The Influence of Salt Loading Density on the Atmospheric Corrosion of Aluminum
Authors: Schaller Rebecca Filardo , Taylor Jason Mark , Schindelholz Eric John 
Published: February 2020

The relationship between NaCl on an aluminum surface and its corrosion rate in humid conditions is explored as a means of better understanding the impact of salt loading on atmospheric corrosion in service environments. Corrosion of aluminum and aluminum alloys under atmospheric exposure has been well documented for outdoor 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.[7,14] The same correlative relationship has been reported in the fewer laboratory studies on this topic.[9-12] Figure 1 exemplifies these trends where mass loss of Al 4N, an Al-Mg alloy (AA 6061), and Al 1100 is correlated with salt deposition rate for field and lab accelerated exposures.[13] 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.
While these studies expose some environmental severity effects and indicate factors that may be controlling corrosion rates, they do not provide insight into the dependence of corrosion rate on salt deposition. Other laboratory studies have elucidated a phenomenon for atmospheric exposures of aluminum, in that corrosion rate diminishes with increased exposure time. Al samples exposed to humid air with 14 or 70 µg/cm2 coverage of NaCl displayed corrosion stifling at 200 h of exposure.[9,10] A thorough explanation to the stifling effect of corrosion over time under fixed NaCl loading conditions has not been fully developed. Possible theories have been put forth, including, the stability or passivation of the surface that shuts down the cathodic region, which is pH and, in turn, CO2 dependent [9,10] or the gettering of NaCl by corrosion product leading to surface drying and depletion of the corrosion aggressor. While there is data supporting the stifling of corrosion with increased ppm of CO2, empirical evidence of pH change is lacking, and there is no evidence to date of NaCl gettering. Also, the hypothesis that increased CO2 results in the stifling of corrosion suggests that corrosion would be independent of NaCl loading density.
This presentation explores the effects of selected NaCl loading densities vs. exposure time of Al 1100 and Al 5N samples with isohumidity tests at a range of set RH. Pure Al samples were selected for study to establish a simplified corrosion system to understand atmospheric mechanisms which could then be expanded to common engineering alloys.
Both macro and micro scale studies are undertaken to elucidate the possible mechanisms leading to corrosion stifling under fixed NaCl loading conditions. High throughput inkjet salt printing is applied to characterize corrosion rate via coupon-level studies. Gravimetry and optical profilometry are utilized to determine mass/volume loss of salt-loaded coupons exposed to humidity and provide indication of conditions leading to corrosion stifling.
Micro scale explorations with in-situ time lapse imaging of micro-pipetted droplets exposed to isohumidity conditions can provide basic indication of electrolyte coverage and stability during exposure. Post-exposure analysis, including optical imaging, pH analysis, SEM, and vibrational spectroscopy will provide characterization of the evolution of electrolyte chemistry and corrosion product mineralogy. 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.

References

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Figure 1