Corrosion of Weld Heat-affected Zones Submersed in Seawater With Indigenous Microorganisms

The corrosion behaviors of different weld heat-affected zones (HAZs) were investigated, comprising an API 5L X65 steel immersed in seawater with an abundant native microbial community. Before tests, HAZ physical simulations were performed at the peak temperatures of 700°C, 860°C, 1000°C, and 1350°C. Throughout 14 days of immersion, various planktonic bacterial groups associated with microbiologically influenced corrosion were metabolically active in the seawater. Desulfovibrio members were present in a considerable number of OTUs. Scanning electron microscopy showed an intense microbial adhesion and corrosion product deposits in all HAZ sub-regions and the control base metal coupon (BM). The electrochemical tests revealed that the corrosion behavior of all HAZ sub-regions and BM were indistinguishable. However, topography analysis showed different behaviors of pitting corrosion. The results revealed that the pitting resistance of the microstructure composed of cementite grains (700°C HAZ) was superior to that of other HAZ sub-regions. On the other hand, the microstructure composed of martensite and austenite grains (860°C HAZ) exhibited the worst pitting resistance of the weld joint, leading to a maximum pit penetration of 2.3 times greater than that pit on the BM. The changes in the HAZ sub-region sensitivity to pitting corrosion were attributed to variations in the microstructural composition.