Corrosion of welded high strength steels immersed in seawater

Corrosion of welded steel joints poses significant costs and operational risks to all steel hulled naval vessels. Although corrosion of welded regions is generally higher than parent metals, there is little current knowledge of the extent of unprotected corrosion expected for the high strength steels and welding methods currently in use or being proposed for the Royal Australian Navy (RAN). In order to better characterise the extent and types of corrosion to be expected in operation, this paper describes the methodology for a multi-year, multi-site, open seawater corrosion trial of welded high-strength ship steels. Four high strength steels and two welding methods used for naval vessels have been used to create standardised coupons exposed in three sites around Australia with different water temperatures and pollution levels. All coupons were scanned using a high-resolution laser scanner before exposure. Following retrieval, all coupons are carefully cleaned to remove marine growth and corrosion products, then weighed to determine total mass loss, visually examined, and laser scanned again. The two laser scans will be compared using a software routine developed to provide corroded surface topography across the parent metal and weld surfaces to allow identification of large populations of pits, including the deepest pits. Optical microscopy is also being performed to allow accurate measurements of pits, which could be associated with Microbiologically Influenced Corrosion.