Corrosion behavior of severely deformed pure and single-phase materials

Abstract

The significant and complex effect of plastic deformation on corrosion behavior involves changes in not only dislocation density but also other metallurgical factors such as grain size, texture, chemical inhomogeneity, phase transformation and residual stress. With the advent of severe plastic deformation (SPD), the effect of plastic deformation on corrosion in the ultrahigh strain range is becoming an important issue. However, our understanding of corrosion properties of SPD materials lags far behind than that of their other properties, e.g. their mechanical properties. In this review, the role of dislocations and grain boundaries generated by SPD was highlighted in pure metals and single-phase materials, where plastic deformation and grain refinement proceed mainly by dislocation activity. Accordingly, the complicated effect of chemical inhomogeneity arising from impurity segregation and precipitation was excluded from discussion, while other implicit effects were included. It is essential to elucidate the effect of so-called ultrafine-grained (UFG) structures which develop progressively to a saturation over a very wide strain range. Unfortunately, the literature mainly compares the corrosion behavior of UFG and coarse-grained (CG) materials, and the degree of perfection of UFG formation and the resultant effects on corrosion vary between studies. The limited number of studies that examines corrosion behavior systematically over a wide strain range suggests that, in most cases, the effect of plastic deformation on corrosion extends into the SPD region gradually, with no anomalous change. That is, SPD improves the corrosion resistance to further degree in a passive environment, whereas it increases the dissolution rate in a non-passive environment. However, several works reported an abrupt change in corrosion resistance, which could be attributed to UFG formation. A marked improvement is observed in FeCr alloys, where passivation becomes more protective owing to UFG formation induced by SPD. In severely deformed materials, structural alterations in dislocations and grain boundaries have a very high impact on the corrosion kinetics because of their closely spaced configuration.