Effect of chemical cleaning on the corrosion behaviour of painted aluminium alloys

Abstract

Painted aluminium alloys are sometimes affected by filiform corrosion. Such a phenomenon which was intensively studied in the last few years is mainly a cosmetic type of attack. Usually it starts at coating defects or at cut edges following an anodic undermining mechanism. As described in the literature, water permeability and elasticity of the coating, oxygen diffusion paths, and presence of salts are the main controlling parameters. However, the metal coating interface is the critical point; then the substrate chemical composition, as well as the type of chemical cleaning or the type of applied chemical conversion coating proved to be fundamental aspects to improve resistance to filiform corrosion. The aim of this work is to study the filiform corrosion behaviour of aluminium alloys surface cleaned using chemically different products (alkaline, acid, combination of the two) and then treated using environmentally friendly pretreatment baths and chromates baths. The selected aluminium alloy was AA6060 type. After powder painting, aluminium samples were tested following the DIN 65472 standard. At the same time electrochemical tests were carried out following a particular procedure on samples exposed for different times to the environment promoting filiform corrosion. Under the experimental conditions used in our tests, electrochemical impedance spectroscopy was shown to be useful and allowed to obtain more quantitative data in a shorter testing time with respect to traditional testing. It was observed that resistance to filiform corrosion is greatly affected by the chemical composition of the aluminium substrate and that chemical cleaning modifies the surface chemical composition of the alloy so modifying its corrosion behaviour.