Electrochemical polarization and passive film analysis of austenitic Fe–Mn–Al steels in aqueous solutions

The corrosion behaviour of the Fe–25Mn–5Al–0.15C alloy in different aqueous solutions of pH −0.8 to 15.3 and the corrosion protection mechanism induced by the presence of Al have been investigated by electrochemical measurements and AES/XPS analysis. The corrosion behaviour of Fe–25Mn–5Al steel has been compared with that of mild steel, cryogenic 9% Ni steel and 1Cr13 stainless steel. The addition of 25 wt.% Mn to mild steel is very detrimental to the corrosion resistance in aqueous solution. Fe–25Mn steel is difficult to passivate, even in such neutral aqueous electrolytes as 1 mol l −1 Na 2SO 4 solution. With increasing Al content of Fe–25Mn–Al steels, the aqueous corrosion resistance increase and the anodic polarization curves exhibit a stable passivation region in 1 mol l −1 Na 2SO 4, 50% HNO 3 and 10–50% NaOH solutions as Al content increases to about 5 wt.%, but it shows no passivation in either 10% HCl or 3.5% NaCl solution. The corrosion resistance of Fe–25Mn–5Al steel in the aqueous solutions tested is comparable to that of mild steel. The outermost surface and the main part of the passive film formed on the surface of Fe–25Mn–5Al steel in 30% NaOH solution are probably the bound water and a mixture of aluminium, iron and manganese oxides including metallic iron and aluminium, respectively.