Characterization of chromatized hot‐dip‐galvanized steel and 55% AlZn‐coated steel using ESCA and AES

Abstract

AbstractChromating of metals has been used for a long time as corrosion protection. In this study two commercial products for chromating, one based on chromates (Iridite 9L‐6) and one with an addition of a polymer (Okem Coat F1), have been analysed on hot‐dip‐galvanized steel, 55% AlZn‐coated steel and pure Zn, Al and Au. The analysis techniques used for this study were electron spectroscopy for chemical analysis and scanning Auger microscopy. With these methods it has been possible to characterize binding energies of different compounds in chromating coatings and to record elemental maps and depth profiles. Coatings formed in the laboratory have been compared with those from production lines. Comparisons of Cr6+/Cr3+ ratios of the chromating coatings formed on different substrates revealed that Cr6+ in a thin coating on Zn is almost completely reduced to Cr3+, while on Al the reducton of Cr6+ is very limited. On hot‐dip‐galvanized steel the Cr6+/Cr3+ ratio is higher than on 55% AlZn‐coated steel. This can be explained by differences in chromate coating thicknesses on hot‐dip‐galvanized and 55% AlZn‐coated steel. The coating formed by Iridite 9L‐6 on 55% AlZn‐coated steel is preferentially found in the interdendritic areas. Okem Coat F1 forms a slightly more homogeneous chromium coating on 55% AlZn‐coated steel as compared to that of Iridite 9L‐6. On the basis of the aqueous chemistry of chromium compounds, models are discussed for the formation of the chromating coatings.