PASSIVITY OF CHROMIUM IN SULFURIC-ACID-SOLUTIONS

The passivity of pure chromium has been studied in sulphuric acid solutions as a function of concentration (1-18 M), acidity, activity of water and the nature of the anions in the electrolyte. The passivity phenomena of this metal have been investigated using anodic polarization measurements and X-ray photoelectron spectroscopy. Chromium undergoes passivation in the whole area of the acid concentration. Its passivation parameters are closely connected with the structure of the electrolyte. In dilute solutions of acid (M < 6) the passivation of chromium proceeds with the participation of water molecules. The XPS measurements reveal that the average composition of the passive film is nearly equal to chromium (111) oxy-hydroxides with a small degree of hydration. The passivity of chromium in concentrated sulphuric acid proceeds with the participation of undissociated acid molecules or HSO4- ions. The anodic layer formed in concentrated solutions also contains sulphur species in lower oxidation states ((S2-). The presence of these species shows that acid molecules undergo reduction in the passivation processes.