Localized corrosion of ultrafine-grained Al-Mg model alloys

The present study investigates passivity and localized corrosion of ultrafine-grained (UFG) binary Al-alloys with 0.5. 1 and 2 wt % of magnesium subjected to three different equal-channel angular pressing (ECAP) pass numbers. The alloys were investigated by electrochemical techniques and surface analyses in different NaCl solutions and compared with their conventionally grained (CG) counterparts. The results of potentiodynamic polarization experiments indicated that the breakdown potential slightly decreased with increasing the ECAP pass number for all alloying series. Major differences are present in the pit transition potential (E-ptp), in that the potential shifts to more negative values with increasing number of passes in all series. EIS measurements showed no differences in corrosion resistance with increasing the number of ECAP passes at the open circuit state compared to their CG counterparts. indicating that passive layer properties are not affected by volume fraction of grain boundaries and dislocation density. Furthermore, serious pitting occurred for the coarse grained alloys in a kind of laterally spreading crystallographic filiform corrosion. In contrast, the UFG material exhibits solely local crystallographic pitting corrosion propagating in depth. The results indicate that UFG Al-Mg alloys tend with increasing pass number to deep localized pitting corrosion which leads to a hindered repassivation behavior compared to its CG counterpart, reflected in the negative shift in E-ptp. (C) 2009 Elsevier Ltd. All rights reserved.