Complex corrosion behavior of carbon steel in water-based drilling fluids: Normal and abnormal regimes

Corrosion of carbon steel in water-based drilling muds (WBMs) is typically controlled through pH adjustment, oxygen scavenging, and the use of film-forming corrosion inhibitors. In this study, unexpected corrosion behavior was observed in certain WBM formulations, where the addition of oxygen scavengers led to an increase in corrosion rate and a markedly reduced response to conventional inhibitors. The corrosion behavior of steel was systematically investigated using electrochemical measurements with a rotating cylinder electrode under controlled temperature and flow conditions. The effects of drilling fluid composition, barite type, pH, salinity, flow rate, dissolved oxygen concentration, corrosion inhibitors, oxygen scavengers, and steel metallurgy were evaluated. Two distinct corrosion regimes were identified in water-based drilling fluids: normal drilling fluids, which respond to conventional corrosion mitigation strategies, and abnormal drilling fluids, which exhibit accelerated corrosion behavior. Conventional WBMs exhibited the expected reduction in corrosion rate following chemical treatment. In contrast, abnormal WBMs showed accelerated corrosion associated with the formation of continuous or patchy zero-valent metallic copper films on the steel surface. The deposited copper originates from residual copper within the steel substrate rather than from drilling fluid solids. The presence of copper promotes galvanic coupling, significantly reducing the effectiveness of film-forming inhibitors. Although this mechanism has not yet been confirmed under field conditions, the findings highlight a previously unrecognized electrochemical corrosion mechanism in water-based drilling fluids and provide evidence of specific redox and interfacial conditions that can trigger abnormal corrosion behavior.