N.H.B. Hamid & E.B. Hamzah - WIT Press...Formability of chromate conversion coating on zinc...

Formability of chromate conversion coating on

zinc electroplated steel sheet

N.H.B. Hamid & E.B. HamzahDepartment of Production and Industrial Engineering,Faculty of Mechanical Engineering, Universiti Teknologi Malaysia,81300 Skudai, Johor, MalaysiaEmail: [email protected]

Abstract

A conversion coating is a coating produced by chemical or electrochemicaltreatment of a metallic surface that gives a superficial layer. This layer which isactually a compound of the metal gives corrosion protection and guarantee paintadhesion. A chromating liquid for forming the chromate film was prepared. Thesolution 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 asurface of zinc-based plated steel sheet of approximately 0.25 mm thickness andthen dried. The chromate film formed on the sheet surface was approximately0.5-1.0 |Lim in thickness. The steel sheet with chromate film was then analysedand mechanically tested. It was found that the chromate solution has betterstability over wide range of pH and the formability of its film is improvedwhence providing a better corrosion resistivity.

Transactions on Engineering Sciences vol 25, © 1999 WIT Press, www.witpress.com, ISSN 1743-3533

34 Surface Treatment

1 Introduction

Chromate films are chemical conversion coatings. The substrate metal participates in thecoating reaction and becomes a component of the coating and it has a profound influenceon 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 morepreferable. In the coating operation a number of concurrent chemical reactions occurwith changing ionic balance and imbalances of resulting concentration and relativeproportion of active ingredients, such as the chromate and sulphate or fluoride ionconcentration, which may affect coating ability/

The basic protective function of chromate conversion coating (CCC) is the evolution ofchromate ions into a humid environment, which due to their inhibiting properties ,protect zinc against corrosion in the damaged areas of CCC/ The course of CCCformation 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 pHrise at the surface-liquid interface. This result in the precipitation of a thin complexchromium-metal gel on the surface, composed of hexavalent and trivalent chromium andthe coated metal itself. The solubility decreases as the coating loses water and asoxidation proceeds, the solubility reaches an optimum for most purposes after at leasttwo days ageing under warm and dry conditions. Excessive dessication by over exposureto 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 particularlycomposition, pH, temperature of bath and time of treatment (immersion). Treatmenttimes, whether by spray or dip, are relatively short, ranging from a few seconds for zincand to a maximum of five minutes for aluminium. Most solutions operate at or nearroom temperature/ The most important factor, decisive for the formation of chromatecoatings is the pH of the chromating solutions/

Zinc and cadmium electroplates are among the metals commercially chromated. CCCimprove 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 makingprotective coatings/


Surface Treatment 35

The aim of this work is to develop a chromate coating which can withstand severedeformation processes and its solution that is stable over a wide range of pH withoutaffecting its aesthetic, corrosion resistance and mechanical properties.

2 The Procedure

The substrates used were low-carbon steel sheets. They were zinc electroplated in a zincalkaline bath to a coating of about 10 p,m before finally being chromated. Thechromating processes were carried out in three different solutions. Solutions Cl andCl 1, were a new formula invented by the investigators and the other was Solution L, anestablished proprietary product. Solution Cl constitute a mixture of Cr^ and Cr at aratio 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 thepresence of 1-10% of Si oil. The parameters under study were pH, between the range of0.8 to 2.0, and dipping time, between the range of 5 to 20 seconds with mechanicalagitation. This will give rise to chromate film of less than 1 jum. The bath and dryingtemperatures were at ambient and 53 °C respectively, these were chosen according tonormal industrial practices. The specimens were then subjected to salt spray test (ASTMB-117), bend test, adhesion test (BS1706), cross-hatch test and suface topographyanalysis.

3 Testing

3.1 Salt spray test. To determine the film corrosion resistivity, salt spray test wasconducted using a media of 5%NaCl atomised at a flowrate of 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, percentagecorroded area and percentage brightness deterioration were measured.

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

3.3 Adhesion test. The chromate film was subjected to a rubbing action with the use ofwhite paper and the presence of chromate film on it indicate poor adhesion. On top ofthis 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 wasobserved 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 wasfound that no chromate film formed on the sheet for pH below 1.8 as indicated in TableL However for solutions Cl and Cll, both show chromating reaction still effectively



operatable even as low as pH 0.8. The performance for both product films are indicatedin Tables 2 and 3 respectively. The corrosion performance as indicated in Figure 1, theweight loss for Cl 1 was the least at pH 2.0. This might be due to the presence of siliconoil. In contrary, the brightness at pH 2.0, Solution Cl gives a better aesthetic value asindicated in Figure 2. From Tables 1-3 it can be concluded that the best pH range to betreated 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 filmproducts. All three exhibit ductile fracture of zinc substrate with firm adhesion ofchromate film without any flaking off phenomena. It can be concluded that, Cl 1 productexhibit the best formability property on zinc substrate with the least amount of cracksfollowed by Cl and L respectively. The cracks that were found on Cll were extremelysmall and narrow compared to those on Cl and L. This points out that chromate filmdoes affect the ductility of the substrate/ Adhesion of the chromate film for all threesolutions 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 pH2.0. Cl possesses the smoothest surface which explains for its brightness as mentionedabove.

5 Conclusions

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

2. Solutions Cl and Cll give a broader effective range of operating parameters such aspH 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 SciencePub., 1993.

7. Strekalov, P.V. & Panchenko, Y.M. Atmospheric corrosion of Zinc and cadmiumchromated coatings on steel in cold, moderate and tropical climate, Protection of

a, 1994, Vol 30, No. 5, 448-458



Immersiontime5

10

15

20

pH

0.81.21.51.820.81.21.51.820.81.21.51.820.81.21.51.82

APPENDICES

Appearance

UU012UU013UU022UU014

% CorrodedareaUU10022UU10021UU100100UU100030

TABLE 1 : SOLUTION L

KEY TO TABLE AND FIGURE

TABLE :APPEARANCE0- VERY BAD1-BAD2 - MODERATE3-GOOD4-VERYGOOD

U - UNSTABLE(NOCCCFILM)

% Brightnessdeterioration

UU10022UU100530UU100705Uu100570

FIGURE LEGEND:

SERIES 1 - C1SERIES 2 - C11SERIES 3 - L

Weight lossx10-4g

UU502812UU473131UU603026UU551223



immersiontlme(sec)

5

10

I— ^ — i

20

Immersiontime(sec)

5

10

15

20

PH

0.8"" 1.2

1.51.820.81.21.51.820.81.21.51.820.81.21.51.82

pH

0.81.2

l_ 1.5 1.820.81.21.51.820.8

L '21.51,820.81.2

' 1.51.82

Appearance

1^~ 2

432133321343312443

Appearance

13433134441344413

j_ 434

TAI

% Corroded area

5100 !50 !5 15 !0 i000 10 i20020 !0 \5300

TABLE 2 : SOLUTION C1

% Corroded area:

0150000510005520002550520

3LE 3: SOLUTION 011

% Brightnessdeteriotation303015551020105Q10102050501020400

% Brightnessdeteriotation

10951022570905260808590501070901070

I Weight lossx10-4g

| 16j 28] 27 1

I 311 34i 35i 21! 25 •! 46

37! 24I 21i 21

28! 34

22I 29! 39I 35

27

Weightlessx 1 Q"*g

4245

; 23i 17i 17

322427

| 17I 12! 22

5725

I 51i 25I 441 291 20i 41I 21!i



"o.

<lii

o:Hi

GO

€0COo

3Ml

S3Gl

Hi

UJnH

OCOUJ§

^


% BRIGHTNESS DETERIORATION

233m

03

O

HZm

Dm

Oz<

i§mC/jO

m

3


Cl Cll

Figure 3 : Bend Test Crack (100X)

- ''4

Cl Cll L

Figure 4 : Surface topography (200X)


N.H.B. Hamid & E.B. Hamzah - WIT Press...Formability of chromate conversion coating on zinc...

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