A CRITICAL-EVALUATION OF ELECTROANALYSIS METHODS FOR THE DETERMINATION OF CHROMIUM SPECIES IN PASSIVATION LAYERS ON TINPLATE BY X-RAY PHOTOELECTRON-SPECTROSCOPY

X-ray photoelectron spectroscopy (XPS) was used to analyse, both qualitatively and quantitatively, the passivation film on tinplate samples subjected to the galvanostatic treatments that are usually employed for the quantitative determinations of metallic chromium (the Aubrun test) and of the total chromium amount (the Britton test). It was concluded that neither method has a rigorous quantitative basis or a specific capability for distinguishing between the chromium species. This is explained by chemical transformations (monitored by means of XPS spectra) which play a dominant role in the overall process, relating the total charge passed during the potential arrest to the actual chromium amounts. Tinplate subjected to potentiostatic treatments was also analysed with a view to assessing the potential dependence of the dissolution of the chromium species in the range of interest. Depth profiles were obtained by alternating XPS measurements with Ar + bombardment sequences; particular attention was paid to the evaluation of possible artefacts induced by Ar + ions, mainly preferential removal and reductive effects. © 1984.