Abstract
Aluminum and aluminum alloys are widely used in many outdoor applications due to their inherent corrosion resistance attributed to the formation of a protective oxide layer. While corrosion rates are generally considered low for aluminum in many atmospheric environments, understanding of the corrosion performance over time is necessary to predict the cost, safety, and esthetics of these materials. The vast majority of the knowledgebase of atmospheric aluminum corrosion is built on environment–response relationships; often based on statistical correlation of corrosion rate data with atmospheric environmental conditions. However, there is still a limited mechanistic understanding of corrosion processes associated with this linkage. This lack in knowledge prevents interpretation and limits the extrapolation of these statistical datasets for prediction purposes. Here, the mechanistic dependence of aluminum corrosion rate on salt loading is explored through complimentary experimental and theoretical analysis relating corrosion rate to electrolyte chemistry, volume and corrosion products. From these results a reaction pathway is proposed for the atmospheric corrosion of aluminum that accounts for the governing effects of CO2 and salt loading on corrosion rate. This reaction pathway provides a new perspective that highlights the importance of the formation and growth of dawsonite (NaAlCO3(OH)2), and the subsequent gettering of sodium from the electrolyte leading to the stifling of corrosion kinetics. This study highlights the importance of accounting for the dynamic physical and chemical state of the electrolyte during corrosion in process models and measurement techniques to better understand and predict atmospheric corrosion behavior.
Highlights
IntroductionTheoretical analyses and laboratory studies under rigorously controlled conditions have demonstrated the governance of electrolyte volume and geometry, and, circumstantially, soluble salt load, on atmospheric corrosion kinetics
Aluminum coupons exposed at 98% relative humidity (RH) and 21 °C with initial salt loading densities of 10, 60, and 125 μg/cm[2] exhibited mass gain measurements that are consistent with previous studies (Fig. 1a).[25]
For the higher salt loadings, the mass gain appeared to decrease slightly below the maximum, which may be due to small losses of corrosion product during measurement
Summary
Theoretical analyses and laboratory studies under rigorously controlled conditions have demonstrated the governance of electrolyte volume and geometry, and, circumstantially, soluble salt load, on atmospheric corrosion kinetics. Salt, such as sodium chloride, imparts electrolyte to the surface through water uptake from the air, primarily via deliquescence (solid–aqueous) phase transitions. In this manner, salt load can control available anode–cathode area through electrolyte coverage and corrosion cell efficiency (volume and geometry); higher salt loads generally increase coverage and volume and tend towards thin, continuous films. Factors affecting efficiency include electrolyte resistance, which can reduce the effective cathode area,[20,21] along with the electrolyte path length for oxygen to reach the surface when oxygen reduction is the rate limiting step.[22,23] While the individual relationships of these factors with electrolyte geometry are generally understood under ideal conditions at the initial stage of corrosion, less is known regarding their combined influence or competition experienced as corrosion advances
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