Effect of Microstructure of X-120 Steel on CO2 Corrosion Inhibition by 2-Mercaptobenzimidazole

The use of 2-mercaptobenzimidazole (2MBI) as corrosion inhibitor for API X-120 pipeline with two different microstructures in a CO2-saturated 3.5% NaCl solution has been investigated. One microstructure was predominantly martensitic and bainitic (steel A), whereas the second one was mainly ferritic (steel B). It was found that 2MBI behaved as a mixed type of corrosion inhibitor and it changed the corrosion-controlled mechanism. In the uninhibited solution, steel A had a dissolution rate higher than steel B. However, in the presence of inhibitor, dissolution rate for steel A was lower than that for B because the film-formed inhibitor is adsorbed in a better way for steel A which has smaller precipitates distributed in a more homogeneous way than that for steel B.