Changes in the composition of the passive layer and pitting corrosion of stainless steel in phosphate—borate buffer containing chloride ions

The influence of the passive layer properties on the pitting corrosion of 316SS was studied in phosphate—borate buffer containing chloride ions by using potential step and potentiodynamic techniques complemented with scanning electron microscopy. The increase of the anodization time in the passive region decreases the nucleation rate and the mean number of corrosion pits formed onthe 316SS surface. Results are explained through changes in the structure and composition of the passive layer during anodization. Two different Cr(III) species can be voltammetrically detected at short anodization times, an outer weakly bound Cr(III) species which is electroaoxidized to soluble CrO 4 2− and an inner Cr(III) species which is electrooxidized to Cr(VI) but retained in the film at potentials lying in the transpassive region. As the anodization time in the passive region increases, the weakly bound Cr(III) species is transformed into another more stable one, probably an iron chromite, which exhibits an electooxidation potential more positive than that of Cr(III) species. The aged passive layer becomes more resistant to pit initiation, due to either a decrease in the density of active sites or a decrease in the nucleation rate constant for pit initiation.