Role of Metallurgy in the Localized Corrosion of Carbon Steel

Localized residual strain develops within the metallurgical texture of 1018 carbon steel from metallurgical processes, such as fabrication, annealing, and shaping. This residual strain results in accelerated localized pitting due to the formation of anodic sites at these locations. Once initiated, micron-sized corrosion pits can coalesce to form sites of potential catastrophic failure. In this contribution, we focus on the localized biocorrosion which initiates and grows in areas of localized strain such as the interfaces between manganese sulfide (MnS) inclusions and ferrite grains in the steel, at grain boundaries between ferrite grains with different crystallographic orientations and at pearlite grains (intergrown cementite (Fe3C) and ferrite), which are readily found in 1018 carbon steel. Here we hypothesize and show experimentally that accelerated biocorrosion in 1018 carbon steel finds its roots in the electrochemical potential difference (micro galvanic cells) generated between the unstrained ferrite iron (α - Fe) and the lattice defects, dislocations and mismatches found at interfaces formed between α - Fe and secondary phases i.e. MnS inclusions, cementite lamellar structures and grain boundaries distributed throughout the 3D network of the carbon steel. This hypothesis is supported by results from multiple micro- and nanoscale imaging and analytical methods obtained from field emission scanning electron microscopy, energy dispersive spectroscopy, electron backscattered diffraction and Auger nanoprobe electron spectroscopy. The morphology and composition of grains in the steel coupons were characterized before and after exposure to suboxic and sulfidogenic environments dominated by aerobic and anaerobic marine organisms. Corrosion processes are demonstrated to initiate in localized areas of high residual strain.