Role of Confinement in Localized Corrosion
To investigate the effects of confinement, a simulated crevice was created by maintaining a tight spacing between the polished surface of a flat pH probe and the metal test specimen, as shown in Fig. 1b. The pH probe and the solution are held against the sample by an inert polyethylene fixture. Sensitized AA5083 was chosen as the test material because of its susceptibility to SCC as a result of intergranular precipitates of Al2Mg3 known as β-phase. The electrolyte was pH neutral 0.1 M sodium chloride solution.
Exposing AA5083 to neutral-pH NaCl solution under confinement in free corrosion conditions triggered a spontaneous localized alkaline corrosion, which resulted in a gradual evolution of the pH from 6.9 to roughly 9. At this higher pH value, the passivation film was damaged and hydrogen evolution that promotes the increase in pH was observed as the counter-reaction to aluminum oxidation. This research presents the effect of different ratios of metal surface area to electrolyte volume on the reaction kinetics, as characterized by the evolution of the solution pH with time. Surface area to volume ratios ranged from 4 cm2/ml to 100 cm2/ml. The results are supported with microscopic analysis and EIS studies.