Effect of microstructure in relation to heat affected zone (HAZ) corrosion of mild steel welds in marine environments

Welded joints in mild steel components are used widely in structural assemblies and infrastructure such as pipelines. It is well known that the weldment region exhibits less corrosion resistance than the parent material. However, discussions remain about the specific mechanics causing the phenomenon. In order to better understand the dominant factors that drive the preferential corrosion effect this paper aimed to investigate the effect on corrosion rates of the microstructural variations in mild steel welds inherently produced in the Heat Affected Zone (HAZ) by simulating the various regions in the HAZ and independently assessing their corrosion rates when immersed in marine waters. Samples from a normalized fine grained mild steel pipe were heat treated to produce a coarse grain microstructure, a spheroidised microstructure, and a quenched and tempered microstructure. Samples of the parent and heat treated microstructures were immersed in natural 20oC seawater. Results are presented and implications discussed as well as the possible influence of the decarburization layer commonly present on hot rolled mild steel products.