The initiation of pitting corrosion on austenitic stainless steel: on the role and importance of sulphide inclusions (original) (raw)
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Abstract
Sensitive electrochemical techniques have been used to detect and follow the evolution of anodic current transients (unstable micropits) which occur at the nano-amp level prior to the onset of sustained pit propagation on stainless steels in neutral dilute chloride solutions. Using two high purity model alloys, one of which was doped with sulphur, commercial purity 304L, and laser surface melting of the alloys, it was shown conclusively, by the correlation of electrochemical results with quantitative metallography, that sulphur-rich inclusions dominate as pit nucleation sites and that the lifetime of unstable pits is related to inclusion particle size. It is proposed that the sulphur derived from the inclusions helps to stabilise pit growth. Laser surface melting (LSM) was shown markedly to improve the pitting resistance of type 304L stainless steel: the nucleation frequency and lifetime of micropits were both notably reduced as a consequence of the laser treatment. LSM also significantly reduced the size of the sulphide inclusions and it is proposed that this was the most important effect leading to the improvement in pitting resistance. The frequency of occurrence of unstable micropits on the alloys showed a consistent trend with the nucleation frequency of propagating (i.e. damaging) pits, which is consistent with the stochastic model for pit initiation previously developed by the authors. Obtaining data on unstable pitting activity thus provides a rapid method for evaluating and ranking alloys in order of pitting resistance.
Publication:
Corrosion
Pub Date:
1992
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