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.
Aluminum and aluminum alloys have been extensively studied under field atmospheric exposure conditions.1-8 These studies characterized effects of environmental severity and exposure factors that may control or enhance corrosion rates. One severity factor, salt deposition rate, results in higher corrosion rates with higher salt deposition rates.7, 9 However, as atmospheric field exposures are open to a wide variety of environmental severity factors, such as salt deposition rates, SO2, UV, Ozone, pH, temperature, humidity, time of wetness, and other industrial pollutants, understanding the reliance of corrosion on one single severity factor often becomes convoluted.3,4,6-8,10-13 Simplified laboratory studies enable establishment of the corrosion rate under more controlled environments and the same correlative relationship between salt loading and corrosion rate has been reported in the fewer laboratory studies on this topic.10, 14-17 Figure 1 exemplifies these trends where mass loss of UNS A91199 (4N pure Al), UNS A96061 (an Al-Mg alloy, AA 6061), and UNS A91100 (Al 1100) is correlated with salt deposition rate for field and lab accelerated exposures.10 For all exposures and materials, corrosion rates increased with increasing deposition rates, except for AA 6061, which showed a stifling in corrosion rate in laboratory exposures above 100 mg/m2d (10 µg/cm2).