THE USE OF X-RAY TOMOGRAPHY IN THE STUDY OF INTERGRANULAR CORROSION ON AIRCRAFT ALUMINIUM ALLOYS

摘要

High-strength aluminium alloys have a microstructure that is highly susceptible to localised corrosion such as intergranular and pitting corrosion. The susceptible microstructure results from the thermo-mechanical processing given to these alloys to develop optimal mechanical properties. Intergranular and pitting corrosion are known to develop on many of the aluminium alloys used on Australian Defence Force (ADF) aircraft. However, there is little information available in the literature covering the conditions that cause atmospheric intergranular corrosion (IGC) and the resulting kinetics. Furthermore, the occurrence and extent of pitting corrosion is generally easily measured, whereas IGC is more difficult to identify and quantify. Historically, experimental investigation of IGC has required destructive, time consuming analysis with results that do not fully describe the extent of attack. Due to increases in computing power, computed X-ray tomography can now be used routinely in the laboratory for the study of metal defects such as corrosion damage. To support the ADF, Defence Science and Technology Organisation (DSTO) has undertaken an experimental program to understand the causality and propagation of IGC of 2024-T351 and 7050-T7451. This study has involved exposure of aluminium alloy specimens contaminated with salts to simulated atmospheric conditions. X-ray micro-computed tomography has been employed to measure the extent of IGC damage and provide visualisation of the networks of intergranular attack in 3-dimensions (3-D). The results showed that the depth and volume of intergranular corrosion increased with time in a 97% relative humidity environment, and also showed the depth and volume of intergranular corrosion increased with increasing relative humidity.