The influence of salt loading density on the atmospheric corrosion of aluminum

Corrosion of aluminum and aluminum alloys under atmospheric exposure has been well documented for outdoor conditions. While these studies expose the effects of environmental severity they do not explicitly establish the dependence of corrosion rate on salt loading. Accelerated laboratory studies have shown that initial corrosion rates are generally higher with higher salt loadings, but, over time corrosion appears to effectively stifle for low loadings of NaCl (<100 μg/cm²) under fixed humidity conditions. This has previously been attributed to the stability or passivation of the surface that is pH and, in turn, CO₂ dependent. Another possible explanation could be the gettering of NaCl by corrosion product leading to surface drying and depletion of the corrosion aggressor. This paper explores the effects of selected NaCl loading densities vs. exposure time of UNS A91100 at both the macro and micro scale to illuminate the possible mechanisms leading to corrosion stifling. Through this work, an understanding of the relationship between corrosion in atmospheric systems versus the variation of a specific environmental severity factor, NaCl loading density, will be further developed.