Pit initiation on austenitic stainless steels

Mechanistic aspects of the initiation and early stages of growth of pitting corrosion on stainless steels in dilute chloride solution are described. Electrochemical techniques have been used to detect and follow the evolution of anodic current transients which occur at the nano-amp level prior to the onset of sustained pitting. These transients are shown to correlate with the occurrence of micropits on the specimen surface. Prior to repassivation, there was no difference in their current-time evolution to distinguish unstable micropits from pits which subsequently grew into stable macropits. The basic shapes of the experimentally-observed transients are shown to be consistent with a simple model of a developing pit. Environmental and metallurgical factors which influence the nucleation and subsequent stability of micropits are identified and described. The imposition of solution flow does not affect the nucleation of pits but turbulent flow stops or inhibits their growth. Using two high purity 304L model alloys, one of which was doped with sulphur, in addition to a commercial purity 304L, it is shown conclusively that sulphur-rich inclusions dominate as pit nucleation sites. A hypothesis is introduced in which sulphur, derived from sulphide inclusions, helps to stabilse the growth of pits. It is proposed that the lifetime of unstable pits is related to inclusion size. These ideas are developed with the aid of experiments on laser treated specimens, in which surface melting was found to refine the inclusion population such that most were only visible in the transmission electron microscope. One consequence of this hypothesis is that below a certain size, sulphur-rich inclusions are too small to nucleate a damaging pit. A stochastic element of the early stages of pitting is suggested to be the availability of sulphur to catalyse the anodic dissolution process in the pit. Molybdenum has been found not to influence pit nucleation, but reduces the dissolution rate in an active pit. The requirement for molybdenum additions to stainless steels in the absence of harmful sulphur-rich inclusions is questioned. In a separate study involving laser processing, surface melting is shown to provide an effective way to prevent intergranular stress corrosion cracking in sensitised 304 stainless steel. The work reported here is consistent with a general class of `feedback' models. The implications of such theories is that the driving force for pit nucleation is the current in the passive state. In the present study it is shown that the passive current and the pitting activity vary with the volume fraction of sulphides in the steel, implying that local instabilities in the passive current occur at sulphide inclusions. Consistent with this, many small pits were associated with sulphide inclusions.