Long-term pitting corrosion of 6060 aluminium alloy immersed in natural seawater

Aluminium alloys are widely used in maritime industries because of their high strength to weight ratio, ease of fabrication and expected corrosion resistance. However, they are susceptible to localised corrosion under specific corrosive environment. Further, information on the long-term corrosion characteristics of aluminium alloys under natural seawater immersed condition is scarce. Hence, this study reports a field investigation on pitting corrosion data of 6060 aluminium alloy immersed for two years in natural seawater with average annual temperature of 20^oC. An Optical Microscope was used to examine pit morphology and to measure pit depths. Cross-section microstructure and chemical composition of pits were investigated by means of Scanning Electron Microscopy and Energy Dispersive Spectrometry. Five deepest pits were measured on each face of a sextuplicate set of coupons. The pit depth data was analysed using extreme value statistics. Results show that the depth of the deepest pits progressed in a ‘step-wise’ manner. Pitting severity and the maximum pit depth increased with the depth of immersion. The results support previous findings indicating changes in corrosion mechanism with time. Similar to the corrosion of steels, this is considered to result from the build-up of corrosion products. The reason for this is discussed and further work is outlined.