Surface Technology, 7 (1978) 361 - 365 361 © Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands

THE INTERFACE IN Al-Zn ALLOY COATING OVER MILD STEEL

SANDIP ROY, V. SIVAN, V. BALASUBRAMANYAN and B. A. SHENOI

Central Electrochemical Research Institute, Karaikudi-6 (India)

(Received February 13, 1978}

Summary

The interface of an A1-Zn alloy coated steel specimen has been analysed by optical microscope and scanning electron microscope studies. The composit ion of the interfacial layer has been established by electron probe microanalysis and X-ray wavelength dispersion analysis.

It is well known that in the formation of metallic coatings by hot-dip processes intermediate alloy layer or layers are formed between the coating and the base material. The nature and number of alloy layers depend, apart f rom temperature, immersion time etc., on the substrate and coated metal combination. In galvanizing, for example, three layers are observed between the coated material and the base material [1, 2] . It is generally agreed that the presence of a thin 3 - 5 gm ductile layer of FeZn 7 next to steel is responsible for the adhesion of zinc to the steel base. In the case of aluminiz- ing, the interface is a single layer of Fe-A1 constituents containing about 45% Fe [3 - 5] .

The present s tudy has been taken up to ascertain whether or not an intermediate layer is present between the steel and A1-Zn coating and, if so, what are the nature, thickness and number of layers. Such a study is impor- tant because the behaviour of the coating on deformation depends on the thickness and nature of the intermediate layer(s).

Experimental

Steel specimens of about 20 gauge (2 in X 1 in) having chemical compo- sitions as shown in Table 1 were cleaned in an alkaline solution containing 10% NaOH and 5% Na2CO 3, washed, pickled in hydrochloric acid at about 60 °C, washed and fluxed for 60 s in a solution containing NH4C1, ZnC12 and ZnO. The fluxed specimens were then dried and preheated by holding over the molten metal bath for 5 s, dipped in a molten alloy bath (composit ion A1

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Fig. 1. Back sca t t e red e lec t ron image of a cross sec t ion of A1-Zn alloy coa t ed mi ld steel (1056 x ).

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Fig. 2. Optical p h o t o m i c r o g r a p h of a cross sec t ion of A l - Z n alloy coa ted mild steel (330 X ).

T A B L E 1

Chemical analysis of steel spec imens

S. No. Carbon Sil icon Su lphur Phosphorus Manganese (%) (%) (%) (%) (%)

1 0 .055 0 .010 0 .015 0 .015 0 .318 2 0 .036 0 .011 0 .011 0 .009 0 .312 3 0 .164 0 .007 0 .036 0 .039 0 .521 4 0 .166 0 .089 0 .023 0 .022 0 .530 5 0 .041 0 .009 0 .012 0 .011 0.311 6 0 .040 Trace 0 .013 0 .002 0 .314

Balance, iron.

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364

TABLE 2

Element/Spectral line Analysing crystal Detector

Fe K~ (1.9374 A) LiF Sealed counter A1 Ka (1.2856 A) Mica Gas flow counter ZnKa (1.4363 A) LiF Sealed

59%, Zn 39.3% and Si 1.5%) for 10 s, slowly removed and cooled in a blast of air [6] .

Cross sections of the specimens were polished with emery using kerosine as lubricant and were finally cloth polished with magnesia powder. The specimens were etched in cold 5% NaOH solution, cut into small pieces of size 0.5 cm 2 and mounted on the specimen holder on the scanning electron microscope. Figure 1 shows the back-scattered electron microphoto- graph taken in a Siemens autoscan scanning electron microscope (SEM). Figure 2 shows an optical micrograph of the same cross section.

The elemental distribution along the cross section was determined qualitatively by X-ray wavelength dispersion analysis. Dispersion analysis was carried out using the same specimen and different regions of the cross section. KAP crystal monochromators were used as detectors for A1 K~ radiation, and LiF crystal monochromators were used as detectors for Fe K~ and Zn K~ radiation. X-ray images of the interface are shown in Fig. 3.

Electron probe microanalysis of the interface was carried out using a Geoscan electron probe microanalyser. The operating voltage was 20 eV and the beam current was 30 pA. The de tec to r~rys ta l scheme shown in Table 2 was chosen for analysis of the major elements in the present study.

In electron probe microanalysis the chemical composit ion of a known compound can be estimated by taking the ratio of the intensities of the characteristic X-ray radiations from the specimen and the standard (spectro- scopically pure A1, Zn and Fe were used as standards in the present study). However, to obtain true chemical compositions, correction factors for absorption, fluorescence etc. have to be applied. The correction factors were applied using a procedure suggested by Khera [7] .

Results and discussion

The SEM image indicates the presence of three layers: the iron base, the interface and the A1-Zn coating. The interface thickness works out at around 6 ~m. However, in contrast to galvanized coatings and in accordance with aluminized coatings, the interface consists of only a single layer. This intermediate layer merges into the outer AI-Zn coating in an irregular boundary. The demarcation into three distinct layers is clearer from the optical micrograph shown in Fig. 2.

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Figure 3 shows the X-ray images of the composite. Figure 3(a) indicates that the iron is present in the interface in large quantities, as is apparent f rom the density of the spots; Fig. 3(b) shows that the quant i ty of alu- minium in the interface is definitely less than that of iron and finally Fig. 3 (c) shows that Zn is present in the interface in very small amounts. The nature of the micrographs remained unchanged for the various regions of the interface. The photographs also indicate that aluminium and zinc have diffused into the mild steel base and that iron has also diffused to the alu- minium-zinc coating. Electron probe microanalysis (EPM) of the interface showed that the percentages of Fe, A1 and Zn were respectively 71.57, 26.58 and 1.85. Analysis of different regions of the interface gave the same reproducible composit ion. This indicates not only that the interface consists of a single layer but that its composit ion is also fixed.

Conclusion

It has been shown that the interface at the A1-Zn/steel junction is a single layer having 71.57% iron, 26.58% aluminium and 1.85% zinc. The composit ion of the layer does not vary along the interface.

Acknowledgment

The authors are grateful to the Physical Metallurgy Division of Bhaba Atomic Research Centre, Bombay, India, for helping in SEM/EPM studies. Thanks are due to Dr. H. V. K. Udupa, Director, CECRI for encouragement during the work.

References

1 John R. Daesen, Galvanizing Handbook, Reinhold, New York, 1946, p. 26. 2 Zinc Development Association (ed.), Proc. 7th Int. Conf. on Hot Dip Galvanizing,

Paris, June 1964, Pergamon Press, London, 1967, p. 187. 3 J. R. Nicholls, Light Met., 27 (1964) 27. 4 K. G. Coburn, Metallurgia, 60 (1959) 17. 5 A. V. Zeerleder, Korros. Metallschutz, 12 (1936) 275. 6 V. Sivan, S. Roy, V. Balasubramanyan and B. Shinoi, Technometrics, 4 (1976). 7 S. K. Khera, BARC (India) Report 696, 1973.