Effect of small amounts of alloyed tin on the electrochemical behaviour of aluminium in sodium chloride solution

Binary model AlSn alloys containing 30-1000ppm (by weight) Sn were investigated by electrochemical polarization in 5wt% chloride solution and subsequent characterization of corrosion morphology. In the homogenized and rolled condition, tin concentration only slightly affected electrochemical behaviour up to 500ppm, and the pitting potentials were all about -0.8V_SCE. However, alloy containing 1000ppm Sn was significantly activated by lowering of the passivity-breakdown potential to -1.38V_SCE. Annealing at 300°C caused significant segregation of Sn to the metal surface, and all specimens, independent of bulk Sn concentration, became nearly similarly active with breakdown potentials around -1.2V_SCE. Corrosion on 300°C-annealed specimens was uniformly distributed by polarization below the bulk pitting potential of -0.76V_SCE. Moreover, the activation effect was temporary, and corrosion was significantly reduced as the segregated Sn was etched away from the surface. Thick oxide, formed during water quenching on high Sn concentration samples, containing 500 and 1000ppm Sn, introduced partial passivation during polarization test. Annealing at 600°C caused increasing activation with increasing Sn concentration, caused by Sn enrichment at the metal surface by dealloying of aluminium during anodic polarization in chloride solution. Corrosion was localized in the form of grain boundary corrosion for alloyed Sn concentration less than and equal to 500ppm and pitting following the triple grain boundaries for 1000ppm.