Characterization of black chromate conversion coating on the electrodeposited zinc–iron alloy

The black chromate conversion coating was prepared on zinc–iron (0.58 wt.% ) alloy deposits by immersing in a proprietary chromate bath. A number of surface analytical techniques, such as X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) coupled with microprobe analysis (energy dispersive X-ray analysis: EDXA), atomic absorption spectroscopy, titration and potentiodynamic measurements were used to characterize the chromate conversion coating. X-ray photoelectron spectroscopy measurements confirmed that the major constituents of the chromate conversion coating were Zn, Cr and O, and a small amount of Cu, Ag, S and Fe was also detected. The chromate conversion coating showed a two-layers structure. The outer layer (the major part of the coating) consisted of Cr2O3, Cr(OH)3, Cr(OH)CrO4, Zn2(OH)2CrO4 and a small amount of absorbed H2O. The inner layer was a transition region where the content of metallic zinc increased and that of metallic chromium and oxygen decreased until a constant composition was reached. Fe was incorporated into the chromate conversion coating and located in the inner layer (adjacent to zinc–iron alloy deposit). Chromium chemical analysis showed that the chromium content of the coating which was the sum of Cr(III) + Cr(VI) was 0.0295 g/m2, and Cr(VI) content of the coatings was 0.02 g/m2. Potentiodynamic measurement was used to evaluated the corrosion performance of chromated and unchromated zinc–iron alloy samples in 3.5% NaCl solutions. It was found that the corrosion resistance of the zinc–iron (0.58 wt.%) alloy deposit was significantly improved after the chromate treatment. SEM observation indicated that the surface morphology was characterized by some ‘dried riverbed’ microcracks. Based these results, a mechanism for the formation of black chromate conversion was proposed, and synergistic effects between chromate conversion coating and zinc–iron alloy deposit on corrosion resistance of steel was also discussed.