INFLUENCE OF NON-METALLIC INCLUSION NATURE AND SHAPE ON THE PITTING CORROSION SUSCEPTIBILITY OF 18CR9NI AND 17CRL1NI2MO AUSTENITIC STAINLESS-STEELS

The influence of the nature and shape of non-metallic inclusions on the nucleation and growth of the pits in aqueous 3% NaCl solutions of different pH values on 18Cr9Ni and 17Cr11Ni2Mo stainless steels is examined. Pits are always localised on sulphides and mixed inclusions. The potential-pH fields which determine the preferential dissolution of the sulphide inclusions are drawn. Sulphide dissolution is a condition necessary but not sufficient for the nucleation of pits since not only the nature of the inclusions but also the shape is effective in determining the probability of pitting nucleation. The higher reactivity of the transverse sections of a stainless steel in comparison with the longitudinal ones is explained by the more unfavourable inclusion shape in that direction. The influence of the inclusion shape becomes less and less important as the bulk solution pH is lower or the etching potentials higher. © 1979.