Localized coating failure of epoxy-coated aluminium alloy 2024-T3 in 0.5 M NaCl solutions: Correlation between coating degradation, blister formation and local chemistry within blisters

Abstract

The role of pH on the nature and rate of the degradation of epoxy coatings on AA2024-T3 panels and subsequent corrosion of the substrate during immersion in NaCl solutions was investigated. In acidic solutions both blister formation and growth are rapid. Blisters become very large (≈1 cm) and new blisters appear to form for a certain time after exposure. Often very small (∼0.1 mm) clear blisters surround these large blisters. Enhanced blister formation is due to irreversibly increased permeability of the coating for chloride ions and protons, the formation of more defect sites within the coating, and the weakening/dissolution of the oxide layer in low pH environments. In neutral pH solutions, coatings fail by forming one, or at most two, active blisters (red in color) within a few days of immersion with the time-to-failure dependent upon coating quality and thickness. Blister growth is a very slow process, and blister diameters rarely exceed a few millimeters even after several weeks. The accumulation of corrosion product within the blister slows down the corrosion rate and blister growth. The chloride concentration in the occluded solutions within the blister is significantly increased over the bulk concentration, and the pH is often in the acidic range. From electrochemical measurements it can be concluded that the anodic and cathodic reactions are confined to the blister and its immediate surroundings, rather than involving more of the surface over which the coating is intact. Based on corrosion morphology it is concluded that replated copper contributes to the overall cathodic reaction.