The initiation of pitting corrosion at MnS inclusions

Abstract

The dissolution of MnS inclusions in stainless steel exposed in 0.5 M NaCl + 0.5 M H 2SO 4 + 0.08% H 2O 2 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 MnCl 2 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 MnCl 2 salt film on to the exposed metal surface preventing repassivation. A critical concentration of MnCl 2 within the cavity must be attained before salt film precipitation will occur. If the critical MnCl 2 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.