Study of chromate coatings obtained on phosphatized steels: a characterization by AES and XPS techniques

Electrogalvanized and phosphatized steel sheets were passivated in chromate baths, and both the composition and temperature of these were varied in order to study the effects of such parameters on the chromate film formation. Scanning electron microscopy (SEM) together with microanalytical electronic spectroscopy techniques such as Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were chosen as the most useful methods for a suitable and careful study of the chromate presence, distribution and thickness. The chromium presence on the treated surfaces is easily detectable by XPS. Moreover it is possible to compare the surface chromium content of differently produced specimens. The detection of chromium is more difficult using AES, due to charging problemS. Nevertheless, by this technique we found that the passive film is not homogeneously distributed on the whole phosphatized surface, but it is localized on randomly distributed areas, the chromate passive layer being very thin (about 4 nm). With respect to corrosion resistance of electrogalvanized steel, it seems that passivation in baths containing only Cr³⁺ is more effective than passivation in baths containing Cr⁶⁺.