Localized corrosion processes in iron and steels studied by in situ Raman spectroscopy

Abstract

The part played by molybdenum in the prevention of pitting corrosion of iron and steels in the presence of chloride ions can be explained using Raman spectroscopy. Here, molybdenum introduced as a molybdate in the electrolyte solution was particularly studied. Different polarization methods were used (potentiostatic polarization before or after the pitting potential, or voltammetric cycling), leading to different pit distributions and sizes. Around the pit, one can discriminate the different molybdate anions, in which Mo has an oxidation state of 4+ or 6+. After the pitting is generalized, there rapidly grows a thick, colloidal and unstable green rust layer (iron hydroxychlorure), which was chemically identified on pure iron using Raman spectroscopy. Beneath this layer, which represents a very agressive medium, the corrosion rate increases dramaticlly, and the sample is rapidly destroyed. On stainless steels, molybdenum (as well as chromium) can be integrated in the green rust, thereby slowing the corrosion rate. When pitting is initiated, the inner layer is formed by tetravalent molybdate, the passivity breakdown being associated with the change in oxidation state of Mo from 6+ to 4+.