THE INITIATION OF PITTING CORROSION AT MNS INCLUSIONS

The dissolution of MnS inclusions in stainless steel exposed in 0.5 M NaCl + 0.5 M H2SO4 + 0.08% H2O2 at the free corrosion potential for 10 s has been studied using simultaneous Auger/EDX analysis and imaging and electron microscopy. Inclusions at different stages of dissolution have been examined. As dissolution of the inclusion progressed, Auger/EDX imaging showed the resulting cavity to have a local solution composition rich in MnCl2 and the exposed bare metal surface was found to repassivate. It is proposed that metallic corrosion and stabilisation of pit growth is critically dependent on the precipitation of a MnCl2 salt film on to the exposed metal surface preventing repassivation. A critical concentration of MnCl2 within the cavity must be attained before salt film precipitation will occur. If the critical MnCl2 concentration is not reached during dissolution of the sulphide inclusion, repassivation of the cavity will result. The probability of salt film formation and consequent pit propagation will be dependent on the size and geometrical shape of the inclusion.