals 

as hopeite and phosphophyllite studied by differential 

thermal analyzer: Curve A: Unmodifed 

hopeite; Curve B: Hopeite with 1.8 wt.% Ni + Mn; 

Curve C: Hopeite with 5.6 wt.% Ni + Mn; Curve D: 

Phosphophyllite) (Reprinted from Surface & Coatings 

Technology, Vol. 30, N. Sato, Effects of heavy 

metal additions and crystal modification on the zinc 

phosphating of electrogalvanized steel sheet, pp. 

171-181 (1987) with permission from Elsevier Science). 

manganese modified zinc phosphate coatings the 

temperature of the endothermic peaks shifts to higher 

temperatures and becomes equivalent to the endothermic 

peak of phosphophyllite (Fig. 7) [326]. 

DSC is used to assess the thermal behavior of 

phosphate coatings. Manganese and nickel modified 

zinc phosphate coatings formed on steel and 

electrozinc coated steel, in the as deposited condition, 

show strong endothermic peaks at temperatures 

below 175 °C associated with the loss of water 

molecules from the phosphate. On the other hand 

the DSC traces obtained for these phosphate coatings 

after stripping the cured paint film exhibit no 

strong endothermic peak over the temperature range 

up to 200 °C, confirming that under paint stoving 

conditions the more labile water molecules have 

been stripped from the phosphate coating (Fig. 8) 

[313]. 

155 

Fig. 8. DSC curves of nickel and manganese modified 

zinc phosphate coating formed on (a) steel and 

(b) electrozinc coated steel. Solid line represents 

before stoving and broken line represents after 

stoving and removal of paint. (Source: R.A. 

Choudhery and C.J. Vance in Advances in Corrosion 

Protection by Organic Coatings, D. Scantelbury 

and M. Kendig (Eds.), The Electrochemical Society, 

NJ, 1989, p.64. Reproduced by permission of 

The Electrochemical Society, Inc.). 

DSC also proves the ability of nickel and manganese 

modified zinc phosphate coating in resisting 

the rehydration compared to unmodified zinc 

phosphate coating. Unmodified zinc phosphate coatings 

obtained on zinc coated steel exhibit two endothermic 

peaks at 103 and 135 °C with a total heat 

flow of 100 J/g. After exposure to 100% RH for 63 

hours, these endothermic peaks appear again, with 

a slight shift in peak temperature towards higher 

temperatures, indicating that the unmodified zinc 

phosphate coating is completely rehydrated (Fig. 

9). In contrast, manganese and nickel modified zinc 

phosphate coating exhibit a much stronger resistance 

to the redhydration process when subjected


Fig. 9. DSC curves of unmodified zinc phosphate 

coating (a) before stoving; and (b) after stoving and 

rehydrated at 100%RH for 63 hours. (Source: R.A. 

Choudhery and C.J. Vance in Advances in Corrosion 

Protection by Organic Coatings, D. Scantelbury 

and M. Kendig (Eds.), The Electrochemical Society, 

NJ, 1989, p.64. Reproduced by permission of 

The Electrochemical Society, Inc.). 

to similar conditions (Fig. 10) [313]. The main limitation 

of DSC is that the phosphate coating has to 

be physically removed from the substrate. 

Handke has demonstrated the use of FTIR to 

study the mechanism of phosphate coating formation 

[327]. FTIR is sufficiently sensitive to distinguish 

between different types of coatings in terms 

of composition and crystallinity [313]. FTIR is also 

sensitive to the degree of hydration of phosphate 

coatings. The FTIR spectra of nickel and manganese 

modified zinc phosphate coating on cold rolled 

steel (CRS) and zinc coated steel (ZCS) under conditions 

namely, before heating, after stoving at 180 

°C for 20 minutes and after exposure to 100% RH 

for 14 days was analyzed (Figs. 11 and 12). The 

assignments of the FTIR spectra for CRS and ZCS 

Fig. 10. DSC curves for nickel and manganese 

modified zinc phosphate coating. Solid line represents 

before stoving and broken line represents after 

stoving and subsequent rehydration. (Source: 

R.A. Choudhery and C.J. Vance in Advances in 

Corrosion Protection by Organic Coatings, D. 

Scantelbury and M. Kendig (Eds.), The Electrochemical 

Society, NJ, 1989, p.64. Reproduced by 

permission of The Electrochemical Society, Inc.). 

are given in Tables 8 and 9, respectively. The FTIR 

spectra of phosphate coatings on CRS and ZCS 

are similar before heating. On heating there are significant 

changes in the FTIR spectra of ZCS while 

the changes for steel are less noticeable. On exposing 

to 100% RH it is evident that phosphate coatings 

formed on ZCS shows greater tendency to rehydrate, 

indicated by the increase in intensity of all 

peaks associated with water vibrations. On the other 

hand phosphate coating formed on CRS exhibit very 

little tendency to rehydrate; the spectrum essentially 

remains constant. 

The infrared and Raman spectra of α-hopeite and 

phosphophyllite are shown in Figs. 13-15 [341]. They 

resemble the room temperature spectra reported by 

Hill and Jones [342] and Hill [343]. The infrared and 

Raman spectra of both compounds display a large 

number of bands. In addition, the infrared and Raman 

spectra of these compounds resemble each other 

very strongly. Hence distinction of the respective 

peaks in phosphate coatings is not easy. However, 

there are some peaks, which are different for these 

compounds, are useful in identifying these compounds. 

For example the Raman bands at 1150, 

1055 and 310 cm -1 are typical for α-hopeite whereas 

the Raman bands at 1135, 1070 and 118 cm -1 are 

useful for identifying phosphophyllite in phosphate 

coating.


Table 8. The assignments of the FTIR spectra of nickel and manganese modified zinc phosphate coating 

on cold rolled steel (after Choudhery and Vance [313]). 

Wavenumber Shape/Intensity Assignment 

(cm -1 ) Before heating After stoving After exposure 

for 20 minutes at 180 °C to 100% 

RH for 14 days 

3536 Sharp/Strong Free O-H Lost All peaks 

3214 V. broad/Strong OH/H 2 O stretching Broadens similar to stoved 

1643 Sharp/Medium-St H 2 O bending Shifts to 1629 sample with 

1145 W. Sharp/Strong P-O stretching Shoulder 1170 only a slight 

appears increase in peak 

and shoulder at height 

1082 now a peak 

1121 Sharp/Strong P-O stretching Shoulder 1170 

appears and 

shoulder at 1082 

now a peak 

1080 Sh./Strong P-O stretching Shoulder 1170 

appears and 


now a peak 


appears and 


now a peak 

997 Sharp/Strong H 2 O rocking Lost 

933 Sharp/Strong P-O stretching – 

643 Sharp/Medium O-P-O bending/ Broadens 

H 2 O waging 

The influence of metal components in hopeite 

films was investigated using Infrared and Raman 

spectra [243]. The IR spectra of hopeite films exhibit 

peaks at 3500 to 3000, 1640, 1200 to 900 and 

640 cm-1 . The peaks at 3500 to 3000 cm-1 correspond 

to the stretching vibration of O-H in H O, the 

2 

peak at 1640 cm-1 to the deformation of H-O-H in 

H O and the three peaks at 1200 to 900 and that at 

2 

640 cm-1 3- to PO in the hopeite films. There are four 

4 

3- basic vibrations for PO , namely, ν1 , ν , ν and ν in 

4 

2 3 4 

which ν and ν are inactive in IR absorption whereas 

1 2 

ν and ν are active. Hence the three peaks at 1200 

3 4 

to 900 cm-1 correspond to the ν mode and that at 

3 

640 cm-1 to the ν mode. A comparison of the IR 

4 

spectra of nickel and manganese modified hopeite 

with unmodified hopeite indicate that the incorporation 

of nickel and manganese into the hopeite films 

157 

3- affects the coordination state of PO and cause 

4 

splitting or shifting of peaks. The three peaks at 

1200 to 900 cm-1 corresponding to the ν mode of 

3 

hopeite film shows splitting and the peak at higher 

wavenumber is shifted by 10 cm-1 towards the lower 

wavenumber. The ν peak of nickel and manganese 

4 

modified hopeite generates a new peak at 580 

cm-1 . 

Laser Raman spectra of hopeite films exhibit four 

peaks in the region between 1150 and 930 cm-1 , 

with the main peak appearing at 996 cm-1 . These 

3- four peaks are due to PO , which has four basic 

4 

vibration modes ν , ν , ν and ν , all of them are 

1 2 3 4 

3- Raman active. If the PO in the hopeite film exists 

4 

as a perfect regular tetrahedron, then only peaks of 

ν and ν should appear. But in practice, there are 

1 3 

3- four peaks. PO in hopeite films affects the sym- 

4


Table 9. The assignments of the FTIR spectra of nickel and manganese modified zinc phosphate coating 

on zinc coated steel (after choudhery and vance [313]). 

Wavenumber Shape/Intensity Assignment 

(cm -1 ) Before heating After stoving After exposure 

for 20 minutes at 180 °C to 100% 

RH for 14 days 

3545 Sharp/Strong Free O-H Lost Water stretching 

3290 V. broad/Strong OH/H 2 O stretching Broadens peaks intensify. 

1635 Sharp/Strong H 2 O bending Shifts to 1649 Spectrum begins 

1135 Sh./Strong P-O stretching Shoulder 1184 to resemble the 

appears and one obtained for 

shoulder at 1026 sample before 

now a peak heating 


appears and 


now a peak 


appears and 


now a peak 

1026 Sh./Strong P-O stretching Shoulder 1184 

appears and 


now a peak 

1003 Sharp/Strong H 2 O rocking Lost 

925 Sharp/Strong P-O stretching – 

638 Sharp/Medium O-P-O bending/ Broadens 

H 2 O waging 

metry of the regular tetrahedron structure by interaction 

with the surrounding crystalline structure so 

that the symmetry will become distorted. As a result, 

the degeneracy of the vibration modes will be 

united and split and hence results in four peaks. 

Three peaks, except for the main peak, correspond 

to the stretching vibration mode of ν , which 

3 

split into three peaks; the main peak around 900 

cm-1 corresponds to the symmetrical stretching vi- 

3- bration of ν . The level of interaction of PO with its 

1 4 

surroundings affects the number of basic vibrations, 

the Raman band or the intensity of the Raman spectrum. 

With the incorporation of manganese or nickel 

in the hopeite films, the Raman band of the main 

peak as well as the three peaks of ν are shifted to 

3 

a lower wavelength. Increasing the metal content in 

hopeite films has an increasing effect on the data of 

Raman band. Raman spectra showed a sensitivity 

to the degree of modification of hopeite films by metal 

components to a greater extent than did IR spectra. 

AFM is used to study the effect of surface preconditioning 

on phosphatability of zinc coated steel. 

AFM is useful in getting a better understanding of 

the activation process [328]. 

Quartz crystal impedance system (QCIS) was 

used to study the formation of phosphate coating 

from a manganese modified low-zinc phosphating 

bath [329]. It involves rapid and simultaneous measurement 

of admittance spectra of the zinc coated 

piezoelectric quartz crystal resonator (PQC). By 

measuring the equivalent circuit parameters and frequency 

shift of the zinc coated PQC resonator, the 

growth kinetics of the phosphating process can be


Fig. 11. FTIR Spectra of nickel and manganese modified zinc phosphate coating formed on steel. (a) Full 

range; and (b) Selected range. The top, middle and bottom spectra represent before stoving, after stoving 

and after stoving with subsequent rehydration, respectively. (Source: R.A. Choudhery and C.J. Vance in 

Advances in Corrosion Protection by Organic Coatings, D. Scantelbury and M. Kendig (Eds.), The Electrochemical 

Society, NJ, 1989, p.64. Reproduced by permission of The Electrochemical Society, Inc.). 

Fig. 12. FTIR Spectra of nickel and manganese modified zinc phosphate coating formed on zinc. (a) Full 

range; and (b) Selected range. The top, middle and bottom spectra represent before stoving, after stoving 

and after stoving with subsequent rehydration, respectively. (Source: R.A. Choudhery and C.J. Vance in 

Advances in Corrosion Protection by Organic Coatings, D. Scantelbury and M. Kendig (Eds.), The Electrochemical 

Society, NJ, 1989, p.64. Reproduced by permission of The Electrochemical Society, Inc.). 

159


Fig. 13. Infrared spectra of α-hopeite and phosphophyllite 

in the region 4000–1400 cm -1 (Reprinted 

from Spectrochimica Acta Part A, Vol. 57, O. 

Pawlig, V. Schellenschlager, H.D. Lutz and R. 

Trettin, Vibrational analysis of iron and zinc phosphate 

conversion coating constituents, pp. 581-590 

(2001) with permission from Elsevier Science). 

monitored. QCIS is also used to measure the change 

in viscoelasticity of zinc phosphate coatings, which 

decreases with increase in phosphating time and 

the concentration of sodium nitrite [330]. 

The utility of GDOES in the analysis of phosphate 

coatings has been attempted by several authors 

[331-334] and phosphate coatings deposited 

on cold-rolled, hot-dipped galvanized, galvanneled, 

electrogalvanized and Zn-Ni or Zn-Fe electroalloycoated 

sheets were characterized. It has been reported 

[331] that the profiles of the elemental distribution 

throughout the film thickness can be obtained 

with high sensitivity. GDOES provides more detailed 

information on the constituent elements, depth, di- 

Fig. 14. Infrared spectra of α-hopeite and phosphophyllite 


from Spectrochimica Acta Part A, Vol. 57, O. 

Pawlig, V. Schellenschlager, H.D. Lutz and R. 

Trettin, Vibrational analysis of iron and zinc phosphate 

conversion coating constituents, pp. 581-590 

(2001) with permission from Elsevier Science). 

rection and the crystal growth process than conventional 

chemical and X-ray methods. Quantification 

of the results has also been attempted in the 

case of GDOES based on the integrated intensity 

of the constituent elements. Maeda et al. [331] have 

suggested the possibility of utilizing this method for 

quantitative determination, after establishing a linear 

relationship between integrated intensity and 

coating thickness of phosphorus and nickel. 

GDOES is considered to be the most appropriate 

method for depth profiling thick phosphate coatings 

[335]. GDOES due to its higher detection sensitivity 

compared to XPS, confirms the adsorption of 

titanium phosphate on zinc coated steel [328].


Fig. 15. Raman spectra of α-hopeite and phosphophyllite 


from Spectrochimica Acta Part A, Vo. 57, O. Pawlig, 

V. Schellenschlager, H.D. Lutz and R. Trettin, Vibrational 

analysis of iron and zinc phosphate conversion 

coating constituents, pp. 581-590 (2001) with 

permission from Elsevier Science). 

AE is also used as a tool to characterize phosphate 

coatings [336-340]. Phosphate coating exhibit 

only a single peak during AE in a ring-down 

count-rate against strain curves. The amplitude distribution 

verses events curve recorded for steel/phosphate 

systems at a strain rate of 10 percent showed 

emissions with amplitudes only below 40 dB. Rooum 

and Rawlings [338, 339] have shown that it is possible 

to assign the source of failure mechanism to 

the peaks of amplitude distribution. Each failure 

mechanism was characterized by events centred 

on specific amplitudes. In their study they attribute 

the peak at 22 dB to the cracking of the hopeite 

needles and the peak at 26 dB to the adhesion fail- 

161 

ure between the phosphophyllite and hopeite. Hence 

it is evident that with the help of AE testing it is 

possible to detect the onset of a process such as 

cracking of the coating and/or adhesion failure and 

the extent to which these failures shall occur. 

3.11. Testing the quality of phosphate 


Methods to evaluate the quality of phosphate coatings 

involve the determination of its physical characteristics 

as well as the performance in corrosive 

environments. Kwiatkowski et al. [344] have reviewed 

the testing methods of phosphate coatings. 

3.11.1. Evaluation of physical characteristics 

(a) Examination of physical appearance. Phosphate 

films on steel may range in colour from light gray 

to dark gray, depending on the type of bath and 

the grade of steel substrate used. After sufficiently 

strong scratching of phosphate coating 

with a fingernail, a white scratch should appear 

on the surface without causing an injury to the 

coating visible to the naked eye. 

(b) Determination of coating thickness and coating 

weight. The local thickness of phosphate coatings 

on steel is usually determined by magnetic, 

electromagnetic and microscopic methods 

[18,19,127,128]. The average thickness is usually 

expressed in g/m 2 or mg/ft 2 . Destructive 

methods of determination of coating weight are 

widely adopted. These methods involve the determination 

of change in weight of a coated specimen 

after dissolution of the coating in a suitable 

medium. Concentrated hydrochloric acid containing 

20 g/l of antimony trioxide is usually used at 

room temperature for this purpose. Other solutions 

commonly used are 5% solution of chromic 

acid at room temperature and 20% sodium 

hydroxide at 90 °C. The difference in weight after 

coating removal is divided by the surface area 

of the work in m 2 to obtain unit coating weight in 

g/m 2 . 

(c) Determination of acid resistance. It is calculated 

as the difference in weight per unit area of the 

panel before phosphating and after stripping off 

the coating; and is expressed in g/m 2 . 

(d) Testing of physical properties. The absorption 

value and hygroscopicity are the important parameters 

relating to the physical properties of 

the phosphate coating. Evaluation of the absorption 

value of the phosphate coating involves the 

measure of the gain in weight per unit area when 

subjected to immersion in diacetone alcohol for


two minutes and allowed to drain the excess for 

three minutes [345]. The gain in weight of the 

phosphated panels, when subjected to a humid 

atmosphere in a closed container at room temperature 

for six hours, gave the hygroscopicity 

of the coating [345]. 

(e) Estimation of porosity. Estimation of the porosity 

of the phosphate coating involves both chemical 

and electrochemical methods. The chemical 

(Ferroxyl indicator) method was based on 

formation of blue spots (Prussian blue) on a filter 

paper dipped in potassium ferricyanide - sodium 

chloride - gelatin mixture when applied over 

the phosphated surface for one minute. The number 

of blue spots per sq. cm gives a measure of 

the porosity of the coating. The electrochemical 

method of determination was based on the measurement 

of the oxygen reduction current density 

when immersed in air-saturated sodium hydroxide 

solution (pH 12). The current density 

values measured at -550 mV, where oxygen reduction 

is the dominant reaction at the uncoated 

areas reveals the porosity of the coating 

[209,210]. Although, both the Ferroxyl test and 

the electrochemical test are used in an industrial 

scale to assess the porosity of phosphated 

steel, the former method is found to be qualitative 

and not very effective in distinguishing the 

porosities of panels phosphated using different 

baths. In contrast, the electrochemical method 

is far reliable besides its simplicity in operation 

and quicker measurements of the porosity. The 

advantage of the electrochemical method has 

been discussed elsewhere [209, 346]. 

(f) Determination of thermal and chemical stabilities. 

The thermal stability of the coating was usually 

determined by calculating the percentage 

loss in weight when the phosphated panels were 

subjected to drying at 120 and 180 °C. 

The chemical stability of the coating, in particular, 

the alkaline stability is very important as it 

determines the effectiveness of the phosphate 

coating as a base for cathodic electrophoretic 

finishes. This is determined by calculating the 

percentage residual coating when immersed in 

alkaline media. Immersion in sodium hydroxide 

solution is recommended to test the alkaline 

stability. Recently, Kwiatkowski et al. [347] have 

recommended a borate buffer solution containing 

0.01M EDTA as the medium to test the alkaline 

stability of the coating and proved its usefulness 

in predicting the same. A similar calculation 

of the percentage residual coating when 

subjected to immersion treatments in buffered 

solutions of varying pH from 2-14 can give an 

insight about ability of the phosphate coatings 

to withstand different chemical aggressions and 

to prove their effectiveness in preventing the cosmetic 

corrosion [219]. 

(g) Evaluation of surface morphology. The surface 

morphology of the phosphate coating is usually 

assessed by scanning electron microscope 

(SEM). This technique reveals the distinct features 

of the crystal structure, grain size of the 

crystallites, the coating coverage and uniformity; 

the parameters that determine the performance 

of the coating. 

(h) Determination of ‘P ratio’. The ‘P ratio’ is usually 

determined by measuring the intensity of the 

characteristic planes of the constituent crystallites 

of the phosphate coating. In a zinc phosphate 

coating, which consists essentially of 

hopeite (Zn (PO ) . 4H2O) and phosphophyllite 

3 4 2 

(Zn Fe(PO ) . 4H2O), the ‘P ratio’ is defined as 

2 4 2 

[199]: 

‘P ratio’ = P 

P + H 

Where P – Intensity of the characteristic planes 

of phosphophyllite and H – Intensity of the 

characteristic planes of hopeite. 

(i) Adhesion measurements. The standard laboratory 

method of estimation of adhesion is the peel 

off test, which involves the determination of the 

extent of adhesion at scribed areas using a pressure 

sensitive adhesive tape. Usually adhesion 

in the dry state will be good since it mainly depends 

on the cohesive failure of the paint film. 

Wet adhesion is of prime importance and is usually 

determined after subjecting the painted panels 

to immersion treatment in de-ionized water 

at 45 °C for 240 hours. Depending upon the extent 

of peeling, rating will be made between 0 

and 5B as per ASTM D 3359-87 [348]. 

3.11.2. Evaluation of corrosion performance 

(a) Rapid preliminary quality control tests. Preliminary 

investigation of coating quality for on-line 

monitoring mainly involves two empirical tests 

of which one involves inspection for rust spots 

after immersion in 3% sodium chloride solution 

for 5-30 minutes and the other is concerned with 

the time taken for metallic copper deposition from 

a copper sulphate-sodium chloride-hydrochloric 

acid mixture. 

(b) Laboratory corrosion resistance tests. The most 

frequently used laboratory tests for evaluating


the corrosion resistance of phosphate coatings 

include (i) immersion test, (ii) salt spray (fog) 

test, (iii) humidity test and (iv) A.R.E. salt droplet 

test. 

(i) Immersion test. This test consists of determining 

the time required for the first appearance of 

corrosion on the basis metal when immersed in 

3% sodium chloride solution. The change in 

weight expressed as g/m 2 for every 24-hour period 

of immersion is also determined and correlated 

to the corrosion resistance of the coating. 

(ii) Salt spray test. Evaluation of corrosion resistance 

of the phosphated and finished panels is 

performed by subjecting them to a salt mist of 

5% sodium chloride solution in a salt spray 

chamber for a specified length of time. The extent 

of spread of corrosion from a scribe made 

on the panel, rated after ASTM B 117-85 specifications, 

is a measure of the corrosion resistance 

of the phosphate coating [349]. 

(iii) Humidity test. This test is used for the evaluation 

of phosphated panels with and without further 

finishes. By subjecting the phosphated panels 

to highly humid conditions (90-95% relative 

humidity) at slightly elevated temperatures (42- 

48 o C), assessment of corrosion likely to be induced 

due to the porosity of the coating can be 

made. The extent of blistering of paints due to 

the presence of soluble salts on phosphated and 

finished panels when subjected for 1000 hours 

in the humidity chamber is also related to its 

corrosion performance. 

Fig. 16. Cyclic voltammogram obtained from zinc phosphate coated steel in 5 wt.% sodium chloride solution 

(pH 6.5; 25 °C) (After Kiss and Coll-Palagos [355]). 

163 

(iv) A.R.E. salt droplet test. This test consists of 

the evaluation of the corrosion resistance of the 

phosphated panels by determining the loss in 

weight after five days of exposure in humid condition 

inside a closed cabinet at room temperature 

with a single spray of synthetic seawater 

on each day [350]. 

(c) Electrochemical methods of testing. The electrochemical 

methods of testing the phosphate 

coating mainly involves the anodic polarization 

studies in 0.6M ammonium nitrate [346,351] and 

AC impedance measurements in 3% sodium 

chloride solution [352-363]. 

Zurilla and Hospadaruk [209] proposed a method 

to assess the porosity of phosphated steel based 

on oxygen reduction current density in air saturated 

0.1 N sodium hydroxide solution. Since cathodic 

reduction of oxygen occurs at the uncoated areas, 

the measure of current density values at a fixed 

potential (–550 mV vs. SCE) reveal the porosity of 

the coating. Kiss and Coll-Palagos [355] utilized 

cyclic voltammetry to evaluate the porosity of phosphate 

coatings. Fig. 16 shows the typical cyclic 

voltammogram obtained for zinc phosphate coated 

steel in 5% sodium chloride solution (pH 6.5; 25 

°C). The voltammogram can be classified into three 

major regions based on the potential values. The 

anodic peak observed between – 550 mV and – 950 

mV is believed to be due to the oxidation of Fe, Fe 2+ 

and/or the formation of a complex compound in the 

pores of the phosphated steel and its intensity is


Fig. 17. Anodic polarization curve obtained for zinc phosphate coated steel in 0.6 M ammonium nitrate. 

related to the porosity of the phosphate coating. 

Kiss and Coll-Palagos [355] termed this peak as 

‘porosity peak’. The peak height at –800 mV or the 

integrated area under this anodic peak was used to 

predict the porosity. 

Anodic polarization study in 0.6 M ammonium 

nitrate solution is one of the recommended methods 

of electrochemical evaluation of corrosion resistance 

of phosphate coatings (Fig. 17) [346]. During 

anodic polarization in 0.6 M ammonium nitrate 

solution, at potentials more negative than –0.33 V, 

phosphated steel undergoes active dissolution. 

Above –0.33 V, the first passivation region occurs 

due to the adsorption of hydroxide ions at the electrode 

surface. The occurrence of the second active 

region is due to the replacement of hydroxide ions 

by phosphate ions available at the electrode/solution 

interface. Replacement of the adsorbed phosphate 

ions by nitrate causes the occurrence of the 

second passive region. Hence it is clear that these 

active and passive regions are the result of the competitive 

and potential dependent adsorption of anions 

at the electrode surface. It should be noted 

here that the appearance of this second current 

density maximum is specific to phosphated steel 

and it is not observed for uncoated steel when tested 

under similar conditions. Hence the value of the 

second current density maximum can be used to 

evaluate the corrosion resistance of different phosphate 

coatings [346]. 

The other important method of evaluation of the 

corrosion resistance of phosphate coatings is the 

electrochemical impedance spectroscopy (EIS). It 

provides a rapid, nondestructive means of evaluating 

corrosion rate and mechanism of corrosion of 

phosphate coatings. Literature reports on the evaluation 

phosphate coatings [357-363] suggest that the 

corrosion behaviour of phosphate coatings in contact 

with the corrosive medium (3.5% NaCl) can be 

explained on the basis of a porous film model since 

the electrolyte/coating-metal interface approximates 

such a model. Accordingly, the phosphated substrates 

are considered as partially blocked electrodes 

when comes in contact with 3.5% NaCl solution. 

This implies that the metal substrate is corroding 

in the same way when unprotected in a much 

smaller area where coverage is lacking. Since the 

capacitive and resistive contributions vary directly 

and indirectly, respectively, with respect to the area, 

based on these measured parameters, predictions 

on the corrosion rate of different phosphate coatings 

can be easily made. Fig. 18 shows the Nyquist 

plot obtained for zinc phosphate coated steel in 3.5% 

NaCl solution exhibiting a semicircle in the high fre-


Fig. 18. Nyquist plot obtained for zinc phosphate coated steel in 3.5 wt.% sodium chloride solution. 

quency region followed by a linear portion in the low 

frequency region. An equivalent electrical circuit 

model, which involves charge transfer resistance 

(R ct ), the double layer capacitance (C dl ) and the 

Warburg impedance (Z w ) can be proposed to simulate 

the behaviour of phosphate coating in 3.5% NaCl 

solution. The change in the values of these parameters 

with time gives an account of how the coating 

degradation has occurred and helps to establish a 

mechanistic pathway. High values of charge transfer 

resistance and low values of double layer capacitance 

signify a coating of better performance. 

The appearance of Warburg impedance suggests 

that corrosion of phosphated steel is a diffusion 

controlled. [352-354,362-365]. 

3.12. Applications 

Phosphate coatings have been put to a wide variety 

of applications; salient among them is the corrosion 

protection, as bases for paint, to provide wear 

resistance and an aid in cold forming of steel 

[18,19,24,127,128,366,367]. 

Phosphate coatings provide an effective physical 

barrier to protect corrosion-prone metals against 

their environment. Due to their insulating nature, the 

phosphate coatings prevent the onset and spreading 

of corrosion. These coatings provide effective 

corrosion protection to ferrous and non-ferrous met- 

165 

als alike. The extent to which these coatings provide 

corrosion resistance is dependent on the thickness 

and weight of the coating used. However, better 

corrosion protection can be achieved by finishing 

these coatings with paints, oils, etc. Phosphate 

coatings provide an effective base for the application 

of paints and this constitutes their most widespread 

application [368]. They can be used as an 

excellent base for more recent methods of paint 

applications such as electrophoretic painting and 

powder coating [369,370] and shown to improve the 

corrosion resistance of steel coated subsequently 

with cadmium, zinc, nickel, etc., both in industrial 

as well as marine atmospheres [371]. The application 

of phosphate coating also improves the adhesive 

bonding of plain carbon steels [372]. 

The mechanism of pyrite oxidation in acidic media 

involves preferential release of iron into the medium. 

The phosphate treatment results in the formation of 

iron phosphate coating and significantly reduces the 

chemical oxidation of pyrite [373]. The formation of 

a phosphate layer on the surface of iron powders 

induces a significant improvement of the oxidation 

resistance in the temperature range of 300-700°C 

[374]. The oxidation resistance of ultrafine copper 

powder is increases by phosphating [375]. Rebeyrat 

et al. [376] based on thermogravimetric experiments 

suggest that phosphating of bulk α-iron significantly 

decreases the gain in weight due to oxidation.


The Nd–Fe–B type magnets are highly susceptible 

to attack from both climatic and corrosive environments, 

which result in corrosion of the alloy and 

deterioration in both its physical and magnetic properties. 

The poor corrosion resistance of Nd–Fe–B 

type magnets in many aggressive environments is 

associated with the presence of ~ 35 wt.% of neodymium 

in their composition. Neodymium, along with 

other rare earth (RE) elements belongs among the 

most electrochemically active metals. The application 

of a zinc phosphate coating proved to be a most 

useful method to improve the corrosion resistance 

of NdFeB magnet [377,378]. The improvement in 

corrosion resistance of NdFeB magnet following the 

application of a zinc phosphate coating is shown in 

Fig. 19. 

Phosphating is a widely used method of reducing 

wear on machine elements and moving parts 

[379,380]. Phosphate coatings function as lubricants, 

in addition their ability to retain oils and soaps 

further enhances this action. Heavy manganese 

phosphate coatings, supplemented with proper lubricants 

are most commonly used for wear resistance 

applications [381]. The manganese phosphates 

widely used in automotive industry are the 

best to improve the ease of sliding and the reduction 

of associated wear of two steel surfaces sliding 

one against the other. The phosphate coatings pos- 

sess no intrinsic lubricating properties but can absorb 

or hold a considerable quantity of lubricant by 

virtue of their porosity [382,383]. This combination 

favours an easier running-in at higher surface pressures 

by forming a non-metallic barrier that separates 

the two metal surfaces and reduces the danger 

of seizure and associated pitting. There is also 

less noise produced at such surfaces and they have 

an in-built capacity, in emergencies, to run dry for a 

limited period [384]. Phosphating increases the sliding 

distance to scuffing as well as the scuffing load, 

whilst marginally reducing the coefficient of friction. 

No advantage was found in phosphating dry sliding 

surfaces. Phosphating reduces the likelihood of 

adhesive wear in marginal or poorly lubricated sliding 

couples. The choice of phosphate coating is 

primarily dependent on the surface finish of the sliding 

counterface; thin coatings are suitable for smooth 

surfaces whereas against rougher surfaces thicker 

coatings are preferred [385]. 

The power used in deep drawing operations, sets 

up a great amount of friction between the steel surface 

and the die. This will decrease the speed of 

drawing operation and the service life of tools and 

dies [386,387]. Application of light to medium weight 

non-metallic zinc phosphate coating to steel surfaces, 

which permit the distribution and retention of 

a uniform film of lubricant over the entire surface, 

Fig. 19. Potentiodynamic polarization curves of NdFeB magnet in 3.5% sodium chloride solution (a) Uncoated 

NdFeB magnet; and (b) Zinc phosphate coated NdFeB magnet.


prevents metal to metal contact and makes possible 

the cold forming and extrusion of more difficult 

shapes, than is possible without the coating 

[85,388-390]. A combination of zinc phosphate and 

lubricant film prevents welding and scratching of steel 

in drawing operations and greatly decreases the 

rejections. Bustamante et al. [391] suggest the 

usefulness of pre-phosphate coating of zinc and zinc 

alloy coated steel sheet in preventing the damage 

during the forming process. 

Phosphate coating is used as an absorbent coating 

in laser surface hardening of steel [392-394]. 

Although, all the three major type of phosphate coatings, 

namely, the manganese, zinc and iron phosphate 

coatings were used as absorbent coatings, 

the former was found to be frequently employed in 

majority of the cases. Besides these, phosphate 

coatings have also been found to be useful as thermal 

control coatings in satellite components [395]. 

3.13. Environmental impact 

The environmental impact of metal finishing operations 

is a matter of serious concern and phosphate 

pretreatment operation is no exception to this. The 

wastes generated from phosphate pretreatment line 

can be broadly classified into liquid wastes arising 

from acid pickling, alkaline cleaning, rinsing stages 

and chromic acid sealing stage, etc. and solid 

wastes arising from the sludges formed during phosphating. 

It is estimated that the total outlet of wastewater 

is about 20,800 g/m 2 and the solid residue 

generated in the whole phosphating process is 

about 49.0 g/m 2 . The phosphating process discharges 

400 g/m 2 CO 2- equivalent mass to the environment, 

which demonstrates that conventional 

phosphating, has some green house effect. Fortunately, 

the influence on human health is not so heavy 

since the average local toxic level (LTL) is estimated 

to be 1.6 ppm/ppm [396]. Unlike the wastewater 

discharge, the environment cannot absorb the solid 

residue. The phosphating sludges are generally considered 

as hazardous waste materials and are, 

therefore, subject to strict regulations as to disposal. 

It is estimated that fifty million pounds of sludge 

results from phosphating operations each year. 

Hence it is not only desirable but also mandatory to 

identify a means for recovering various sludge components. 

Phosphating sludge, in general, has 20 wt.% iron, 

10 wt.% zinc, 1-3 wt.% manganese,


ing the particle size of the dried sludge to less than 

about 20 mesh. The dried and ground phosphate 

sludge has been found to be an excellent lubricant 

additive, which is suitable for use in lubricant formulations 

designed for the metal treatment, metal forming 

and industrial lubrication. 

United States Patent 4986977 [402] suggests a 

method for treating the phosphating sludge with an 

aqueous base to achieve a pH greater than 10 that 

results in precipitation of iron hydroxide. The iron 

hydroxide is recovered and the aqueous phase is 

acidified to a pH of 7-10 to cause precipitation of 

zinc hydroxide. 

Baldy [403] suggests that the phosphating 

sludge generated from automotive pretreatment process 

may contain up to 25 wt.% of oil and removal 

of oil is essential for the full recovery of all useful 

components. The removal of oil is accomplished by 

the addition of H 2 O 2 to an acidic dispersion of the 

sludge that displaces the oil from the remaining 

mixture. According to him, the recovery of zinc phosphate 

sludge involves the following four stages: 

Phase I is the separation of oil from the sludge. 

Phase II is the extraction of zinc, manganese and 

nickel from the sludge following digestion with phosphoric 

acid. Phase III is the conversion of the iron 

phosphate residue from phase II to pigment grade 

iron oxide and sodium phosphate that may be acidified 

to an iron phosphate concentrate. 

The use of phosphating sludge in the process of 

clinker production is suggested as one of the possible 

mode of reclamation of such waste by Caponero 

and Tenorio [404]. Their study proves that an addition 

of up to 7.0% of phosphating sludge to the raw 

cement meal of Portland cement did not cause any 

damage to the clinkerization process. X-ray diffraction 

analysis shows that there is no significant modification 

in the yielded clinker proportional to the 

sludge additions, neither is there atypical phases 

formed with additions up to 5.0% of phosphating 

sludge. Differential thermal analysis of the mixture 

with up to 7.0% dry sludge additions does not show 

any significant difference from the analysis of the 

cement clinker raw meal. Additions of the phosphating 

sludge up to 5.0% significantly modify only the 

zinc content of the clinker that was produced. The 

major element of the sludge, zinc, shows an average 

of incorporation of 75%. 

4. SUMMARY 

This review outlines the various aspects of phosphating. 

Although, numerous modifications were 

proposed recently, on the deposition technologies 

to achieve different types of coatings and desirable 

properties such as improved corrosion resistance, 

wear resistance, etc., phosphate conversion coating 

still plays a vital part in the automobile, process 

and appliance industries, as it has unique advantages 

in the cost-wise placement among all the 

emerging deposition technologies and pays off the 

finisher with a handful of profit. 

ACKNOWLEDGEMENT 

Financial support given by the Council of Scientific 

and Industrial Research, New Delhi, India, is gratefully 

acknowledged. The author expresses his sincere 

thanks to Dr.S. Srikanth, Scientist-in-charge, 

NML Madras Centre and Prof. S.P. Mehrotra, Director, 

National Metallurgical Laboratory, 

Jamshedpur, for their constant support and encouragement. 

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surface pretreatment by phosphate conversion coatings – a review

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