Ellipsometric Study of Surface Films Grown on Austenitic Stainless Steel in Chloride Solutions

Surface films grown on AISI Type 304 steel were studied ellipsometrically at open circuit potentials in 5 and 35% MgCl₂ solutions at 20, 60, and 90 C, and in 0.1 N, 1N, 3N HCl, and 5N H₂SO₄ + 0.5N NaCl solutions at 20 C. In contact with 35% MgCl₂ at 90 C, similarly as in contact with HCl and H₂SO₄ + HCl solutions at 20 C, the previous oxide film was found to transform into a moderately protective layer of corrosion products which favors the nucleation and propagation of stress corrosion cracks. On the contrary, in 5% MgCl₂ at 20 to 90 C, and in 35% MgCl₂ at 20 to 60 C, the previous highly protective oxide film does not undergo any deep transformation, and SCC is absent.