The controlling role of sodium and carbonate on the atmospheric corrosion rate of aluminum

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 CO 2 and salt loading on corrosion rate. This reaction pathway provides a new perspective that highlights the importance of the formation and growth of dawsonite (NaAlCO 3 (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.

Revealing the growth kinetics of atmospheric corrosion pitting in aluminum via in situ microtomography

“…The trend in cumulative volume loss observed in Fig. 1 is consistent with previous studies of Al under fixed salt load and constant humidity conditions: after some period of initial corrosion upon humidity exposure 22,23 , measurable loss effectively ceases. This occurred after similar exposure times (100-500 h) in both this study and in those of refs.…”

“…Schaller et al found the time at which corrosion stifled strongly correlated with observed dry-up of electrolyte for NaCl droplets on AA1100 at 98% RH 23 . This was attributed to corrosion stifling by a reaction of the cathode electrolyte with atmospheric CO 2 during corrosion which precipitated sodium aluminum carbonate compounds that dry the surface 23,24 .…”

“…Understanding how pits evolve under these conditions is critical to developing robust damage models for corrosive environments 20 . Laboratory studies of aluminum corrosion at longer times (weeks to months) have shown that volume loss and attack depth can reach a limiting value under a fixed amount of salt for high humidity conditions [21][22][23] . Schaller et al found the time at which corrosion stifled strongly correlated with observed dry-up of electrolyte for NaCl droplets on AA1100 at 98% RH 23 .…”

“…Laboratory studies of aluminum corrosion at longer times (weeks to months) have shown that volume loss and attack depth can reach a limiting value under a fixed amount of salt for high humidity conditions [21][22][23] . Schaller et al found the time at which corrosion stifled strongly correlated with observed dry-up of electrolyte for NaCl droplets on AA1100 at 98% RH 23 . This was attributed to corrosion stifling by a reaction of the cathode electrolyte with atmospheric CO 2 during corrosion which precipitated sodium aluminum carbonate compounds that dry the surface 23,24 .…”

“…As a first step towards evaluating the relationship between pit growth, microstructure, and the evolving surface environment, this study applied XCT to characterize the evolving kinetics and morphology of pitting attack over 96 days of exposure for a 99.9% Al wire loaded with 60 μm/cm 2 sodium chloride salt particles at 84% RH. This material and environment were chosen both for their pedagogical simplicity and for their relationship to previous work by the authors on aluminum atmospheric corrosion kinetics 23 . Focusing on this system provides insights into the effects of corrosion-induced electrolyte changes on atmospheric corrosion kinetics without the complicating factors of multiple phases or inclusion types typical in many engineering alloys.…”