The effects of sulfides on the passivity of carbon steel in organic solutions

Corrosion is known to initiate at sulfide inclusion in aqueous solutions. The effects of sulfides on the passivity and the breakdown of passivity of 99.999% iron and 1018 carbon steel was investigated in two aprotic nonaqueous solvents: (1) propylene carbonate (PC) with LiClO4 or LiAsF6 as supporting electrolytes and (2) dimethoxyethane (DME) with LiAsF6 electrolyte. Carbon steel was also examined in PC-DME/LiAsF6 mixtures. Sulfides do not appear to interfere with passivation when it is provided by solvent adsorption in either nonaqueous solvent. Sulfides may lower the solvent oxidation potential and decrease the anodic potential at which breakdown initially occurs in either pure solvent. Sulfides also influence the kinetics of repassivation above the solvent oxidation potential of either solvent when passivation is provided by salt film formation or electropolymerization. The bleeding of sulfur or sulfide from inclusion sites where electropolymerization has occurred in PC/DME mixtures undermines attempts at passivation and promotes instability leading to anodic current increases with time and further corrosion of carbon steel. Published by Elsevier Science Ltd