Comparison of the morphology and corrosion performance of Cr(VI)- and Cr(III)-based conversion coatings on zinc

The morphology, composition and corrosion performance of conversion coatings generated on zinc in Cr(VI) and Cr(III) treatments have been studied and compared. With an immersion time of 1 min, the coatings produced in a Cr(VI) bath were about 400 nm thick, whereas the coatings produced in a commercial Cr(III) bath were about 90 nm thick, determined by spectroscopic ellipsometry. By means of XPS, it was found that about 40% of the chromium in the outermost layer of the coating generated in the Cr(VI) bath was in the Cr(VI) oxidation state, while about 60% was in the Cr(III) state. No Cr(VI) was detected in the coating produced in the Cr(III) bath. The corrosion behaviour of zinc in 0.01 M NaCl solution (pH=6) after both types of surface treatment was studied using d.c. polarisation and a.c. impedance techniques. The behaviour of untreated zinc surfaces in de-aerated 0.01 M NaCl solutions containing different concentrations of dichromate was also studied. The coatings generated in the Cr(III) bath inhibited the corrosion of zinc to a significant degree, but less effectively than the coatings formed in the Cr(VI) bath. The difference in the corrosion protection given by the two coating types was attributed to the difference in the coating thickness and to the availability of mobile Cr(VI) species which can repassivate flaws.