LOCALIZED CORROSION PROCESSES IN IRON AND STEELS STUDIED BY IN-SITU RAMAN-SPECTROSCOPY

The part played by molybdenum in the prevention of pitting corrosion o f iron and steels in the presence of chloride ions can be explained us ing Raman spectroscopy. Here, molybdenum introduced as a molybdate in the electrolyte solution was particularly studied. Different polarizat ion 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 dif ferent molybdate anions, in which Mo has an oxidation state of 4+ or 6 +. After the pitting is generalized, there rapidly grows a thick, coll oidal and unstable green rust layer (iron hydroxychlorure), which was chemically identified on pure iron using Raman spectroscopy. Beneath t his layer, which represents a very agressive medium, the corrosion rat e increases dramatically, and the sample is rapidly destroyed. On stai nless steels, molybdenum (as well as chromium) can be integrated in th e green rust, thereby slowing the corrosion rate. When pitting is init iated, the inner layer is formed by tetravalent molybdate, the passivi ty breakdown being associated with the change in oxidation state of Mo from 6+ to 4+.