Metallurgical investigation of cracked Al–5.5Zn–2.5Mg–1.5Cu aluminium alloy valve

(in lingua inglese)

The component was anodised more than five years ago and after qualification keptin store. The presence of numerous pits on the valve face, especially nearby cracked region was indicative of anodised layer inability to prevent corrosion. The anodised layer was not intact throughout. There were many locations all along the crack, indicating devoid of anodised layer. It seems as if the anodised layer was mechanically damaged/or deteriorated due to long storage period, which facilitated corrosion to take place. In fact, under such condition the anode to cathode area ratio will be drastically low, resulting in very high anodic current density and in order to balance during the electro chemical reactions, the corrosion will be severe.