INFLUENCE OF NI, MO, AND CR ON PITTING CORROSION OF STEELS STUDIED BY RAMAN-SPECTROSCOPY

Corrosion layers grown on stainless steels (AISI 302, 316), polarized at the pitting potential in alkaline solutions containing chloride, were identified by Raman spectroscopy. Several binary alloys such as Fe-10Ni, Fe-6Mo, and Fe-18Cr, were studied to separate the influence of the different alloy elements. A comparison was also made with the layers formed on these alloys by voltametric cycling (which allows the thickening of the passive layer), the anodic reverse potential being in this case below the pitting potential. The parts played by the two main elements, Cr and Mo, are described. After the passivity breakdown, i.e., once the chromia layer has broken down, the inner layer is formed by tetravalent molybdate, the pitting initiation being related to the passage of Mo from valency 6+ to valency 4+. In the outer green rust layer, bichromate ions fill the sites available for Cl- and then slow down the ingress of chloride.