Pitting corrosion resistance of ultrafine-grained aluminum processed by severe plastic deformation

The effect of equal-channel angular pressing (ECAP) on the pitting corrosion resistance of Al and Al-Mg alloy was investigated by means of polarization curves in solutions containing 300 ppm of Cl- and by surface analysis. The potentials for pitting corrosion of Al and Al-Mg alloy were clearly shifted in the noble direction by the ECAP process, indicating that this process improves resistance to pitting corrosion. SEM observations revealed that pitting corrosion Occurred near impurity precipitates and that the size of the impurity precipitated decreased as a result of the ECAP process. The time-dependence of corrosion potential and the polarization resistance determined using the AC impedance technique suggest that the ECAP process increases the rate of formation of Al oxide films. The improvement in pitting corrosion resistance of Al and Al- Mg alloy by ECAP appears to be attributable to a decrease in the size of impurity precipitates and an increase in the rate of formation of Al oxide films.