ELECTROCHEMICAL POLARIZATION AND PASSIVE FILM OF A CRYOGENIC AND NONMAGNETIC STEEL IN AQUEOUS-SOLUTION

The corrosion behaviour of the austenitic steel Fe-23Mn-4Al-5Cr-0.3C i n different aqueous solutions of pH -0.8 to 15.3 and the corrosion pro tection mechanism induced by adding Al or Al and Cr have been investig ated by electrochemical measurements and AES/XPS analysis. The corrosi on behaviour of Fe-Mn base steel have been compared with those of mild steel, cryogenic 9 % Ni steel, stainless steels 1Cr13 and 1Cr18Ni9Ti. The addition of manganese to mild steel is very detrimental to the co rrosion resistance. Fe-25Mn steel passivates with difficulty even in s uch neutral aqueous electrolytes as 1 M Na2SO4 solution. The addition of 5 % aluminum to Fe-25Mn steel confers passivity to the steel in neu tral or oxidizing, chloride-free solution. The addition of 5 % Cr to F e-Mn-Al steel further improves resistance to corrosion. The passivity of Fe-23.5Mn-4Al-5Cr-0.3C steel in aqueous electrolytes tested is supe rior to that of 9 % Ni steel and approximate to that of 1Cr13 stainles s steel. The corrosion resistance is probably imparted by a thin barri er film of oxides. The outer part of the passive film formed on the su rface of Fe-23.5Mn-4Al-5Cr-0.3C steel in 1 M Na2SO4 solution is enrich ed in Al3+, Cr3+ and Fe3+, and this means that the film is probably ma de up of a mixture of Al2O3, Cr2O3 and Fe2O3.