A review of filiform corrosion and its prevention on polymer-coated aluminium alloys

Filiform corrosion (FFC) is an underpaint corrosion form frequently observed on metals with a high band gap oxide, e.g., aluminium and magnesium. Macroscopically, FFC shows as filaments propagating at a metal/coating interface. Electrochemically, an anodic active region is followed by a cathodic tail and a region where solid corrosion products precipitate. Two forms of FFC are observed, (i) fast anodic undercutting at the metal/coating interface and (ii) successive pitting. This work reviews experimental evidence for FFC mechanisms especially on 3xxx and 6xxx rolled and extruded aluminium alloys and the effect of the surface composition and microstructure. Recent evidence suggested that the hydrogen evolution and related bubble detachment near the anodic region in the head are more important for coating detachment than previously envisaged. For efficient FFC protection, surface pretreatments such as anodising and the sequence of etching, desmutting and conversion coating are crucial. Industrial focus has recently been on FFC on aluminium with a large content of post-consumer (a.k.a. end-of-life) scrap.