Chromate conversion coating treatments for electrodeposited zinc-iron alloy coatings from an acidic sulphate bath

The composition, microstructure and morphology of a zinc-iron alloy deposit obtained from an acid sulphate bath have been investigated by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that under the optimised conditions, the bath produced smooth, uniform, bright, fine-grained zinc-iron alloy deposits having the desired 0.4-0.8 wt. % Fe. The zinc-iron alloy deposit was identified as h.c.p.eta - phase and showed an highly preferred orientation. A low concentration chromate treatment has been successfully developed and applied to electrodeposited zinc-iron alloy coatings with 0.4-0.8 wt.% Fe. The corrosion resistance of the chromate conversion coatings on zinc-iron alloy deposits has been assessed using the neutral spray tests and electrochemical measurements. The results indicated that the corrosion resistance of the zinc-iron alloy deposit was significantly improved and strongly depended on iron content of the alloy deposit after chromate passivation treatment. The composition depth-profile of the chromate conversion coating was measured by sequential use of X-ray photoelectron spectroscopy (XPS) and sputter-etching. It was found that the major constituents of the chromate conversion coating were zinc, chromium and oxygen, and a small amount of iron was also detected According to analysis of the XPS data, the chromate conversion coating on the zinc-iron alloy deposit is mainly composed of oxides and hydroxides of zinc and chromium. SEM investigation indicated that the surface morphology of the chromate conversion coating was characterised by some 'dried riverbed' microcracks.