Long-term marine pitting corrosion of AlMgSi aluminium alloys

AlMgSi aluminium alloys (i.e. 6000 series) are one of the so-called 'marine alloys' with good performance in aggressive marine environments. Advantages include high strength-to-weight ratio and good corrosion resistance. They are commonly used for modern small to medium size ship construction. There remains a need for more explicit long-term corrosion information for accurate lifetime prediction. This paper reports 2-year field exposure data for 6060-grade aluminium in temperate immersion, tidal and splash zone exposures. Duplicate sample strips were recovered from the tidal and atmospheric splash zones respectively. Corrosion losses were estimated and the surface morphology and chemistry of the corroded surfaces were investigated by scanning electron microscopy and energy dispersive spectroscopy. Extensive pit depth measurements were made using a bright light optical microscope and differential focus technique. The results show that the 6060-grade aluminium alloy suffered pitting corrosion and intergranular corrosion simultaneously in the marine environments. Crystallographic corrosion morphology was observed in some of the extreme pits. The AlMgSi alloy samples were most resistant to corrosion under tidal condition but less resistant in the splash zone. The most aggressive environment was seawater immersion at 0.64m depth. A statistical analysis using extreme value theory indicates that after 2 years' exposure the pitting corrosion mechanism of the maximum pit may be different from that for shallower pits. The reasons for this and the implications are discussed.