J': - Virginia Tech

7

Phosphate Stabilization by Non-Chromate Post-Rinse Treatment

by

Tae-Ho Yoon

Thesis submitted to the Faculty of the

Virginia Polytechnic Institute and State University

in partial fulfillment of the requirements for the degree of

Master of Science

J. L.0 Lytton

in

Materials Engineering

7 W. J':" van Ooij, Jhairman

/

June, 1987

Blacksburg, Virginia

J. G. Dillard \

Phosphate Stabilization by Non-Chromate Post-Rinse Treatment

by

Tae-Ho Yoon

W. J. van Ooij, Chairman

Materials Engineering

(ABSTRACT)

Zinc phosphate conversion coating has been applied to improve the corrosion resistance and

paint adhesion. However, zinc phosphate crystals dissolve in a highly alkaline environment,

which reduces the corrosion resistance of base metal. To improve the phosphate stability in

a highly alkaline environment, a post-rinse treatment has been applied to phosphate coating

by rinsing with an aqueous solution which contains certain anions or cations. Chromate-

post-rinse treatment is the most widely used method and has shown a great improvement in

corrosion resistance. But, due to the environmental problems caused by chromate ions,

non-chromate post-rinse treatment should be developed, which has equal or better corrosion

resistance than does chromate post-rinse treatment. In this research, inorganic silicate with

addition of Ca2+, Ba2 +, Ni2+, Mg2 +, has been extensively evaluated together with silane sol-

ution, y -aminopropyltriethoxysilane ( y -A.P.S., NH2(CH 2) 3Si(OC2H5) 3), which was applied after

the post-rinse treatment. The evaluation was carried out by the highly advanced surface

analysis techniques such as SEM/EDX, AES, SIMS, and XRD and polarization measurements.

Acknowledgements

The author would like to express his gratitude to Dr. W. J. van Ooij for his contributions

and suggestions throughout this research effort and to Amchem Products Inc. for their spon-

sorship.

The author would also like to extend his thanks to Dr. J. L. Lytton, and Dr. J. G. Dillard for

serving on his committee. The author wishes to acknowledge the helpful suggestions from

Dr. Woo Jin Choi, Dr. Hee Young Lee and Regina H. Kim.

The author wish to express special thank to Bob McGrew at Colorado School of Mines for

the SEM and the EDX works.

Finally, the author would also like to express his sincere thanks to his parents, Mr. and

Mrs. D. S. Yoon, and Mr. and Mrs. C. K. Lee for their support.

Acknowledgements iii

Table of Contents

Chapter I. Introduction ................................................... 1

Chapter II. Literature Review .............................................. 4

2 - 1. Phosphate Conversion Coatings. . .................................... 5

2 - 1 - 1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 - 1 - 2. Mechanisms of Phosphate Conversion Coatings. . . . . . . . . . . . . . . . . . . . . . 6

2 - 1 - 3. Effects of Phosphate Coatings on Corrosion Protection. . . . . . . . . . . . . . . . . 7

2 - 1 - 4. Dissolution of Phosphate Coatings. . .............................. 8

2 - 2. Post-rinse Treatment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2 - 2 - 1. Introduction .................................................. 9

2 - 2 - 2. Effect of Post-rinse Treatment on Corrosion Protection. . . . . . . . . . . . . . . . . 9

2 - 2 - 3. Possible Non-chromate Post-rinse Treatment. . . . . . . . . . . . . . . . . . . . . . . 10

2 - 3. Silane Coupling Agent. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2 - 3 - 1. Bonding Theory of Silane Coupling Agent. . . . . . . . . . . . . . . . . . . . . . . . . . 12

2 - 3 - 2. Chemistry of Silane Coupling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2 - 4. Principle of Surface Analysis Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2 - 4 - 1. Basic Principles ............................................. 15

Table of Contents iv

2 - 4 - 2. Scanning Electron Microscope. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2 - 4 - 3. Energy Dispersive X-ray Analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2 - 4 - 3. Auger Electron Spectroscopy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2 - 4 - 4. Secondary Ion Mass Spectrometry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Chapter Ill. Experimental ................................................ 22

3 - 1. Sample Preparation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3 - 1 - 1. Raw Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3 - 1 - 2. Experimental Conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3 - 1 - 3. Solution Preparation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . 27

3 - 1 - 4. Experimental Procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3 - 2. Sample Analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3 - 2 - 1. SEM/EDX Analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3 - 2 - 2. AES/SIMS Analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3 - 2 - 3. X-Ray Diffraction Analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3 - 2 - 4. Polarization Measurement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Chapter IV. Results and Discussion ••••••••••••••••••••.•.••.•.••.••.••.••• 35

4 - 1. SEM/EDX Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4 - 1 - 1. Post-rinse Treatment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

4 - 1 - 2. Variability and Brittleness. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

4 - 1 - 3. Silane Treatment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

4 - 1 - 4. Heat Treatment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

4 - 2. AES/SIMS Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

4 - 3. X-ray Diffraction Analysis Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

4 - 4. Polarization Measurement Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

Chapter V. Summary and Conclusions • • • • • • • • • • • • • • • • • • . • • • • • • . • • • • • • • . . . • • 91

Table of Contents v

Bibliography 96

Vita 100

Table of Contents vi

List of Illustrations

Figure 1. Variety of signals produced by electron bombardment. . . . . . . . . . . . . . . . . . 16

Figure 2. General procedure of treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Figure 3. Schematic diagram of treatment setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Figure 4. Schematic diagram of polarization measurement setting. . ...... : . . . . . . . . 34

Figure 5. Leaching of phosphate in a solution of pH 12.0. . . . . . . . . . . . . . . . . . . . . . . . 37

Figure 6. Leaching of phosphate in a solution of pH 12.5. . . . . . . . . . . . . . . . . . . . . . . . 38

Figure 7. Phosphate crystals of as-received sample . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Figure 8. Phosphate crystals after treatment in a solution of pH 12.0 for 30minutes(a) and one hour(b). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Figure 9. Phosphate crystals after treatment in a solution of pH 12.5. for 10(a) and 20 minutes(b). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Figure 10. Effect of waterglass on the stabilization of phosphate in a solution of pH 12.0 for 30 minutes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Figure 11. Dissolution of phosphate crystals after treatment in solution WG (a) and WZ (b) for 30 min. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Figure 12. Change of silicon content by the post-rinse treatment and followed by immersion test for 10 and 20 min. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Figure 13. Effect of post-rinse treatment on the phosphate stabilization in the immersion test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Figure 14. Effect of zinc sulfate on the stabilization of phosphate in solution WZ. . . . . . . 48

Figure 15. Effect of calcium on the phosphate stabilization . . . . . . . . . . . . . . . . . . . . . . . 50

Figure 16. Morphology change by the treatment in· solution WC (a) and WB (b) at pH 12.0 for 30 min. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Figure 17. Dissolution of phosphate crystals in the immersion test for 10(a) and 20 min.(b) after treatment in solution WC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

List of Illustrations vii

Figure 18. Effect of the post-rinse treatments in solution WB, WCN, WCM and WCB on the phosphate stabilization in the immersion test. . . . . . . . . . . . . . . . . . . . . . . . 55

Figure 19. Dissolution of phosphate crystals of samples treated in solution WB in the immersion test for 10 (a) and 20 min.(b). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Figure 20. Variability of P, Si and Ni in as-received sample. . . . . . . . . . . . . . . . . . . . . . . 60

Figure 21. Relationship between P, Si and Ni in as-received sample. . . . . . . . . . . . . . . . 61

Figure 22. Effect of treatment on variability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

Figure 23. Effect of immersion test on variability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

Figure 24. Cracks formed by the punching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Figure 25. Change of silicon content by the silane treatment and followed by the immersion test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

Figure 26. Effect of the silane treatment on the phosphate stabilization in the immersion test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

Figure 27. Effect of the silane treatment on the dissolution of phosphate crystals in the immersion test for 10 (a) and 20 min.(b)(pretreated in WCB) . . . . . . . . . . . . . 69

Figure 28. Effect of heat treatment on the phosphate stability in the immersion test. 72

Figure 29. Degradation of phosphate crystals by heat treatment followed by the immersion test for 10 (a) and 20 min.(b). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

Figure 30. AES spectrum of as-received sample. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

Figure 31. AES spectrum of the sample treated in solution WG for 30 min. . . . . . . . . . . . 76

Figure 32. AES depth profiling spectrum of the sample treated in solution WG. . . . . . . . 77

Figure 33. AES spectn,im of the sample treated in solution WC without water rinse. 78

Figure 34. Effect of water rinse on the retention of elements in the coating surface. 79

Figure 35. Depth profiling result of the sample treated in solution WC for 30 min (no rinse). 80

Figure 36. AES spectrum of the sample treated in solution WCB. . . . . . . . . . . . . . . . . . . 81

Figure 37. AES spectrum of the sample treated in solution WCB followed by the silane treatment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

Figure 38. SIMS survey spectra of the sample treated in solution WCB for 30 min. . . . . . 85

Figure 39. SIMS spectrum of the sample treated in solution WCB followed by the silane treatment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

Figure 40. X-ray diffraction spectra of the samples after post-rinse treatment followed by the immersion test for 20 min. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

List of Illustrations viii

Figure 41. Cathodic polarization curves of post-rinse treated and/or silane treated samples and as-received smple. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

Figure 42. Proposed model of thin film formed by the post-rinse treatment in solution WB. 94

List of Illustrations ix

List of Tables

Table 1. Analytical characteristics of AES (70). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Table 2. Analytical characteristics of SIMS (74) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Table 3. The specification of sample panel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Table 4. List of chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Table 5. Solutions used in post-rinse treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Table 6. Average and standard deviation value of the variability treatment. . . . . . . . . . 59

List of Tables x

Chapter I. Introduction

Recently, the need to improve corrosion resistance of automotive steel became more

important than ever due to hostile environment such as the increasing use of de-icing salts in

winter (1). A number of investigations have been carried out to improve corrosion resistance

of automotive steel with more research focusing on the pre- and post-treatment (2, 3) rather

than developing new anti-corrosive materials. Powerful surface analysis techniques have

become increasingly valuable for corrosion studies, for example SEM/EDX, AES and SIMS, in

combination with other tools (4).

Zinc (6-9) and its alloy coating (10-13) have been widely applied to steel surface to im-

prove corrosion resistance by the sacrificial action of zinc to steel. The phosphate conversion

coatings have also been extensively used by themselves or in combination with other coatings

to improve the corrosion resistance (14-16), which provide good barrier for corrosion pro-

tection and base for paint coating. However, phosphated and electrocoated steel corrodes

readily in salt spray, scab and filiform conditions. Recent studies using modern surface

analysis tools such as XPS, AES and EDX have indicated that such corrosion is initiated by the

transformation in the phosphate layers (17,18).

Chapter I. Introduction 1

Since phosphate coatings have some degree of porosity (19, 20) and damage from han-

dling, corrosion cell can be established under the paint coating. As corrosion reaction go on,

the pH of corrosion cell increases generally upto 12-14. As reported in literature (21, 22)

phosphate coatings dissolve in a highly alkaline environment; hence corrosion protection by

phosphate coatings reduce. The stabilization of phosphate coatings and hence the corrosion

protection can be further enhanced by the post-rinse treatment in a solution which contains

certain cations or anions, such as CrH, Pb2+ or Cro~- (17, 18, 21). This treatment forms a

protective skin with a much reduced solubility by ion exchange of certain cations or anions

with either Zn2+ or POl- ion. This treatment was found to be effective only at high pH condi-

tion (salt spray test). The post-rinse treatment with solution containing chromate ions has

shown great improvement of corrosion protection (19), however, due to the environmental

problems caused by chromate ions, non-chromate post-rinse treatment should be developed,

which has equal or better corrosion resistance than chromate post-rinse treatment.

It is certain that corrosion protection can be improved by reducing the dissolution of

phosphate coating in a highly alkaline environment. In this research, to develop non-chromate

post-rinse treatment widely used corrosion inhibitors were tried as a post-rinse treatment

agents, such as silicate, calcium, barium and magnesium, and nickel which is well known

coating material. Silicate which is inexpensive, non-toxic, well known corrosion inhibitor (25,

26) and causes no serious problems was extensively evaluate together with other ions. The

silane treatment was also performed to improved the paint adhesion and reduce the dissol-

ution of phosphate. The analysis of the samples was carried out with SEM/EDX, AES and

SIMS, and confirmation of results was made by the application of XRD and polarization

measurements.

It is proposed to optimize the conditions in terms of pH, temperature, time, concentration

of the silicate and the presence of other ions in the solution by observing the effect of these

factors on the rate of phosphate attack. The objectives of this research were as follow;


1. to understand the mechanism by which zinc and zinc-iron phosphate exchange cations

and/or anions in highly alkaline media and to formulate a model,

2. to develop a post-rinse treatment which is fast, cheap, effective at both high and low pH,

and which does not involve the use of chromium or chromate ions by using the funda-

mental knowledge acquired from step 1,

3. to further improve the corrosion protection by incorporating a suitably selected silane in

the inorganic silicate film.


Chapter II. Literature Review

To improve the corrosion resistance of automotive steel panels, numerous research has

been carried out on the various aspects of phosphate conversion coating and post-rinse

treatment. The availability and usefulness of highly advanced surface analysis techniques

have led to the fundamental studies of the mechanism of corrosion protection by the post-

rinse treatment and phosphate coating. The silane treatment has emerged as a very impor-

tant method in enhancing paint adhesion to metal or inorganic coating and corrosion

protection.

Due to the demand a good quality coating to minimize the corrosion damage, numerous

versions of phosphate coating system and post-rinse treatment technique have been devel-

oped and some have been utilized widely in the industries. To have a better knowledge of

the protection mechanism, it is important to understand the basic principles of phosphate

coating and post-rinse treatment.

Chapter II. Literature Review 4

2 - 1. Phosphate Conversion Coatings.

2 - 1 - 1. Introduction.

The protective coatings are applied to the steel surface to enhance the corrosion re-

sistance, durability and appearance. A number of protective coating systems have been de-

veloped and can be classified into three groups; (i). metallic coatings, (ii). non-metallic and

inorganic coatings and (iii). organic coatings. In recent years, however, a mixture of metallic

and inorganic coating has been introduced (27).

Prior to the application of protective coatings, it is necessary to clean the surface of steel

very carefully to enhance the adhesion of coating to the base metal(28, 29). The effect of

surface contamination on coating has been investigated (19, 30). The adhesion of coating to

the metal surface is a key to the coating effectiveness and thus, the surface preparation be-

comes even more critical for coatings applied in corrosive areas.

The phosphate conversion coatings, belonging to the inorganic coating category, have

been applied to steel surfaces to improve the corrosion protection since the beginning of this

century. Since then, a number of investigations have been carried out to improve the quality

and application technique of phosphate coatings. The phosphate coatings are usually applied

to provide a barrier for corrosion protection and a base for paint coating, and are readily ap-

plied to the articles of all size by spray or immersion. The general aspects of phosphating

were reviewed by Lorin (31).


2 - 1 - 2. Mechanisms of Phosphate Conversion Coatings.

All conventional phosphate coating processes are based on dilute phosphoric acid sol-

ution of iron, manganese or zinc phosphate either separately or in combination. The chemical

reactions are proposed by several authors (31-33). Zinc phosphate coating is accomplished

by treating the materials in an aqueous solution of acidic zinc phosphate, Zn3(H2P04h to form

a thin film of zinc phosphate crystals, Zn3(P04h , on the surface by spraying or dipping.

The formation of the phosphate crystal in an aqueous solution of acidic zinc phosphate

is represented by the following reactions.

However, when a steel panel is immersed in the phosphate solution its surface is im-

mediately attacked by the free phosphoric acid. Thus ferrous phosphate and hydrogen are

produced as follows;

If the steel panel is coated with zinc (no source of iron), equation (4) may be replaced by the

following reaction,


Reaction (5) produces a acidic zinc phosphate solution and the driving force of reaction (2),

which shifts the reaction toward the right, is the neutralization of free phosphoric acid by re-

action (4) and (5). Therefore, the relative amount of free phosphoric acid is very important in

order to obtain a precipitation of phosphate coating(34).

In the phosphating process, chemical activators such as nickel ions are generally added

to make phosphate crystals finer (35). O~idizing agents are also added to accelerate the

overall reaction. Since the chemical activator absorbs on the surface, called surface condi-

tioning, prior to phosphate coating, they provide the sites for crystal nucleation.

2 - 1 - 3. Effects of Phosphate Coatings on Corrosion Protection.

In the automotive industry, the zinc phosphate coatings are generally used. The zinc

phosphate coatings usually consist of two types of crystals; hopetite (Zn3(P04) 2-4H20) and

phosphophyllite (Zn2Fe(P04}z-4H20). The former is more likely obtained by spray and the latter

by immersion. Even though phosphophyllite is more desirable due to its superior alkaline

resistance (36), it is hard to get phosphophyllite even in immersion application if the steel

surface was already treated with other coatings, like zinc.

The phosphate coatings provide a barrier for the corrosion protection. Protectability of

the phosphate coatings without sealing treatment is not good enough because usually the

phosphate coatings have some degree of porosity and defects (20, 21). It is generally ac-

cepted that corrosion protection of phosphate coatings is directly related to the thickness and

porosity of coating, and the ratio of phosphophyllite to hopetite. The porosity of phosphate

coatings has been investigated (37).


2 - 1 - 4. Dissolution of Phosphate Coatings.

Recent studies have shown that the phosphate coatings dissolve in a wide range of pH

(19,38). At high pH the phosphate ions are leached out of the phosphate crystals, while at

moderate pH zinc and phosphate ions dissolve, and at very low pH the entire phosphate

coating dissolves. The phosphate dissolution is due to the corrosion reaction which increases

the pH of the corrosion cell beneath the coating(39, 40).

Due to the porosity and the damage from the handling, the corrosion cell might be es-

tablished beneath the coating by the diffusion of oxygen, water and ions. As the corrosion

reactions proceed, the pH of the corrosion cell increases due to the OH- ions produced by

cathodic reaction. The increased pH has an adverse effect on the corrosion protection be-

cause of the dissolution of phosphate coatings in a high alkaline solution which results in a

greater surface porosity and additional coating defects (41), hence less corrosion protection

(42).

Since the dissolution of phosphate coatings is directly related to corrosion protection,

the corrosion resistance can be improved by preventing the dissolution of phosphate, espe-

cially in a high pH solution.


2 - 2. Post-rinse Treatment.

2 - 2 - 1. Introduction.

The post-rinse treatment is another very important procedure to obtain increased cor-

rosion protection. After phosphating, the water rinse is a necessary step to remove the sol-

uble salts and unreacted phosphate ions. The improved corrosion resistance can be obtained

by water rinse which contains suitable corrosion inhibitors; certain cations or anions. In the

water rinse step, care should be taken to ensure that the water itself is sufficiently free from

the harmful salts.

The most commonly used compound in the post-rinse treatment is chromate which has

shown an excellent corrosion protection properties (18). Even though trivalent chromium is

regarded as the safer form and its use as a final rinse was more widely accepted by the in-

dustry than hexavalent compound, recently chromic acid has been labeled as a suspected

carcinogen by the EPA. As a result, metal product finishers are being forced to take another

look at the safer form of chromium or develop new materials for the post-rinse treatment.

2 - 2 - 2. Effect of Post-rinse Treatment on Corrosion Protection.

The post-rinse treatment is usually applied by immersion in a solution or spray, which

contains the desired ions. This treatment forms a rarely soluble film on the phosphate coating

by incorporating with ions in solution. The formation of the film seems to be related to the ions

used and phosphate coating itself. Even though the phosphate coating may look complete and

uniform, there is a certain amount of porosity in the coating. The porosity may come from the


poor surface preparation and/or the phosphate itself. Two mechanisms are proposed to ex-

plain the improvement of corrosion protection by the post-rinse treatment; the chemical and

the physical effects.

These two effects can be explained by the chromate post-rinse treatment. During the

post-rinse treatment, the chromate ions precipitates into the pores and on to the surface of

phosphate crystals. It is assumed that the precipitates adhere to the coating surface phys-

ically and act as a gel that fills the pores and results in a sealing effect by forming a thin film

of chromate compound which acts as an effective barrier for corrosion protection.

The action of post-rinse treatment is not confined solely to the precipitation of the insol-

uble film on the coating surface but also involves reactions with the phosphate coating itself.

The solution penetrates the coating to some degree and reacts with the phosphate ions in the

crystal resulting in the formation of a very insoluble compound, for example, chromium

phosphate. The insoluble phosphate film covers the coating and thus provides a sealing ef-

fect. Hence, it is thought that almost inhibiting effect is attributed to physical absorption of

precipitate, and chemical reactions provide improved adhesion of film to coating.

2 - 2 - 3. Possible Non-chromate Post-rinse Treatment.

Even though chromate post-rinse treatments have been applied to a wide range of in-

dustrial metals and show a great improvement in corrosion protection, they should be re-

placed by non- chromate post-rinse treatments due to the possible environmental problems

caused by chromate. For this purpose, several kinds of materials have been tried such as

inorganic silicate and other metallic ions . However, no treatment has yet been developed to

replace the chromate post-rinse treatment. In this research, to develop non-chromate post-


rinse treatment, well known corrosion inhibitors were tried as a treatment agent, such as

silicate in combination with calcium, barium, magnesium and nickel. The corrosion inhibition

of those elements has been reviewed (43).

Silicate is inexpensive, non-toxic and causes no serious environmental problems. It is

very stable in acidic solution except in hydrofluoric acid. Silica and silicate have been used

for corrosion protection because they provide a good barrier and act as inhibitors (44,45). One

of the widely used soluble silicates is sodium silicate, waterglass Na2 O-nSi02• The aqueous

solution of this is more or less polymeric depending on the ratio of silica to alkali. Soluble

silicate has been reviewed extensively (46) and recently, the effects of silicate on human

health have also been studied (47).

Calcium is one of the metallic elements used as a corrosion inhibitor (48,49) and calcium

phosphate has already been commercialized. This is the fifth abundant element in the earth's

crust and not toxic. Barium, belonging to the alkaline earth element group like calcium, has

almost the same chemical characteristics as calcium , but most barium compounds are toxic

except barium sulfate. Barium salts have been also used as a corrosion inhibitor (50).

Magnesium is another element in alkaline earth group. Magnesium has good resistance

to atmospheric attack and to certain chemicals because of its ability to acquire a protective

film, insoluble hydroxide film (51). Due to this ability, it has been used as a corrosion inhibitor

(52). However, despite its resistance to alkaline environment. it is susceptible to acids and

does not promote the formation of an insoluble film in an acidic environment.

Nickel has been used as anti-corrosive coating material, such as Zn-Ni alloy coating

(53,54), and as an activator in phosphating (35). Nickel is not readily oxidized in air at ordinary

temperatures. Nickel and its salts are not generally considered poisonous except nickel

carbonyl which is extremely toxic.


2 - 3. Silane Coupling Agent.

In recent years, the use of silane coupling agents has increased dramatically in the in-

dustries to obtain high strength plastic composites and to improve the bond strength of or-

ganic coatings to inorganic surfaces (55-61) and corrosion protection (62,63). In the

automotive industry, paint delamination is one of the major problems and is generally as-

sumed to be alkaline saponification of the polymer adjacent to the interface of metal as a re-

sult of cathodic corrosion reaction. The paint adhesion to metal or to inorganic coatings is

very important in corrosion protection. A number of investigations have been carried out and

the the application of silane coupling agents has dramatically increased recently.

2 - 3 - 1. Bonding Theory of Silane Coupling Agent.

Since minute proportions of coupling agents at the interface have a great effect on ad-

hesion promotion, it is necessary to understand the mechanism of adhesion promotion by a

silane coupling agents and the fundamental nature of them. The function of silane coupling

agent is to provide a stable bond strength between two different surfaces.

Several adhesion theories have been proposed but chemical bonding theory is the old-

est and predominant (64). Coupling agents contain two different types of reactive groups that

provide an ability to bond chemically to inorganic surfaces and organic polymers. The bond-

ing to the surface is made by chemical bonds, usually covalent bond, but, sometimes, van der

Waals forces or ionic bonds. Assuming that all this occurs, the coupling agents may act as

a bridge to bond the metal surface to paint coating. The bridge could be expected to lead to

strong chemical bonds.


2 - 3 - 2. Chemistry of Silane Coupling.

Silane coupling agents have two functional groups : organo and inorganic silicate to form

a bridge between inorganic substrate and organic polymer. The performance of

organofunctional silanes depends on the stable link between the organofunctional group (R)

and hydrolyzable group (X) in compounds of the general formula X3-SiR (64,65).

The organofunctional groups (R) usually react with the polymer surface while the

hydrolyzable groups (X) are merely intermediate in formation of silanol groups for bonding to

inorganic surfaces. In many commercial applications the silanes are pre-hydrolyzed and ap-

plied from dilute aqueous solution. Therefore, the nature of hydrolyzable group is not impor-

tant to the performance of the coupling agent, if adequate moisture is available for hydrolysis.

Silane coupling agents interact with receptive inorganic surfaces forming strong chemi-

cal bond at the interface. Receptive inorganic surfaces are characterized by the presence of

hydroxyl groups attached to certain elements, such as silicon and aluminum, in the formation

of hydrated oxides having absorbed water of hydration as their outermost layer. The coupling

agent is first converted to the reactive silanol form by hydrolysis;

R SiX3 + 3H20 -+ R-Si(OH)3 + 3HX

This reaction may occur on the surface or already brought in a previous step during prepa-

ration of solution of the coupling agent. The reaction is rapid in the presence of slightly

acidified water around pH 3 to 6.

The silanol form of the coupling agent now reacts with hydroxyl group in the inorganic

surface. Self condensation of the silanol forming siloxane polymer is an important side re-


action. When silica or alumina is present in the surface layer, the coupling reaction is readily

obtained since they are inherently receptive to coupling with silane.

-OH ----OH+(OH)3 Si(CH2)3 R_. ~-OH

OH 0Si(OH)2 (CH2)3 R+ H2 0 OH

Glasses, silicates and other Si-OH bearing siliceous materials share receptivity in pro-

portion to their silicon content. The coupling reaction of silane to the organic surface is easier

than to the inorganic surface. The organic chemistry predicts the formation of chemically

covalent bonds between the organofunctional group of silane and the reactive species in the

polymer matrix.

~ OH /1 I /Si-0-Si-R /~ I ~ OH

~ OH /o I ~1-0-Si-R ,,~ I ~ OH

2 - 4. Principle of Surface Analysis Techniques

In recent years, surface analysis techniques in combination with traditional tools for

corrosion research have become widely adapted to examine and solve many problems. It is

believed that such use will increase with time. A number of review papers are available on


the applications and basic principles of surface analysis techniques (66-75). In this research,

several surface analysis techniques were used such as SEM, EDX, AES, SIMS. In this section

the basic principles of these instruments will be reviewed briefly.

2 - 4 - 1. Basic Principles

In the instrument chamber, a sample is bombarded with the electron or ion beam which

was accelerated through an electric field and hence has acquired kinetic energy. The dissi-

pation of electron beam yields a variety of signals for the analysis of sample as depicted

schematically in Fig. 1. Auger and secondary electrons escape from the surface layers of the

sample. The secondary electrons which emerge from near the sample surface are detected

to form an image of the sample in the SEM while Auger electrons are for the AES analysis.

The characteristic X-rays having much greater escape depth than electrons, provide ele-

mental information; therefore, they are used for EDX or WDX to identify the elements.

2 - 4 - 2. Scanning Electron Microscope.

SEM is by far the most powerful instrument for obtaining topological information. In the

corrosion research SEM is widely applied usually in combination with EDX, WDX or others

such as AES. The principle of and the applications of this on corrosion research have been

published (66,67). A fine probe electron beam is used for rastering the sample. The SEM

image is obtained by scanning the beam across the sample and by detecting the low energy

secondary electrons or high energy backscattered electrons returning from the surface of the

sample. The detection of electrons is achieved by means of a scintillator -photomultiplier ar-

rangement, that is positively biased by several hundred volts. The signal is fed to a cathode


Cathodoluminescence (visible light)

lncldlent Electron Beam

Bremsstrahlung

Characteristic X-rays

-.

Specimen Current

Auger Electrons

Secondary Electrons

Elastically Scattered Electrons

Transmitted Electrons and lnelastlcally Scattered Electrons

Figure 1. Variety of signals produced by electron bombardment.


ray tube (CRT) and surface topology and elements give rise to different signal intensity

;therefore, contrast is obtained on the CRT.

The SEM has a remarkable depth of field and better resolution compared to optical

microscopy providing a three-dimensional image. One of advantages is large samples can be

analyzed. However, the major problem is the charging of insulating materials which can be

overcome by applying thin conductive coating on the surface of the sample.

2 - 4 - 3. Energy Dispersive X-ray Analysis.

When the primary beam bombards the sample, the characteristic x-ray as well as

electrons are generated. However, it was not utilized until the solid state detector (Li or Si)

had become available to resolve the x-ray peaks of adjacent elements in the periodic table.

In EDX, the characteristic x-rays are detected by the liquid nitrogen-cooled Si (or Li )

detector. The energy of x-rays are translated proportionally to the pulse mode which is fed

to a multichannel analyzer. By accumulating the pulse, x-ray spectrum is processed by a

computer.

EDX techniques have many advantages, such as a satisfactory compromise between a

multi-element character, ~conomy, speed and ease of operation. Also EDX offers a fairly uni-

form detection limit across a large portion of periodic table. However, it is at most a semi-

quantitative analysis technique. The details of EDX have been reported in the literature

(68,69).


2 - 4 - 3. Auger Electron Spectroscopy.

During the past decades, the technique of AES has emerged as one of the most widely

used analytical tools for acquisition of the chemical information. The fundamental mechanism

involved in AES are ionization of core level atom by the bombardment of electrons. Auger

electrons escape from near the surface of the sample and are characteristic of the target

materials, and the measured energy of Auger electrons is not dependent on the primary en-

ergy. Since Auger electrons have well defined energies, they are manifested as small peaks

in the total energy distribution function { N{e)). The differential spectrum is usually recorded

by removing the background.

The peak positions are characteristic of particular elements allowing qualitative analysis

of all elements except H, He and sometimes Li. AES is particularly sensitive to light elements

and the analytical characteristics of AES are listed in Table 1(70). The depth profiling in AES

can be performed during the analysis. The one disadvantage is that quantification is difficult.

The mechanisms and applications of AES have been reported in detail {71,72).

2 - 4 - 4. Secondary Ion Mass Spectrometry.

SIMS is one of the true surface analysis techniques, which has higher sensitivity, ability

to detect all elements and information from the outermost layers {73-75). The principle

mechanism of SIMS is almost the same as that of AES. In SIMS, however, an ion beam instead

of an electron beam is used for bombardment, which is usually an inert gas ion such as Ar+,

o;, Ga+ or cs+. By bombardment with an ion beam of 2 to 20 keV, considerable disruption

occurs by transfer of momentum to lattice atoms, which result in the eruption of atomic and

molecular fragments from the several atomic layers deep, consisting of mainly neutral and a


Table 1. Analytical characteristics of AES (70).

Kinetic energy : 50 - 2500 eV Energy range Escape depth : 20 A0

Peak location : ± 1eV

Chemical information Marginal

Elements : Z > 2 Elemental sensitivity Signinificity : good

Sensitivity variations : SOX

Absolute : ± 30 percent Quantitative analysis Relative : ± 5 percent

Detection limit : 0.005 monolayer Matrix effect : some

Vacuum : 10-1 - 10- 11 torr Depth profiling: yes, rapid, multiplex

Other aspects X-Y resolution : 0.5µ Elemental mapping : yes Speed : fast, most spectra takes minutes Sample destruction : particularly bad for organics


few percent of positively and negatively charged fragments. In SIMS analysis, only positively

or negatively charged ions are collected to form the mass spectrum which gives rise to the

peaks presenting elemental species and cluster ions.

SIMS has two variations depending on the operating conditions; dynamic and static

mode. Even though SIMS is a very sensitive surface analysis technique, it has limitations.

Charge builds up when insulators are analyzed, which could be eliminated by compensating

the charge ionization with appropriate low energy electron flux. The analytical characteristics

of SIMS are shown in Table 2 (74).


Table 2. Analytical characteristics of SIMS (74)

Spectral range 1 - 500 amu

Analysis depth 40 A0 (dynamic), monolayer (static)

Chemical information Mass spectrum from surface layer

Element - all Elemental sensitivity Specificity - good (some overlap)

Sensitivity variation - 10- 5

Absolute - not possible Relative - ± 50 percent

Quantitative analysis Detection limit - 10- 4 percent nomolayer Matrix effect - severe

Vacuum : 10- 5 torr of ionizing gas Depth profiling : Yes, rapid, 'dynamic' SIMS

Other aspects X - Y resolution : 1 µ with ion microprobe Elemental mapping : As a microprobe Speed : fast, most spectra take minutes Sample destruction : yes, sputtering of surface


Chapter Ill. Experimental

The phosphate ions are leached out in a high pH solution (32), which is directly corre-

lated with the corrosion protection (34) and paint adhesion failure (25). In this investigation,

a non-chromate post-rinse treatment was applied to the phosphate coating in order to reduce

the amount of leaching of phosphate ions in a high pH solution; hence to improve the corro-

sion protection and paint adhesion.

Seven different solutions were tried as post-rinse treatment agent. After the post-rinse

treatment, some samples were immersed in a KOH solution of pH 12.5 to see how effective

this treatment was, named as an immersion test, and others were submitted to the silane

treatment or the heat treatment. The general procedure of the experiment is shown in Fig.

2. The variability and brittleness of the phosphate coating were also evaluated before and after

the post-rinse treatment. Heat treatment was performed at 110° C for 24 hours and at 180° C

for 30 minutes after the post-rinse treatment. The silane treatment was applied to the selected

samples.

For the characterization of the treated samples, the surface analysis techniques, such

as AES and SIMS, were applied, and X-ray diffraction analysis and polarization measurements

Chapter Ill. Experimental 22

POST RINSE TREATMENT

SI LANE TREATMENT

ANALYSIS

Figure 2. General procedure of treatment

Chaper Ill. Experimental

HE AT TREATMENT

23

were also used for further evaluation. However, most of analysis was performed by Scanning

Electron Microscopy(SEM) and Energy Dispersive X-ray Analysis (EDXA or EDX).

3 - 1. Sample Preparation.

3 - 1 - 1. Raw Materials.

The as-received steel panel was cold-rolled, electrogalvanized, type GM-16-20E, and one

side sprayed phosphated using Granodine 902, which is the same material used in the auto-

motive industry. The panels were supplied by the project sponsor, Amchem Porducts Inc. of

Ambler, Pa.. The specification of the panels is listed in Table 3. The sample panels were

sheared to 1x2 inch rectangular pieces and a hole of 1/8 inch in diameter was punched at the

top center of th·e sample.

3 - 1 - 2. Experimental Conditions.

During the treatments, a magnetic stirrer, Corning Pc-351, was used at a speed of 240

RPM. All samples were treated in 500 ml of solution at 25° C in 600 ml beaker. In the post-

rinse treatment, pH of all solutions was adjusted to 12.0 and in the immersion test, to 12.5. A

pH-meter, Beckman 4500, was calibrated with buffer solutions of pH 7 and 10 prior to the ex-

periments. The treatment was carried out for 30 minutes in the post rinse treatment, for 10

or 20 minutes in the immersion test and for 5 minutes in the silane treatment, respectively.

The pH of all solutions was adjusted by dissolution of the reagent KOH. Usually, two samples

were treated simultaneously but sometimes they were treated for different time duration. Fig.

3 shows the schematic diagram of the treatment setting.


Table 3. The specification of sample panel

Material Cold rolled steel

Thickness 0.030 inches

Size 4x12 inches

Hardness B 60-45 Re

Carbon SAE 1008

Coating 0.54 mils Electrogalvanized

Phosphate Granodine 902 sprayed


Water in

®0

Figure 3. Schematic diagram of treatment setting 1. Thermometer 2. Sample 3. Heat control 4. Speed control

Chapter Ill. Experimental

Water out

0®

26

The heat treatment was carried out for the elected samples either at 180° C for 30 min-

utes or at 110° C for 24 hours. Before the heat treatment, the oven was calibrated and stabi-

lized by waiting for one hour. The immersion test was performed before or after the heat

treatments at both temperatures. After the heat treatment, the samples were cooled to room

temperature.

3 - 1 - 3. Solution Preparation.

The seven different solutions were prepared for the post-rinse treatment, as listed in

Table 2. The waterglass solution was first made by diluting waterglass with deionzed water

to 0.05M concentration and then futher diluted to the molarity needed. The other solutions

were prepared by dissolving the chemical(s) into the 0.005M waterglass solution. The general

procedure of preparing the solutions is as follow except that of the silane solution which will

be described later.

1. Weighing the exact amount of chemical(s)

2. Dissolving the chemical(s) in a solution of 0.005M waterglass

3. Adjusting pH and temperature

The research or reagent grade chemicals were used, which are listed in Table 5. To get

the optimun result, one or more chemicals were dissolved in the 0.005M waterglass solution.

When the chemical(s) were dissolved completely, pH and temperature were adjusted to 12.0

and 25° C.


Table 4. List of chemicals

Chemical Grade Manufacturer

Waterglass 40-42 Be Fisher

Zinc Sulfate Reagent Fisher

Calcium Nitrate 99.5 percent Mallinckrodt

Nickel Sulfate 97.1 percent Baker

Magnesium Sulfate 99.5 percent Baker

Barium Nitrate 99.4 percent Baker


Table 5. Solutions used in post-rinse treatment

Solution Chemical Rinse

WG 0.005M waterglass Yes

wz 0.005M waterglass + 0.01 M zinc sulfate Yes

WC 0.005M waterglass + 0.005M calcium nitrate Yes/No

WB O.OOSM waterglass + 0.005M barium nitrate No

WCB Solution WC + 0.005M barium nitrate No

WCN Solution WC + O.OOSM nickel sulfate No

WCM Solution WC + 0.005M magnesium sulfate No


A wide range of molarity was tried to find the optimum molarity of solution such as

waterglass, 0.005M waterglass + zinc sulfate and 0.005M waterglass + calcium nitrate sol-

ution. For the immersion test, the solution of pH 12.5 was made by dissolving KOH in

deionized water.

y -Aminopropyltriethoxysilane ( y-A.P.S.) was used for the preparation of the silane

solution. y -A.P.S. was hydrolized by 100 parts of y -A.P.S., 5 parts of acetic acid and 25 parts

of deionized water, and stirred for about 30 minutes until a clear solution was obtained. This

silane solution containing about 30 percent of active ingredient was diluted with deionized

water again to 0.5 percent of active solution (76).

3 - 1 - 4. Experimental Procedure.

The post-rinse treatment was carried out right after the solutions were prepared to avoid

the deterioration, such as precipitation. As soon as the post-rinse treatment was completed,

some samples were rinsed with deionized water and dried by air blown at room temperature

while others were just dried in air. The post-rinse treatment was followed by either the

immersion test, the heat treatment, the silane treatment or the analysis. The general proce-

dure of post-rinse treatment is as follow.

1. Treating samples for 30 min. in solution

2. Rinsing with deionized water ( optional; see Table 5 )

3. Drying by air blown at room temperature


4. Immersing some of the samples in a solution of pH 12.5 for 10 and 20 min. ( others sub-

jected for analysis, heat treatment or silane treatment)

5. Repeating step 2) and 3)

6. Analyzing

The brittleness test of the phosphate crystals was carried out for selected samples. The

samples, 1/8 inche, were punched from the sample panels before and after the post-rinse

treatment in solution WC, and after the immersion test for 10 min.

Heat treatment was carried out for the samples treated in a solution WC and WCB at 110

0c and at 180 °c for 24 hours and 30 minutes, respectively. In the first stage of heat treatment.

Heat Treatment I, the treatment steps are i) post-rinse treatment, ii) immersion test, iii) heat

treatment, while in the second stage, Heat Treatment II, the steps ii) and iii) are switched.

After the heat treatment, the samples were cooled down to room temperature in air.

The silane treatment was applied to the samples treated in solution WC, WB and WCB.

The samples were treated in 0.5 percent silane solution for 5 minutes and dried by air blown

at room temperature. After the silane treatment, the samples were aged at room temperature

for 24 hours before further treatment. Some of the silane treated samples were analyzed and

others were immersed in a solution of pH 12.5 for 10 and 20 minutes and rinsed, dried and

analyzed.


3 - 2. Sample Analysis.

3 - 2 - 1. SEMIEDX Analysis.

The samples for the SEM/EDX analysis were prepared by punching a piece , 3/8 inches

in diameter, from the treated sample panels and were carbon coated with Denton Vacuum

Evaporator, DV-502, to avoid charging problem since the zinc phosphate is not a good con-

ductor. SEM, Jeol JAX-840 Scanning Microanalyzer, combined with a Tracoer Northern EDX

analysis sytem, TN-5500, was used at 15 kV. The EDX data was acquired for 60 seconds of

live acquisition time from the area of 1.2 mm x 0.9 mm. The EDX analysis was performed three

times per sample. The spectra obtained were analyzed quantitatively and qualitatively.

3 - 2 - 2. AES/SIMS Analysis.

For a better understanding of mechanism of phosphate stabilization by post-rinse treat-

ment, AES/SIMS as well as SEM/EDX were applied in this research. A Perkin-Elmer PHl-610

Scanning Auger Microprobe was used for AES/SIMS analysis of selected samples, which was

equipped with a quadrupole mass analyzer with a mass range of 1 to 511 amu. A defocused

beam was used for a survey scan of AES at 7kV, and depth profiling was performed with

Ar+ ion sputtering. In SIMS analysis, 1 kV Ar+ ion source was used for the survey scan.


3 - 2 - 3. X-Ray Diffraction Analysis.

The X-ray diffraction analysis was utilized to ascertain the protection of phosphate

coating by the treatment. The diffraction spectra were obtained from the samples which were

immersed in a solution of pH 12.5 for 20 minutes after the post-rinse treatment and/or the

silane treatment. The spectra were obtained by using Cu Ka radiation with a Rigaku-200. The

experimental conditions were 40kV, 80mA, 2 degree per minute of scan rate and time con-

stant of 1.

3 - 2 - 4. Polarization Measurement.

An open "L" shaped plastic cylinder was used for sample mounting where a 0-ring

gasket was placed between the cylinder and the sample surface to avoid leakage of salt sol-

ution. At the open end of the cylinder, 1.54 cm2 of treated sample surface was exposed to an

aerated and stirred 5 percent NaCl solution for one hour before measurements. The anodic

and cathodic polarization curves were obtained with Potentiostat, Princeton Applied Research

Model-173, in a broad potential range at a scan rate of 1mV per second. A saturated calomel

electrode ( S.C.E.) and a platinum electrode were used as a reference and a counter

electrode, respectively. The schematic diagram of polarization measurement setting is shown

in Fig. 4.


w E

R E

c E

Figure 4. Schematic diagram of polarization measurement setting. I = Ampere meter E = Volt meter


Chapter IV. Results and Discussion

The phosphate stability in a highly alkaline solution was improved by the post-rinse

treatment, especially in the solutions containing Ca2+ and Ba2+. Barium added waterglass

solution showed the best result in the prevention of phosphate dissolution in the immersion

test. The silane treatment, applied to post-rinse treated sample, improved the phosphate

stability further. However, the heat treatment did not reduce the dissolution of phosphate

crystals in the immersion test but rather accelerated.

The data from the SEM/EDX and AES/SIMS indicated that thin film had formed by the

post-rinse treatment and the silane treatment. The thin film, which should be a silicate com-

pound film containing zinc and other metallic ions, increased the stability of phosphate due

to the very low solubility and high alkaline resistance. The results from x-ray diffraction and

polarization measurements confirmed that the leaching of phosphate crystals was reduced

and corrosion resistance was improved by the post-rinse treatment as well as by the silane

treatment.

Chapter IV. Results and Discussion 35

4 - 1. SEM/EDX Results.

4 - 1 - 1. Post-rinse Treatment.

The leaching of phosphate ions was observed by SEM/EDX. The relative percentage of

phosphorus in a coating layer decreased rapidly with time in a high pH solution. Fig.5 and 6

show the change of relative percentage of phosphorus in coating layer with time in a solution

of pH 12.0 and 12.5 respectively. Almost all phosphate crystals were leached out in an hour

in a solution of pH 12.0 and in 30 minutes in a solution of pH 12.5, respectively.

Fig. 7 shows the phosphate crystals of as-received sample before any treatment. The

needle-like crystals, ( hopetite, Zn3(P04) 24H20 ), are presented, which are usually produced

by spray application (6). As expected , the dissolution of phosphate crystals was observed in

the solution of pH 12.0 and 12.5 as shown in Fig. 8 and 9, respectively. These micrographs

show that all phosphate crystals disappeared in an hour in a solution of pH 12.0, and in 30

minutes in a solution of pH 12.5. The leaching of phosphate ions is directly related to pH and

length of treatment time. Therefore, the investigation aimed at the prevention of phosphate

dissolution in an alkaline media by non-chromate post-rinse treatment.

The samples treated in a wide molarity range of the waterglass solution of pH 12.0 for

30 minutes showed that the leaching of phosphate ions varied depending on the molarity of

waterglass. Fig. 10 shows the phosphate stabilization in the waterglass solution treatment.

The leaching of phosphate decreased with increasing molarity of waterglass up to the molarity

of 0.0075 and increased at the higher molarity than 0.0075M. The relative percentage of silicon

increased with the waterglass treatment from about 2 percent of as-received sample to as

high as 7.5 percent, but the increasement of silicon did not depend on the molarity of

waterglass in the treatment solution. It is suggested that only certain amount of silicate


20

-~ 0 -0 -0..

10

0 15 30 45 60 Tl ME(mi n)

Figure 5. Leaching of phosphate in a solution of pH 12.0.


30 ,...----------------------

20

-~ ... 0 -a..

10

0 5 10 TIME(min)

Figure 6. Le~ching of phosphate in a solution of pH 12.5.


15 20

38

Figure 7. Phosphate crystals of as-received sample


a

b Figure 8. Phosphate crystals after treatment in a solution of pH 12.0 for 30minutes(a) and one

hour(b).


a

b

Figure 9. Phosphate crystals after treatment in a solution of pH 12.5. for 10(a) and 20 minutes(b).


(silicon) precipitated on the phosphate crystals regardless of the molarity of waterglass in the

solution and silicate content is related to some degree to the stabilization of phosphate coat-

ing in a high pH solution.

The waterglass solution (0.005 M) was chosen for the later post-rinse treatments since

the result obtained at this molarity was almost same as that obtained at the molarity of 0.01

and 0.0075. The degree of destruction of phosphate crystals in the waterglass treatment

showed the same trend as that of the leaching of phosphate ions in the wide molarity range

of waterglass solution.

The relative percentage of phosphorus of the sample treated in a solution WG, 0.005 M

waterglass solution of pH 12.0, for 30 minutes was about 22 percent and that of as received

sample immersed in a solution of pH 12.0 for 30 minutes was about 7 percent, while that of

as-received sample without any treatment was around 28 percent. Fig. 11-a shows the

morphology of the sample treated in a solution WG for 30 minutes.

The dissolution of phosphate crystals by the post-rinse treatment in a solution WG for

30 minutes was much less than that by immersion in a solution of pH 12.0 for the same time

duration. The addition of waterglass reduced the dissolution of phosphate crystals. The

waterglass treatment improved the stabilization of phosphate coating in a solution of pH 12.5,

especially when the concentration of waterglass was optimum. One of the graphs in Fig. 12

shows the change of silicon content by the treatment in solution WG followed by the

immersion test. The relative percentage of silicon increased significantly with the waterglass

treatment. It suggests that the increasing silicon content improved the stabilization of

phosphate ions in a high pH solution. The possible mechanism of this is a formation of a thin

film of silicate or silicate compound with zinc and/or phosphate ions in solution. Due to the

higher alkaline resistance of the thin film and hence, the reduced phosphate dissolution, better

corrosion protection would be obtained.


:E ~(Jl f"T1 :::a G> r l> CJ) CJ) -3· 0 (1)

~o 0.

c;I -

(Jl

P ( Q f 0/o)

0

0

(JJ 0

Figure 10. Effect of waterglass on the stabilization of phosphate in a solution of pH 12.0 for 30 minutes.


a

b Figure 11. Dissolution of phosphate crystals after treatment in solution WG (a) and WZ (b) for 30

min.


Si ( 0 f 0/o) 0 N (>J

l> ::u

~ G')

N CJ) :: : :.:::::·1----

() z ::u () I I II 111 z CD z

I I II Ill

~ D () '1) '1) -0 3: CJ)

:I: :I: 0 CJ)

N N ~ () z CJ)

() CD

. 01 01 :::0 N z 0 0 CJ)

~ ~ rii 11111 11 11 I z

Figure 12. Change of silicon content by the post-rinse treatment and followed by immersion test for 10 and 20 min.


Even though the phosphate stabilization was improved dramatically by the treatment in

solution WG for 30 minutes, this treatment improved the phosphate stabilization slightly in the

immersion test for 10 minutes but not for 20 minutes compared to the result from the as-

received sample. The results of the immersion test of the samples treated in solution WG are

shown in Fig. 13. The phosphate crystals of sample treated in solution WG totally disappeared

in 20 minutes in the immersion test. The immersion test shows that the post rinse treatment

in solution WG is not effective enough to protect the phosphate coating in a high pH solution.

On the other hand, the silicon content decreased slightly by the immersion test of the

samples treated in solution WG but was still high as shown in Fig. 12. It indicates that the

post-rinse treatment in waterglass solution had a limited effect on phosphate stabilization in

a high pH solution and that the leachability of phosphate ions in a high pH solution is different

from that of silicon.

Fig. 14 shows the results of treatment in a solution of varied molarity of zinc sulfate

added 0.005 M waterglass solution at pH 12.0 for 30 minutes. The relative percentage of

phosphorus decreased slightly with the increasing zinc sulfate molarity. However, Fig. 11-b

shows that the phosphate crystals of samples treated in a solution WZ for 30 minutes were

destroyed less than those treated in solution WG for the same time. Even though the relative

percentage of phosphorus decreased with the increasing zinc sulfate molarity, the phosphate

crystals did not follow the same trend as that shown in the waterglass treatment. It is as-

sumed that the zinc ions in the solution adhered to the surface of the sample, which decreased

the relative intensity of phosphorus.

The results of the immersion test of the samples, treated in solution WZ for 30 minutes,

are shown in Fig. 13. Even though the relative percentage of phosphorus decreased, the

treatment slightly improved the phosphate stabilization in the immersion test compared to the

treatment in solution WG. After the immersion test for 20 minutes, the relative percentage of


-~ 0 +-0 -a..

20

10

0 5

-0- ·AR -0- WG -0- WG+ZS -ts- WG+CN(R) -0- WG+CN

10 TIME(min)

15 20

Figure 13. Effect of post-rinse treatment on the phosphate stabilization in the immersion test.


()1

N -z n en c-ro ,, l> -I rr1 -3 0 (1)

x 0 ~ -

()1

N 0

N ()1

P (at 0/o)

0 N 0

0

Figure 14. Effect of zinc sulfate on the stabilization of phosphate in solution WZ.


(JJ 0

48

phosphorus was about 1 percent; however, all the crystals disappeared. The post-rinse treat-

ment in solution WZ did not have a positive effect on the stabilization of phosphate coating in

a solution of pH 12.5.

After the treatment in solution WZ for 30 minutes, almost the same amount of silicon was

detected as that in the sample treated in solution WG as shown in Fig. 12. However, the silicon

content decreased significantly in the immersion test for 20 minutes but not less than 1 per-

cent. The improvement of phosphate stability might be explained by the same hypothesis as

that used in the waterglass solution treatment; the formation of a thin film of silicate or silicate

compound. The addition of zinc to waterglass solution increased the phosphate stability very

little in a high pH solution. The reachability of silicate (silicon) was increased by addition of

zinc in the immersion test.

The phosphate stabilization of samples treated in 0.005 M waterglass + varied molarity

of calcium nitrate solution of pH 12.0 for 30 minutes is presented in Fig.15. The relative per-

centage of phosphorus was greatly improved up to that of the as-received sample, and did

not depend on the molarity of calcium nitrate in a solution. Even a very small amount of

calcium added to waterglass solution of pH 12.0 protected the phosphate completely.

The phosphate crystals of samples treated in solution WC look similar to those in the

as-received sample, shown in fig. 16-a. However, the silicon content was less than that in the

sample treated in solution WC, while the calcium content was increased; the amount of in-

crease in the calcium content is much less than the amount of decrease in silicon. Calcium

together with silicate is very effective in improving the phosphate stability in a high pH sol-

ution. It is suggested that a thin film formed by calcium added waterglass solution treatment

is much more effective in the phosphate stabilization than that formed by waterglass or zinc

added waterglass solution treatments. The added calcium seems to decrease the solubility


0

nCJl )> r ()

c 3:: -~o -i ::0 l> ~ m -3 01 0 CD )C

~N 0

0

P (at 0/o) N 0

Figure 15. Effect of calcium on the phosphate stabilization


0

(>J 0

~ 0

50

of thin film and increase the alkaline resistance ;hence, increase the stability of phosphate

ions in a high pH solution.

As described in the experimental section, two different options were applied to the

samples treated in solution WC; deionized water rinse and no rinse. The observed differences

between the deionized water rinsed sample and the non-rinsed sample after the post-rinse

treatment in solution WC were the relative percentages of silicon and calcium, and the degree

of the stabilization of phosphate ions in the immersion test. The less relative percentages of

phosphorus and silicon were detected with deionized water rinse, therefore, the less stability

of phosphate ions in the immersion test.

The post-rinse treatment in solution WC is much more effective in the stabilization of

phosphate in the immersion test than the treatments in solution WG or WZ, which is shown in

Fig. 13. It is also shown that rinsing with deionized water gave poorer protection. Even after

20 minutes in the immersion test, the non-rinsed sample showed about 5 percent of

phosphorus while the rinsed one showed only about 0.5 percent. The addition of calcium to the

waterglass solution improved the phosphate stability even in the immersion test for 20 min-

utes.

The leaching of the phosphate crystals in the immersion test of the sample treated in

solution WC for 30 minutes (non-rinsed) is shown in Fig. 17. The non-rinsed sample showed

some crystals even after the 20 minute immersion test, while the rinsed one did not show any

crystals (micrographs are not included, see Fig. 9-b). This is due to the increased resistance

of the thin film to alkali by addition of calcium and much less protection is obtained by

deioninzed water rinse which removed added elements partly on the surface of phosphate

coatings.


a

b Figure 16. Morphology change by the treatment in solution WC (a) and WB (b) at pH 12.0 for 30

min.


.a

b Figure 17. Dissolution of phosphate crystals In the Immersion test for 10(a) and 20 min.(b) after

treatment in solution WC.


As shown in Fig. 12, the silicon content of the non-rinsed sample decreased slightly while

that of the rinsed sample decreased significantly by the immersion test. This also indicates

that the silicon content is related to the stabilization of phosphate in a high pH solution. The

relative percentage of silicon of the non-rinsed sample treated in solution WC is less than that

of samples treated in solution WG and WZ. It could be due to the added calcium which de-

creased the reactivity of silicate in solution to the phosphate coating.

Solution WB was made by substituting calcium with the stoichiometric amount of barium

nitrate in solution WC. All samples treated in solution WB were dried by air blown at room

temperature without deionized water rinse. The relative percentage of phosphorus detected

by the EDX analysis is similar to that from the sample treated in solution WC, which is almost

the same as that of the as-received sample. The treatment in barium added waterglass sol-

ution (WB) completely prevented the dissolution of phosphate crystals in a solution of pH 12.0,

just as in the calcium added waterglass treatment. In the immersion test, however, the barium

added waterglass solution treatment showed the best protection of the phosphate coating than

any other treatment, as presented in Fig 18. The micrographs of the smples treated in solution

WB followed by the immersion test are shown in Fig. 19. Even after 20 minutes of immersion,

fairly good phosphate crystals were observed and about 13 percent of phosphorus was de-

tected by EDX analysis.

The silicon content after treatment in solution WB was almost the same as that after

treatment in solution WC, and was decreased slightly by the immersion test, as shown in Fig.

12. The decreased dissolution of silicate(silicon) from the phosphate coatings in the

immersion test seems to arise from the added barium and can be related to the phosphate

stability. The results of the post-rinse treatments in solutions WCB, WCM, and WCN ·followed

by the immersion test are also shown in Fig. 18. These post-rinse treatments did not show

any difference among themselves in the relative percentage of phosphorus, which was same

as that of the as-received sample, consequently, they prevented the dissolution of phosphate


-~ 0 ... 0 -CL

20

10

0

-0- WG+BN -6- WG+CN+NS -<)- WG +CN +MS -0- WG+CN+BN

5 10 15 20 TI ME(m in)

Figure 18. Effect of the post-rinse treatments in solution WB, WCN,WCM and WCB on the phosphate stabilization in the immersion test.


a

b Figure 19. Dissolution of phosphate crystals of samples treated in solution WB in the immersion

test for 10 (a) and 20 min.(b).


crystals completely. However, The immersion test showed significant differences in terms of

the relative percentage of phosphorus and the phosphate crystals.

The samples treated in solutions WCM and WCN showed poorer results than those

treated in solution WC in the immersion test. However, the samples treated in solution WCB

showed the same result as those treated in solution WC in the immersion test. Even though

the separate use of calcium and barium improved the stability of phosphate coating in the

immersion test, as discussed before, the improvement from the mixture of these did not ex-

ceed that obtained by the use of barium nitrate alone. This suggests that barium nitrate could

not make insoluble film if calcium nitrate is dissolved in the same solution or vice versa.

These results also indicate that magnesium sulfate and nickel sulfate had a negative effect

on the stabilization of phosphate in a high pH solution.

The analytical data by SEM/EDX reveal that the best post rinse treatment carried in this

research is the treatment in solution WB, 0.005M waterglass + 0.005M barium nitrate solution

of pH 12.0. The possible mechanism to explain this result is the formation of silicate com-

pound film on the phosphate coating by the post-rinse treatment. The improved stability of

phosphate coating in a high pH solution is due to the higher resistance of film to alkaline

solution and the sealing effect of the film.

The different results from the different treatments are related to the nature and the re-

activity of the chemicals. Some chemicals, such as barium, make a virtually soluble film with

silicate, which has higher alkaline resistance, while others such as nickel do not. One thing

to be pointed out here is that nickel and magnesiulTI, which are well known corrosion

inhibitors like calcium, did not improve the stability of phosphate coating in a high pH solution.


4 - 1 - 2. Variability and Brittleness.

During the experiment, it was found that the relative percentage of phosphorus varied

from one sample to another. Therefore, the analysis with SEM/EDX was carried out on ten

randomly selected sample panels. The samples were treated in solution WC and immersion

tested for 10 minutes. The analysis was carried out at all stages of the treatments.

In the study of as received samples, the relative percentage of phosphorus varied from

25.68 to 30.26. Other statistical data are listed in Table 6. The analytical data show that the

percentage of phosphorus was high whenever those of nickel and silicon were high. Among

them, silicon was more directly related to the phosphorus content. Fig. 20 shows the vari-

ability of phosphorus, silicon and nickel of as-received sample. The relationships between

these elements are shown in Fig. 21 where a slight relationship exists between phosphorus

and silicon, and nickel.

The relative percentage of elements in the phosphate coating was changed by the

post-rinse treatment in solution WC for 30 minutes, which is shown in Fig.22. After the post-

rinse treatment, the relative percentage of phosphorus and nickel was unchanged, but the

relative percentage of silicon and calcium increased.

The results of the immersion test for 10 minutes after the post-rinse treatment in solution

WC for 30 minutes are shown in Fig. 23. The relative percentage of phosphorus and silicon

decreased by more than 50 percent based on the average value. However, that of nickel, less

than 1 percent, did not decrease. As a result, it is believed that not only phosphorus but also

silicon was leached out in the immersion test. The same relationships among phosphorus,

silicon and nickel exist after treatment in solution WC as those in the as-received samples.


Table 6. Average and standard deviation value of the variability treatment.

As-received

p Si Ni Ca

AVG. 27.61 1.75 0.80 0.17

S.D. 1.58 0.11 0.10 0.02

Waterglass +Calcium nitrate

p Si Ni Ca

AVG. 28.01 2.21 0.75 0.36

S.D. 1.37 0.43 0.18 0.12

After immersion test for 10 min.

p Si Ni Ca

AVG. 9.61 1.08 0.53 1.09

S.D. 3.04 0.18 0.05 0.04


J\)

(>I CJ)

~~ -c r 01 fT1

z cm ~ CD -J fT1 :::0

CD

a

0 0

0

P (at 0/o) N 0

(>J 0

J\) (>J

S i a N i { O t 0/o )

Figure 20. Variability of P, SI and Ni In as-received sample.


~ 0

~~9 zcn-c -· -·

60

S i a N i ( a t 0/o)

-c -~ 0 -

t> ~~ z Cf) -· -·

Figure 21. Relationship between P, Si and NI in as-received sample.


0

N

(>J

CJ) l>~ ~ -0 r 01 f'T'1

Zm c ~ CD -J rT'1 :0 CX>

0

0

0

P (at 0/o) N 0

(>J 0

N (>J

Si a Ni (at 0/o}

~ 0

~~9 ~~-0

Figure 22. Effect of treatment on variability: Variability of P, Si and Ni after treatment in a sol-ution WC for 30 min. at pH 12.0.


0

0

P(at 0/o)

0

N (>I

Si Ni (at 0/o}

N 0

Figure 23. Effect of Immersion test on variability: Variability of P, Si and Ni after treatment in a solution WCB followed by the immersion test for 10 min.


The phosphate stability in a high pH solution is, in some degree, related to the content

of silicate(silicon) and calcium in the coating layers. Due to the limited detectability of EDX,

which is only a semi-quantitative analysis technique, it is difficult to give full credit to the re-

sults, especially on the content of nickel and calcium which are less than 2 percent.

When the samples were analyzed with SEM, some cracks were found on the edges of

the sample. In Fig. 24, the cracks in the phosphate crystals of as-received sample is shown.

Since the samples were hole punched to be analyzed in the SEM, the cracks in the phosphate

crystals were formed by the punching. The formation of cracks indicated that the phosphate

crystals were brittle. The number of and length of crack per unit area were decreased slightly

by the post-rinse treatment in solution WC, 0.005 M waterglass + 0.005 M calcium nitrate

solution of pH 12.0. This is probably due to the post-rinse treatment which decreased the

brittleness of phosphate coatings.

Since cracks increase the surface area of phosphate crystals and provide more chances

for the solution to access the steel surface, it should have a negative effect on the stabilization

of phosphate in a high pH solution. Therefore, more investigation is needed to understand the

mechanism.

4 - 1 - 3. Silane Treatment.

The silane treatment was applied to the samples treated in solutions WC, WB and WCB.

The analytical data from EDX did not show any change with the silane treatment in the relative

intensity of the elements present except that of silicon which increased with the silane treat-

ment as expected. However, the increased silicon content by the silane treatment is less than

1.5 percent.


Figure 24. . Cracks formed by the punching.


5

4

-3 ~ 0

..... 0 -·- 2 (/)

0

I I POST RINSE

~ SI LANE I: : : I pH 12.5 ICM ~ pH 12.5 20M

WG+CN WG+CN+BN WG+BN

Figure 25. Change of silicon content by the silane treatment and followed by the Immersion test.


The change of the relative percentage of silicon by the silane treatment followed by the

immersion test is presented in Fig. 25. The relative percentage of silicon increased slightly

with the silane treatment. The highest increase was obtained with the sample treated in sol-

ution WB. There should be a relationship between the amount of silane precipitated on the

phosphate crystals and the elements present on the phosphate crystals. The immersion test

of the silane treated samples decreased slightly the content of silicon. Compared to the results

from the post-rinse treatments in solution WC, WB and WCB followed by immersion test, the

stability of film formed by silane treatment is generally higher than that formed by the post

rinse treatments. The treatment in solution WB followed by silane treatment showed the

highest silicon content in the immersion test for 20 minutes. It is assumed that a thin silane

layer was formed on the surface of the sample by the silane treatment, actually on the thin film

of silicate compound coating according to the hypothesis, and that the thin film containing

barium had a higher reactivity to silane than any other elements used in this experiment since

the dissolution of silane decreased in the immersion test when only barium was added.

The leaching of phosphate ions in the immersion test was reduced significantly by the

silane treatment as shown in Fig. 26. The relative percentage of phosphorus was still more

than 20 percent even after 20 minutes immersion test. The samples treated in solution WB

and WCB showed better protection than those treated in solution WC after the silane treat-

ment. Even though the treatment in solution WCB followed by the silane treatment showed the

less silicon content than that in solution WB followed by the silane treatment, the protection

of the phosphate coating by these treatments was almost the same in the immersion test. The

added barium might produce a highly insoluble film with silane; hence, the samples treated

in solution WCB and WB followed by the silane treatment showed almost the same result in

the immersion test. The film formed by the post-rinse treatment in cooperating with the film

by the silane treatment had a higher alkaline resistance than that by post-rinse treatment by

itself.


-~ 0 .... 0 -

30 r--------r-------------

20

10

0

-0--• • -6--

WG+CN WG+CN~SLN

WG+CN+BN ~sLN WG+BN~SLN

5 10 TIME (min)

15 20

Figure 26. Effect of the silane treatment on the phosphate stabilization In the immersion test.


a

b

Figure 27. Effect of the silane treatment on the dissolution of phosphate crystals in the immersion test for 10 (a) and 20 min.(b)(pretreated in WCB)


The phosphate crystals of samples treated in solution WB followed by the silane treat-

ment and immersion test are shown in Fig 27. After the immersion test for 20 minutes, the

phosphate crystals were slightly destroyed. It indicates that the silane treatment was effective

on the phosphate stabilization in a high pH solution.

4 - 1 - 4. Heat Treatment.

The results from the heat treatment of samples treated in solution WC, O.OOSM

waterglass + O.OOSM calcium nitrate, are shown in Fig. 28. The analytical data with EDX dis-

played that the relative percentage of phosphorus from the Heat Treatment I at 180° C is little

higher than that from at 110° C. The results obtained at both temperatures are similar to that

from the immersion test without the heat treatment. However, the results from the Heat

Treatment II showed different phosphate content and morphology of the phosphate crystals

from the samples without the heat treatment and those from the Heat Treatment I. The rela-

tive percentage of phosphorus decreased by 3 to 4 percent with the Heat Treatment II com-

pared to that without the heat treatment. The samples treated in solution WCB showed the

same trend as those treated in solution WC at 110° C and at 180° C.

The micrographs of samples, treated in a solution WC followed by Heat Treatment II at

110° C for 24 hours, are shown in Fig 29. The phosphate crystals were changed by heat

treatment at 110° C as well as at 180° C. However, the micrographs taken from the Heat

Treatment I did not show any change compared to those taken from the samples immersion

tested only, which are not included here.

The results indicate that the heat treatments at 180° C and at 110° C after the immersion

test did not affect the phosphate coating while the heat treatment before the immersion test


did have a significant effect. It is suggested that the heat treatment had evaporated the water

in the coating layers which might result in the degradation of the phosphate coating. It was

reported (34) that the zinc phosphate coatings heated in the absence of air lose their corrosion

resistance between 150 and 163° C. At these temperatures, about 75 percent of the water of

hydration is lost and it results in a volume decrease of the coating which causes voids and

thereby lowers the corrosion resistance. The loss of water from the zinc phosphate coating

heated in air is 10 to 20 percent higher than the loss on heating in the absence of air. The

change of phosphate crystals by heating, as shown in Fig. 29, should be due to the loss of

water.

The results obtained from this experiment are not consistent with the literature as de-

scribed. The degradation of phosphate coatings by Heat Treatment II at both temperatures

was observed. On the other hand, the Heat-Treatment I at 180° C did not show any degrada-

tion of the phosphate coating. The difference in observation between literature and experiment

is probably due to the post-rinse treatment which decreased the transition temperature of the

phosphate coating but is not certain.

4 - 2. AES/SIMS Results.

The analysis by AES and SIMS was carried out only on selected samples. The survey

spectra of AES were taken from the samples treated in solution WG, WC, WCB and as-received

sample, and depth profiling spectra were taken from the samples treated in solution WG and

WC. The SIMS analysis was performed only on the sample treated in solution WCB before

and after the silane treatment. The AES spectra are shown in Fig. 30 to 37 and those of SIMS

in Fig. 38 and 39, respectively.


30 ------,-----r-----,-----,

20

-~ +-0 -a..

10

0 5

-0---0--• --6-•

10 TIME( min)

WG+CN pH -.1 so0 c 1so0 c~pH pH-+110°C 110°c~pH

15

Figure 28. Effect of heat treatment on the phosphate stability in the immersion test.


20

72

a

b Figure 29. Degradation of phosphate crystals by heat treatment followed by the immersion test

for 10 (a) and 20 min.(b).


In the survey spectrum of as-received sample, as shown in Fig. 30, the data from AES is

fairly consistent with that from EDX. The silicon peak not present in the AES spectrum might

be explained by the sensitivity factor of silicon or by that silicon is not in the outermost surface

layers.

However, the samples treated in solution WG for 30 minutes showed clear silicon peaks

which are shown in Fig. 31, while some other peaks such as calcium and iron disappeared.

It also shows that the peak intensity of phosphorus decreased with the treatment in solution

WG, which was fairly consistent with the EDX results. The increased intensity of the silicon

peak by the waterglass treatment supports the hypothesis of the thin film formation and also

indicates that the thickness of the film is ,less than the probing depth of AES at the exper-

imental condition.

The result from depth profiling of the sample treated in solution WG is shown in Fig. 32.

The silicon content decreased significantly in the first three minute sputtering and decreased

slightly after that. This fact clearly indicates that the silicon content was increased by the

waterglass treatment. It is assumed that silicon might diffuse to some degree to the surface

layer of phosphate coating and/or react with ions in the solution, such as phosphate, zinc or

iron ions, which led to the formation of thin film of silicate compound. Based on the depth

profiling data, the thickness of the film should be few hundred A0

The survey spectra of samples treated in solution WC without and with deionized water

rinse are shown in Fig. 33 and 34, respectively. A much higher silicon peak was recorded from

the sample without deionized water rinse and the calcium peak was also detected. The depth

profiling data of sample treated in solution WC, 0.005 M waterglass + 0.005 M calcium nitrate,

and dried by air blown at room temperature is shown in Fig. 35. The calcium peak increased

sharply until about 1.5 minute sputtering time and decreased sharply, while the silicon peak

showed the opposite trend.


IT1 ~

r IT1 () -i :0 0 CX> z rn z rn :0 -G>f\) -< -Cl) < )(

0 o-- ())

N

0

N :J

11 Cl)

d!/dE ~

z

Figure 30. AES spectrum of as-received sample.


fT1 r o fT1 () -; ::uoo 0 z fT1 z f"Tl-:::oN G)

-< -Cl)

< )( (j) 0 0 -

dl/d E ~

Figure 31. AES spectrum of the sample treated in solution WG for 30 min.


A. C, ( 0/o) p ~ co rv N

(j) 0 -J _..., ~ ~ ~ ~ 0

' ' ' \ I

(JJ I (/) I

""O I c I -i I

-i O'> I rn I ::0 I

I -i I 3: (.!) I rn I - I 3 I :::> I I -

N I I I I (/'I -0 I I

U1 I

Figure 32. AES depth profiling spectrum of the sample treated in solution WG.


0

~ fTI r rn () _, :::0 CX) 0 z fTI z fTl-:::0 N G') -< -ct> < )< -_()')

0 0 -

N 0

0

N ::::>

en

"'O

dI/dE ~

z

Figure 33. AES spectrum of the sample treated In solution WC without water rinse.


~ rn r fT1 (") -i ::a 0 00 z rn z rn ::a -G> I\) ~ -Cl) < x o_ Om - '

dl/dE ~ (j)

0

CJ)

Figure 34. Effect of water rinse on the retention of elements in the coating surface.


A C. (0/o)

°' <.O . . <.O . .

ow <.O <.O -----

en VJ \ \

"U \ c \

I -f I -f mm \

I

::0 I

-f I I - I ~

~<.O I I I 3

I :J - I

I I

f\) I "U () (./)

I I 0 -·

. I

(Jl

Figure 35. Depth profiling result of the sample treated In solution WC for 30 min (no rinse).


dl/dE N ~

~

rr1 r 0 IT1 CD () 0

-i :::0 CX> 0 z

N rr1 :J

z 1"11-:::0 N G) -< -Cl> < x -en ~ 0 0 -

N o--~~~~--~~~-'---~~~~--~---'

Figure 36. AES spectrum of the sample treated in solution WCB.


~ rn r rn () -; :::0 ooo z rn z rn :::0 N G> -< -Cl> < >< --om 0 -

N

dI/dE N ~

0

()--~~~~--~~~----~~~~~.-~-

Figure 37. AES spectrum of the sample treated In solution WCB followed by the silane treatment.


These data indicate that the thin film formed by the treatment in solution WC consists

of two parts; the silicon rich outer layer and the calcium rich inner layer of the film. This is

supported by comparison with the spectra in Fig. 33 and 34, where the sample rinsed with

deionized water gives rise to the less intense peak of silicon than the sample without rinse,

while the peak intensity of calcium in both spectra is alike. The two layer film formation might

be due to the different reactivity of silicon(silicate) and calcium to the phosphate coating.

The spectra of the sample treated in solution WCB and this treated in solution WCB fol-

lowed by the silane treatment are shown in Fig 36 and 37, respectively. A fairly large barium

peak was detected from the sample treated in solution WCB. However, it disappeared by the

silane treatment. According to these results, the treatment in solution WCB increased the peak

intensity of barium compared to the treatment in solution WC, and the silane treatment after

the treatment in solution WCB reduced the intensity of this peak. The reduced intensity of

barium peak by the silane treatment should be the result of silane film formation on the

silicate compound film.

Since AES is a surface analysis techniques with a probing depth of 10 to SO 0A, the

spectra taken by AES indicates that the thin film of silicate(silicon) compound, consisting of

calcium, barium, phosphate, zinc and Iron, was formed by the post-rinse treatment and the

thin layer of silane film was also formed by the silane treatment. Even though the formation

of the thin film by the treatment is well known, the mechanism of the film formation and the

protection of phosphate in a high pH solution need more investigation in detail.

The SIMS spectra of samples treated in solution WCB, and in the silane solution after

treatment in solution WCB are shown in Fig. 38 and 39 respectively. In the spectrum of

sample treated in solution WCB, 0.005 M waterglass + 0.005 M calcium nitrate + O.OOSM

barium nitrate, the peak intensity of silicon and calcium were relatively low and that of barium


was relatively high. This may be due to the different sensitivity factor of these elements since

the data taken by EDX showed almost the same amount of calcium and barium.

In the spectrum of the sample treated in solution WCB followed by the silane treatment,

the intensity of silicon increased while that of barium and calcium decreased compared to that

in the spectrum of the sample treated in solution WCB. Since the probing depth of SIMS is

usually 3 to 10 A0 , the silane layer should be thicker than 10 A0 because the barium peak was

not detected in SIMS analysis of the silane treated sample.

Based on the data taken by SEM/EDX and AES/SIMS, which are in mutual agreement,

the formation of the thin film by the post-rinse treatment and the silane treatment is confirmed.

The assumed thickness of films are few hundred A0 by the post-rinse treatment and several

A0 by the silane treatment.

It is generally accepted that a coherent oxide film on the coating surface was formed by

the chromate post-rinse treatment, which provides the passivation. The increased stabiliza-

tion of phosphate coating in this experiment could be explained by the same theory. The thin

film formed by the post-rinse and silane treatment provides a passivation as well as high alkali

resistance. Based on the AES/SIMS results, it is proposed that the film formed by the post-

rinse treatment in solution WB consists of Ba, Zn and Si03 • As a result, the proposed model

of film is the compound of those elements; (Ba, Zn)Si03-Zn(OH), which has very low solubility

in alkaline solution and hence, protects the phosphate coatings from the alkaline attack.


6

en

ci 4

(j 2

6

cri

.4 a..

(.)

2

0

H+ Na+

0 20

100 120

K+

ca+

+ + K+C3H5

+ Si(C~)

40 60 A. M. U.

+ + BaO BaOH

\ ,/

140 160 A. MU.

80 100

180 200

Figure 38. SIMS survey spectra of the sample treated In solution WCB for 30 min.


6 H+

en

a.: 4

u 2

0

6

cL 4

2

0 100

~+

CH+ 3

20

liO

Si+

SiH+ + + C3H5+K

+ K fiCH:

Si OH Fe+

+ SiH

40 60 A. M U.

120 A. M.

130 u.

80 100

140 150

Figure 39. SIMS spectrum of the sample treated in solution WCB followed by the silane treat-ment.


4 - 3. X-ray Diffraction Analysis Results.

X-ray diffraction spectra are shown in Fig. 40. Many zinc phosphate and zinc peaks were

recorded in the spectrum of the as-received sample. However, most zinc phosphate peaks

disappeared in the spectrum of the sample immersion tested for 20 minutes, while the peak

intensity of zinc increased ( at 20 = 36°). After the treatment in solution of WZ, WC and WCB

followed by the immersion test, the intensity of zinc phosphate peaks increased from treat-

ment in WZ to WCB while that of zinc peaks decreased. The silane treatment followed by the

immersion test increased the intensity of zinc phosphate peaks more than any other treat-

ments did, as expected according to SEM/EDX and AES/SIMS results.

Therefore, it is corroborated again that the post-rinse treatments carried out in this re-

search retarded the leaching of phosphate ions in a solution of pH 12.5. The treatment in sol-

ution WCB showed the best result, which was further improved by the silane treatment. Even

though the EDX analysis revealed that there is more than 20 percent of phosphorus after the

immersion test, the peak intensity in the XRD spectrum is low. This might be resulting from

the reduced X-ray intensity by the film on the surface, which was formed by the post-rinse or

silane treatment.

4 - 4. Polarization Measurement Results.

The cathodic polarization curves of samples treated in solution WG, WZ, WC, WB, and

in the silane solution after treatment in solution WB, and as received sample are shown in Fig.

41. Even though the anodic polarization curves, which are not shown in this report, were alike,

the cathodic polarization curves were different from one another depending on the treatment.


INTENSITY

I\) f\) (>J

CD -0.. (1)

cO .., Cl> (1) -

f\) ~

(>J

VJ CX>

Figure 40. X-ray diffraction spectra of the samples after post-rinse treatment followed by the Immersion test for 20 min. 1) as-received(no treatment). 2) 0.005M waterglass treated, and immersion test. 3) 0.005M waterglass and 0.01M zinc sulfate, and immersion test. 4) 0.005M of waterglass and calcium nitreate, and immersion test. 5) 0.005M of waterglass, calcium nitrate and barium nitrate solution treated and immersion tested. 6) silane treatment of #5.


The lowest dissolution phosphate coating in a pH 12.5 solution was obtained by the

treatment in solution WB followed by the silane treatment, which was slightly better than that

obtained by the treatment in solution WB only. The improvement by the silane treatment was

noticeable but not to a great extent in the polarization measurement even though the stability

of phosphate crystals was greatly improved. The results from the other post-rinse treatments

listed in the order of descending improvement are the samples treated in solution WC, WZ,

WG and the as-received sample.

Since the phosphate stabilization is directly related to the corrosion protection, it is as-

sured that there should be better protection after the post-rinse treatments according to the

data revealed by SEM/EDX and AES/SIMS. The polarization measurements confirmed that the

post-rinse treatments improved the corrosion protection by depressing the reaction of oxygen

depolarization.


POTENTIAL vs S. C. E. ( mV x -100) ~ N 0 CD

r 0 G')

("") c :::0 . :::0 ~ f"Tl z --i 0 f"Tl z c.n ---i -< -:J (J1 l> ~ 3 "' -

Figure 41. Cathodic polarization curves of post-rinse treated and/or silane treated samples and as-received smple. 1). as-received. 2). O.OOSM waterglass 3). O.OOSM waterglass and 0.01M zinc sulfate. 4). O.OOSM waterglass and calcium nitrate. 5). 0.005M waterglass and barium nitrate. 6) .. silane treatment of #5.


Chapter V. Summary and Conclusions

The dissolution of the phosphate crystals in a highly alkaline environment, which is

mainly caused by the corrosion reaction under the organic coating, has a negative effect on

the corrosion resistance and paint adhesion. This problem is wide-spread in industry, and

many investigations have been carried out in order to understand the mechanism of dissol-

ution and to minimize this effect. One of widely utilized methods is post-rinse treatment with

a solution which contains metallic or non-metallic ions. Due to the environmental problems

generated by the use of chromate ions, which showed great improvement of corrosion pro-

tection in a highly alkaline solution, the chromate post-rinse treatment should be replaced by

the non-chromate post-rinse treatment.

The objective of this research was the development of a non-chromate post-rinse treat-

ment with equal or better corrosion protection than the chromate post-rinse treatment. The

basic consideration was that zinc phosphate could be exposed to an alkaline solution con-

taining certain additives which would precipitate onto the phosphate crystals and arrest fur-

ther attack of the crystals. Silicate (sodium silicate) was extensively used due to low cost of

Chapter V. Summary and Conclusions 91

materials. The silicate treatment would produce a silicated and silanized phosphate which

reduced the sensitivity of phosphate coating to alkaline environment and in addition, provide

a highly stabilized phosphate-paint interface against hydrolysis and rehydration. The addition

of other ions such as Ca2+, Ba2+ , Ni2+ and Mg2+ to waterglass solution was utilized.

The analysis of treated samples were carried out by SEM/EDX, and AES/SIMS were uti-

lized to understand the mechanism of prevention of phosphate dissolution in highly alkaline

environment. The additional data by XRD and polarization measurement revealed that the

post-rinse treatment as well as the silane treatment are effective in stabilization of phosphate

coating and in corrosion protection. The observed results are as follow.

1. The post-rinse treatment in a solution of O.OOSM waterglass + O.OOSM barium nitrate

showed the best result in the immersion test, followed by the treatment in a solution of

O.OOSM waterglass + O.OOSM calcium nitrate and O.OOSM waterglass + O.OOSM calcium

nitrate + O.OOSM barium nitrate. (by SEM/EDX)

2. A further improvement was obtained by the silane treatment. The samples treated in

solution of O.OOSM waterglass + O.OOSM barium nitrate and of O.OOSM waterglass +

O.OOSM calcium nitrate + O.OOSM barium nitrate showed almost the same result which

were much better than that taken in step 1). (by SEM/EDX)

3. The analytical data from AES/SIMS showed that (a) the thin silicate compound film was

formed by the post-rins treatment, (b) silane is covering the surface of the sample, and

(c) added elements are present in the surface layer, which were removed partly by the

deionized water rinse.

4. The heat treatment both at 180° C and 110° C did not improve the stability of phosphate

but had a negative effect.


5. X-ray diffraction analysis confirmed that the dissolution of phosphate coating was reduced

by the post-rinse treatment and also by the silane treatment.

6. The polarization measurement also confirmed that the post-rinse treatment improved the

corrosion protection. The cathodic polarization( oxygen reduction current) was improved

by the waterglass itself and more with additional ions such as Ca2+ and Ba2 +. The silane

treatment also led to a noticeable improvement in its own right.

From the results obtained in this research, the recommended treatment is that of 0.005M

barium added 0.005M waterglass solution which showed best results. However, it has been

reported that all barium compounds except barium sulfate are toxic. Therefore, the utilization

of barium added solution needs more investigation on the possible health problems. The ex-

perimental results can be accommodated in a simple model which is based on the solubility

parameter of the zinc phosphate in alkali and of films precipitated on to the crystals.

Zn3(P04h-4H2 0 dissolves readily at pH 12.0 and even faster at pH 12.5 under formation

of K3P04 and K2Zn02• In the presence of waterglass, however, a film of zinc compound is de-

posited in situ onto the phosphate. This film has to be dried in air in order to protect the

phosphate from the alkaline attack, e.g. in our test condition at pH 12.5. In the presence of

Ba2 + ions in the waterglass a slightly different film is formed with the tentative composition

(Ba, Zn)Si03-Zn (OH)z based on surface analysis data. This film is of much lower solubility

than ZnSi03, in agreement with zinc chemistry. The film has to be dried , too, in order for it

to form completely. The proposed model of thin film formed by the post-rinse treatment in

solution WB, 0.005fv1 waterglass + 0.005M barium nitrate, is shown in Fig. 42.

The silane treatment seems to lead to a surface compound which includes Zn, based

on surface analysis. Since silane needs OH- ions to react, it is reasonable to assume that it

reacts with the Zn(OH)z component of the surface film, forming a NH2(CH2)JSi(OC2H5h -0-Zn


( Ba, Z n) Si 0 3 - Zn ( OH )2

L~~--

·~····: . .... ;_ :..,-·.: " .... :--·:,-~·-·--·;· .... ·;.-:··--·~,,-~ .. :.-,-: .. '---. =. : · , - • , · · / , : s· T E E L.:.. · , , ·.. ~ .... .:. · , , '\ "··" J • ,. ···-,·.·.· .·,-•.: : --.·' .. ,:"~.,·:.~•,;. .i-.'

, I • • • I, t" • ;, • "' " ' •• • • I• "' r I • _. •: ~· • I"' - "' I r ,.I ,._,, " - • • "' ,, _ • ' • • .. • 1 • • ,. • • •" "' ,- • , • I 1 " " ,, ,

Figure 42. Proposed model of thin film formed by the post-rinse treatment in solution WB.


structure. Upon drying for at least 24 hours, this surface film crosslinks two-dimensionally. In

this treatment, the presence of the inorganic silicate is essential. It reduces the sensitivity of

the phosphate to alkali and also provides reaction sites for the silane.

The analytical data revealed that the treatment modifies a standard zinc phosphate

coating so as to improve its alkali resistance and hence its corrosion resistance. They also

provided a surface that will bond chemically to an electrocoat. This will reduce the sensitivity

of the interface to hydrolysis. In addition, the oxygen reduction sensitivity is less than that of

the standard.

However, more work needs to be done to understand the mechanism of film formation

better and to optimize the treatment for practical applications. The suggested works are as

follows;

1. Extensive testing of the recommended treatment in a variety of corrosion conditions (salt

spray, scab, water soak, cyclic tests, atmospheric tests, etc.)

2. By varying the parameters such as time, temperature and concentrations, the treatment

time should be reduced from 30 minutes to approximately 1-2 minutes without any loss

of performance.

3. Dip and spray versions should be tested on other phosphates and other substrates.

4. The mechanism of the exchange and Si-uptake should be studied in more detail as well

as role of other metallic ions.

5. The studies on the possible health or environmental problems are required in detail be-

fore utilization of this treatment in automotive industries.


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