The effect of environment on intergranular corrosion kinetics in aircraft aluminium alloys

Sub-surface corrosion, known as intergranular corrosion, is a significant cause of structural damage in ageing civilian and military aircraft. The cost associated with the downtime and maintenance of corrosion-related failures is estimated in the order of tens of millions of dollars per annum for the Australian Defence Force alone. In order to reduce this cost, a knowledge base on how corrosion initiates and propagates is being developed to improve maintenance practices. The prediction of when corrosion will occur for aircraft alloys in service, such as AA2024-T351 and AA7050-T7451, requires kinetic data of intergranular corrosion under thin-layers of moisture (containing salts), and exposed to a range of humidity. Synchrotron X-ray tomography is a non-destructive and relatively new technique to visualise and quantify the extent of sub-surface corrosion attack. This technique supersedes traditional methods which involved painstaking metallographic sectioning of materials, which revealed only limited information. The present computed tomography study showed depth and volume of intergranular corrosion increased with increasing relative humidity and time. Furthermore, in situ studies were useful in determining the relationship between microstructure, and corrosion initiation and propagation. Environmental effects, such as the time-of-wetness, and the concentration of salt and dissolved oxygen, on intergranular corrosion kinetics and morphology are examined.