Formability of Chromate Conversion Coating on Zinc Electroplated Steel Sheet

Formability of chromate conversion coating on zinc electroplated steel sheet

N.H.B. Hamid & E.B. Hamzah

Department of Production and Industrial Engineering, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81300 Skudai, Johor, Malaysia

Email: [email protected]

Abstract

A conversion coating is a coating produced by chemical or electrochemical treatment of a metallic surface that gives a superficial layer. This layer which is actually a compound of the metal gives corrosion protection and guarantee paint adhesion. A chromating liquid for forming the chromate film was prepared. The solution is a mixture of Cr* and Cr^ at a ratio of 1.5 to 2.0, l-20g/l ofHiSO^ 1- 20g/l of HNO], 1-20 ml NaOH and 1-10% Si oil. The solution was applied on a surface of zinc-based plated steel sheet of approximately 0.25 mm thickness and then dried. The chromate film formed on the sheet surface was approximately 0.5-1.0 |Lim in thickness. The steel sheet with chromate film was then analysed and mechanically tested. It was found that the chromate solution has better stability over wide range of pH and the formability of its film is improved whence providing a better corrosion resistivity.

34 Surface Treatment

1 Introduction

Chromate films are chemical conversion coatings. The substrate metal participates in the coating reaction and becomes a component of the coating and it has a profound influence on the properties of the coating. The solutions for chromating are acidic/

The pH of the operating solution may varied over a fairly wide range of acid pH, satisfactorily in the pH range of about 1.1 to about 2.3, but pH 1.6 - 2.1 is more preferable. In the coating operation a number of concurrent chemical reactions occur with changing ionic balance and imbalances of resulting concentration and relative proportion of active ingredients, such as the chromate and sulphate or fluoride ion concentration, which may affect coating ability/

The basic protective function of chromate conversion coating (CCC) is the evolution of chromate ions into a humid environment, which due to their inhibiting properties , protect zinc against corrosion in the damaged areas of CCC/ The course of CCC formation involves oxidation of the metal ions to the solution and evolution of hydrogen. The reaction taking place during chromating are as follows:

Zn + HzSO, -» ZnSC>4+ % 3H: + 2N&,Cr2O7 -> 2Cr(OH)s + 2Na,CrO4 2Cr(OH)s + NazCrO, -> CrfOH^CrO, + 2NaOH

The CCC are formed because the metal surface dissolves to a small extent, causing a pH rise at the surface-liquid interface. This result in the precipitation of a thin complex chromium-metal gel on the surface, composed of hexavalent and trivalent chromium and the coated metal itself. The solubility decreases as the coating loses water and as oxidation proceeds, the solubility reaches an optimum for most purposes after at least two days ageing under warm and dry conditions. Excessive dessication by over exposure to high temperatures leads to total insolubilation and, what is worst, to film cracking/-

The colour and thickness of CCC vary with conditions of chromating, and particularly composition, pH, temperature of bath and time of treatment (immersion). Treatment times, whether by spray or dip, are relatively short, ranging from a few seconds for zinc and to a maximum of five minutes for aluminium. Most solutions operate at or near room temperature/ The most important factor, decisive for the formation of chromate coatings is the pH of the chromating solutions/

Zinc and cadmium electroplates are among the metals commercially chromated. CCC improve corrosion resistance and appearance of metals and adhesion of organic top coat. About a half of the world consumption of zinc (> 4 million tons) is used for making protective coatings/

Surface Treatment 35

The aim of this work is to develop a chromate coating which can withstand severe deformation processes and its solution that is stable over a wide range of pH without affecting its aesthetic, corrosion resistance and mechanical properties.

2 The Procedure

The substrates used were low-carbon steel sheets. They were zinc electroplated in a zinc alkaline bath to a coating of about 10 p,m before finally being chromated. The chromating processes were carried out in three different solutions. Solutions Cl and Cl 1, were a new formula invented by the investigators and the other was Solution L, an established proprietary product. Solution Cl constitute a mixture of Cr^ and Cr ^ at a ratio of 1.5 to 2.0, 1-20 g/1 HjSCX 1-20 g/1 HNO,, 1-20 ml/1 10%NaOH and Cl 1 with the presence of 1-10% of Si oil. The parameters under study were pH, between the range of 0.8 to 2.0, and dipping time, between the range of 5 to 20 seconds with mechanical agitation. This will give rise to chromate film of less than 1 jum. The bath and drying temperatures were at ambient and 53 °C respectively, these were chosen according to normal industrial practices. The specimens were then subjected to salt spray test (ASTM B-117), bend test, adhesion test (BS1706), cross-hatch test and suface topography analysis.

3 Testing

3.1 Salt spray test. To determine the film corrosion resistivity, salt spray test was conducted using a media of 5%NaCl atomised at aflowrat eo f 10ml per 80cm^ per hour. The humidity was kept at 95-100% RH with temperature of 35 °C and pressure at 20 psi. The pH was maintained in the range of 6.9-7.0 for six hours. The weight loss, percentage corroded area and percentage brightness deterioration were measured.

3.2 Bend test. To determine the film formability bend test, was carried out with the smallest standard mandrel diameter of 1mm. The bend cracks were analysed under 100X magnification.

3.3 Adhesion test. The chromate film was subjected to a rubbing action with the use of white paper and the presence of chromate film on it indicate poor adhesion. On top of this the cross-hatch scratch test was also conducted to analyse the degree of adhesion.

3.4 Surface topography. The film surface degree of porosity and smoothness was observed under 200X magnification.

4 Results and Discussion

It was observed that the proprietary product L is not stable below pH value of 1.8. It was found that no chromate film formed on the sheet for pH below 1.8 as indicated in Table L However for solutions Cl and Cll, both show chromating reaction still effectively

36 Surface Treatment

operatable even as low as pH 0.8. The performance for both product films are indicated in Tables 2 and 3 respectively. The corrosion performance as indicated in Figure 1, the weight loss for Cl 1 was the least at pH 2.0. This might be due to the presence of silicon oil. In contrary, the brightness at pH 2.0, Solution Cl gives a better aesthetic value as

indicated in Figure 2. From Tables 1-3 it can be concluded that the best pH range to be treated for dipping time of 5-20 seconds, for all the three solutions is between 1.8 to 2.0.

Bend tests results indicated in Figure 3 shows the nature of crack of Cl, Cl 1 and L film products. All three exhibit ductile fracture of zinc substrate with firm adhesion of

chromate film without any flaking off phenomena. It can be concluded that, Cl 1 product exhibit the best formability property on zinc substrate with the least amount of cracks followed by Cl and L respectively. The cracks that were found on Cll were extremely small and narrow compared to those on Cl and L. This points out that chromate film does affect the ductility of the substrate/ Adhesion of the chromate film for all three

solutions was good and no peeling off phenomena observed after both the rub and cross- hatch scratch tests. Figure 4 shows the degree of surface topography smoothness at pH 2.0. Cl possesses the smoothest surface which explains for its brightness as mentioned above.

5 Conclusions

1. The chromating solutions Cl and Cll produce a good adhesion and formability quality of chromate conversion coating.

2. Solutions Cl and Cll give a broader effective range of operating parameters such as pH from 0.8 to 2.0 and immersion time 5 to 20 seconds.

3. Solution Cl 1 gives the best surface topography and excellent corrosion resistivity.

References

1. Brumer, H, Chromating, Product Finish, 1993, 206-211.

2. Lawrence, R.C, Composition and process for forming improved, non-cracking chromate conversion coatings, Patent Number 5,268,0452, Dec. 1993. 3. Sarmaitis and Rozovsky, Inter. Congress Metal Corr., Vol 1, 390-395, 1984. 4. Beistek and Weber, Electrolytic and Chemical Conversion Coatings, 1976. 5. Farid Hanna, H.. Noguchi and 1. Kotani, Metal Finishing, 17-19, July 1989.

6. Fred,W.E. & Melvin, R.J. Chromate Conversion Coatings, Metal Finishing 61st Guidebook and Directory Issue, Volume 91, No. lA,pp 413-424, Elsevier Science Pub., 1993. 7. Strekalov, P.V. & Panchenko, Y.M. Atmospheric corrosion of Zinc and cadmium chromated coatings on steel in cold, moderate and tropical climate, Protection of a, 1994, Vol 30, No. 5, 448-458

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APPENDICES

Immersion pH Appearance % Corroded % Brightness Weight loss time area deterioration x10-4g 5 0.8 U U U U 1.2 U U U U 100 1.5 0 100 50 1.8 1 2 2 28 2 2 2 2 12 10 0.8 U U U U 1.2 U U U U 1.5 0 100 100 47 1.8 1 2 5 31

2 3 1 30 31 15 0.8 U U U U 1.2 U U U U 1.5 0 100 100 60 1.8 2 10 70 30 2 2 0 5 26 20 0.8 U U U U u 1.2 U U U 1.5 0 100 100 55 1.8 1 0 5 12 2 4 30 70 23

TABLE 1 : SOLUTION L

KEY TO TABLE AND FIGURE

TABLE : FIGURE LEGEND: APPEARANCE 0- VERY BAD SERIES 1 - C1 1-BAD SERIES 2 - C11 2 - MODERATE SERIES 3 - L 3-GOOD 4-VERYGOOD U - UNSTABLE (NOCCCFILM)

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immersion PH Appearance % Corroded area % Brightness I Weight loss tlme(sec) deteriotation x10-4g 5 0.8 1 5 30 | 16 "" 1.2 ^~ 2 10 30 j 28 1.5 4 0 ! 15 ] 27 1 1.8 3 5 5 I 31 2 2 0 ! 5 1 34 10 0.8 1 5 1 10 i 35 1.2 3 5 ! 20 i 21 1.5 3 0 i 10 ! 25 • 1.8 3 0 5 ! 46 2 2 0 Q 37 I— ^ — i 0.8 1 0 1 10 ! 24 1.2 3 0 i 10 I 21 1.5 4 20 20 i 21 1.8 3 0 5 28 2 3 2 0 ! 34 20 0.8 1 0 ! 50 22 1.2 2 0 \ 10 I 29 1.5 4 5 20 ! 39 1.8 4 30 40 I 35 2 3 0 0 27

TABLE 2 : SOLUTION C1

Immersion pH Appearance % Corroded area % Brightness Weightless time(sec) : deteriotation x 1 Q"*g 5 0.8 1 0 10 42 1.2 3 15 95 45 l_ 1.5 ^ 4 0 10 ; 23 1.8 3 0 2 i 17 3 2 0 2 i 17 10 0.8 1 0 5 32 1.2 3 5 70 24 1.5 4 10 90 27 1.8 4 0 5 | 17 2 4 0 2 I 12 5 15 0.8 1 60 ! 22 L '2 3 5 80 57 1.5 4 20 85 25 1,8 4 0 90 I 51 2 4 0 50 i 25 20 0.8 1 2 10 I 44 1.2 3 5 70 1 29 ' 1.5 j_ 4 50 90 1 20 1.8 3 5 10 i 41 2 4 20 70 I 21 ! TAI3LE 3: SOLUTION 011 i

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Figure 3 : Bend Test Crack (100X)

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Figure 4 : Surface topography (200X)