Understanding of localized corrosion behavior induced by composite inclusions with Ca, Mn, and RE-containing sulfide part of low-alloy steel in deaerated environments

• Localized corrosion induced by three kinds of inclusions with different sulfides part is studied. • CaS part serves as a micro-anode and results in the formation of a cathodically protected region around the inclusion. • Circular shallow pit forms around Al 2 O 3 -MnS and (RE)AlO 3 -(RE) 2 O 2 S inclusions because of the sulfide part serving as micro-cathodes. • Larger and deeper pits are formed around (RE)AlO 3 -(RE) 2 O 2 S inclusion as compared to that initiated by Al 2 O 3 -MnS inclusion. This study investigates influences of sulfide type within oxide-sulfide composite inclusions on localized corrosion behavior of low-alloy steel in deaerated soil solution. Three distinct inclusion types were identified: MgO·Al 2 O 3 -CaS, Al 2 O 3 -MnS, and (RE)AlO 3 -(RE) 2 O 2 S, exhibiting similar physical properties but distinct electrochemical behavior compared to steel matrix. MgO·Al 2 O 3 -CaS inclusion initiated corrosion through micro-galvanic coupling, with electrochemically dissolved CaS creating a circular protected region. Conversely, Al 2 O 3 -MnS and (RE)AlO 3 -(RE) 2 O 2 S inclusions induced circular shallow pits, attributed to that sulfide phases acted as micro-cathodes accelerating interfacial corrosion. Notably, RE-oxysulfide inclusions generated significantly larger pits (both diameter and depth) than MnS-containing counterparts, attributed to their larger dimensions enhancing galvanic corrosion effects. These findings establish critical relationships between inclusion chemistry/morphology and localized corrosion patterns.