Composition analysis of the plating on electrolytically treated steel sheets in chromic acid solution

The structure and composition of the chromium plating on Tin Free Steel (TFS) were investigated and analyzed with scanning electron microscopy (SEM), energy dispersive analysis of X-ray (EDS), X-ray photoelectron spectroscopy (XPS) and coulometric electrolysis method. The plating under investigation was manufactured by a two-step electrolytic reduction from a chromic acid solution. Experimental results showed that the plating on TFS had a two-layer structure. Moreover, two valences of chromium, three and zero, were detected in the plating and no hexavalent chromium was found in it. Correspondingly, all chromium in the outer layer was trivalent and existed as the form of Cr₂O₃ and Cr(OH)₃, the content of which was about 6.62 mg/m², while the pure metallic chromium was the only form of chromium found in the inner layer with the content of about 57.75 mg/m². In addition, the amount of O existed as M-OH was twice as that of M-O in the above-mentioned hydrated chromium oxide layer.