Understanding Initiation and Evolution of Pitting Corrosion in 7075-T6 Aluminum Alloy by Four-Dimensional X-Ray Microtomography

Abstract

X-ray microcomputed tomography was conducted on an AA7075-T651 sample immersed in a 3.5 wt% NaCl solution to provide time-resolved measurements of localized corrosion. A nondestructive, volumetric analysis of pitting sites and local microstructural features followed, and quantitative results were combined with 2D and 3D visualizations. During alternating immersion periods, pitting was initiated at cathodic intermetallic inclusions and continued throughout the study. Rates of pit growth varied as a response to the decoupling of inclusions from the matrix, resulting in a start-and-stop trend that was observed in a significant number of examined sites. When a sample with a higher extent of cold rolling was examined with the same procedure, a finer and more homogeneous distribution of inclusions correlated with an increase in the maximum and mean pit depth. This change was attributed to the more frequent exposure of subsurface inclusions by localized corrosion, a phenomenon that mitigated the passivating effect of decoupling. Additionally, the effects of continuous immersion vs. alternate immersion were examined for metallurgically identical samples. Intermittent drying during the alternate immersion period destabilized the passive layer, increasing mean pit depth, while continuous immersion for 20 d produced a uniform and protective layer of corrosion product.