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AISI 416 is a high sulphur martensitic stainless steel with elliptical shaped sulphide inclusions over 2 μm in length. After laser melting the sulphide inclusions became spherical and reduced in size to below 0.5 μm in diameter. SEM investigations showed that the total number of inclusions increased dramatically and it was calculated that the total volume of sulphur in the stainless steel had not changed during the laser treatment. The pitting potential was increased by over 200 mV with an inverse linear dependency on the average inclusion size. The frequency of metastable pitting events decreased by a factor of 100.

It has long been accepted that manganese sulphide favours pitting on stainless steels. However, there are different standpoints on the most important mechanism for pit initiation; due to dissolution of sulphide inclusions, chromium depletion around the inclusion or mechanical rupture of the passive film by metal chlorides. Analysing the pitting potential and metastable pitting rates on different grades of stainless steels has rationalised the effect of sulphide content on pitting corrosion resistance. In situ atomic force microscopy (AFM) has been used in conjunction with conventional electrochemical techniques for imaging real time pit initiation events.