Study of aluminium filiform corrosion by using electrochemical techniques

Filiform corrosion of aluminium alloys has increased drastically in the last 10 years, in particular when manufactured articles are working in marine or industrial environments. This type of corrosive attack usually starts at coating defects or at cut edges following anodic undermining or cathodic delamination as corrosion mechanisms. As described in the literature, main controlling parameters are: water permeability and elasticity of the coating, oxygen diffusion paths, presence of salts, substrate chemical composition. In particular the metal coating interface is the critical point. It is well know that aluminium paint bonding is enhanced by treating the metal surface in chromate or chromate-phosphate baths and this kind of conversion coatings have also a beneficial effect in preventing filiform corrosion. Aim of this work is to study the filiform corrosion behaviour of aluminium samples treated using environmentally friendly pretreatment baths and to compare these results with that of chromate treated samples. In the same time together with the use of traditional testing procedure of the filiform corrosion we tried to develop and use a new testing method based on Electrochemical Impedance Spectroscopy with the aim to obtain more quantitative data in a shorter testing time. Under the experimental conditions used in our tests, Electrochemical Impedance Spectroscopy showed to be useful and allowed to obtain the same results obtained following the DIN 65472 standard. The resistance to filiform corrosion was greatly influenced by wet adhesion, when it is poor the filament grows rapidly producing also a lateral detachment.