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 0, 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, Zn,(OH)(2)CrO4 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 2 chromium content of the coating which was the sum of Cr(III) + Cr(VI) was 0.0295 g/m(2), and Cr(VI) content of the coatings was 2 0.02 g/m(2). 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. (C) 2003 Elsevier Science B.V. All rights reserved.