Effect of calcium on X65 carbon steel pitting in saturated CO₂ environment

Micro-pits could propagate during in-service life in facilities such as crude oil storage tanks in which calcium salts are present. Calcium is known to influence pit initiation and propagation but explanation on pitting kinetics on low carbon steel remains debatable. More so, knowledge is lacking on how different CaCO3 polymorphs influence pitting in low carbon steel. The effect of calcium on pitting in UNS K03014 (X65) carbon steel was investigated using potentiostatic induced pits in an environment typical of crude oil storage tanks and liquid aquifers. SEM, EDX and XRD investigations were performed to profile formed corrosion products followed by surface profilometry to determine the extent of pit growth. It was revealed that Ca2+influence general and pitting corrosion while the kinetics of pit propagation depends on the nature of the calcium carbonate polymorph that forms on steel surface. Pit propagation was high when amorphous CaCO3 deposit formed at pH 3.8 but pit growth retarded as aragonite started to grow on the steel surface.