Filiform corrosion of aluminium sheet. II. Electrochemical and corrosion behaviour of rare substrates

Coiled rolled aluminium substrates of the AA1000, 3000 and 5000 series in various tempers were characterized by corrosion potential (E-corr) and porentiostatic measurements in an acidic chloride solution. The time dependence of these data indicated the presence of an active surface layer on the annealed substrates that were susceptible to filiform corrosion (FFC) after painting. This layer was electrochemically more active than the bulk of the material whose E-corr was stable at the well known critical pitting potential (E-c) of the alloy. The E-corr of an active substrate was more negative than the E-c of the bulk rnetal immediately after exposure to the test solution. rising to the bulk E-c with immersion time, as the active layer dissolved in the solution. Analogously, potentiostatic measurements at E-c indicated high anodic current transients, decaying with time to lower steady-state values after an equivalent amount of metal (of the order 1 mu m in thickness) was removed from the surface. The reactivity of the layers, quantified in this manner, depended on the chemical composition of the substrates in the same way as the susceptibility of the layers to FFC. (C) 1998 Elsevier Science Ltd. All rights reserved