Dissolution kinetics of the sulfide-oxide complex inclusion and resulting localized corrosion mechanism of X70 steel in deaerated acidic environment

Dissolution kinetics of the sulfide-oxide complex inclusion and its surrounding matrix of X70 steel in deaerated acidic environment was investigated. The dissolution process can be divided into three stages including the initiation stage, galvanic corrosion stage, and the corrosion propagation stage. Chemical dissolution of CaS generates H2S and deposits, providing the condition for the formation of the galvanic couple in which the occluded micro-pits serve as anode and the surrounding matrix serves as cathode. The presence of CaS and appropriate electrolyte chemistry are indispensable for the formation of galvanic couple, of which the cathode/anode area ratio is about 50.