Passivity Breakdown and Crack Propagation Mechanisms of Lean Duplex (UNS S32001) Stainless Steel Reinforcement in High Alkaline Solution Under Stress Corrosion Cracking

Abstract

The passivity breakdown and subsequent stress corrosion cracking (SCC) of Type 2001 lean duplex stainless steel (UNS S32001) reinforcement were investigated in a highly alkaline environment containing chlorides at a low temperature. Electrochemical analysis and mechanical testing were utilized to characterize the passive film development. Fractographic analysis was performed, correlating microstructure and corrosion performance, to reveal preferential crack paths. A chloride threshold below 4 wt% Cl− for a high alkaline environment was elucidated, with pitting susceptibility factor values close to unity, having a threshold critical areal cation vacancy concentration for passivity breakdown close to the 1013 cm−2. Pit initiation leading to passivity breakdown and crack nucleation in 4 wt% Cl− was triggered for stresses above σy, developing a low-frequency peak (0.1 Hz to 0.01 Hz) of the cracking process. Current peak deconvolution demonstrated passivity breakdown was triggered by the intensification in the rate of Type II transient and exposure time, while an increase in transient amplitude was related to the crack propagation. The α phase served as a nucleation site for pits, whose propagation was arrested at the γ phase. Predominant intergranular-SCC morphology through the α/γ interface was developed following anodic dissolution given the more active nature of the α phase (most active path); minor transgranular-SCC propagated through γ phase when high-stress concentration was reached, corresponding to slip-step dissolution.