Abstract
X10Cr18Ni9Ti stainless steel was plasma nitrided at 600°C for 9 h and then subjected to various chemical passivation treatments for improvements in corrosion resistance. Polarisation curves and polarisation resistance were measured in a solution of 0.1 M Na2SO4 with a pH of 3.0 or 6.4. The elemental concentration in the surface films was determined by AES. It was found that the corrosion resistance of the nitrided steel was significantly improved by passivation in a hot NaOH/KNO3 solution or a molten NaNO3/KNO3 mixture, followed by oil impregnation. AES analysis showed that the outer layers of the surface films were composed mainly of Cr nitrides and oxides after the HNO3 treatment, of Fe and Ni oxides after the NaOH/KNO3 treatment, and of Fe oxides after the molten NaNO3/KNO3 treatment. Films with the ratio O/metal≅0.8 were probably a mixture of the metal and oxides. The formation of thick oxide-containing films on nitrided stainless steel was possible due to the high reactivity of the chromium-depleted matrix and of nitrides, such as Cr2N.
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