Corrosion behavior and mechanism of Cr–Mo alloyed steel: Role of ferrite/bainite duplex microstructure

This study presents a systematic investigation of the corrosion behavior and corrosion mechanism of a novel ultralow-carbon Cr–Mo alloyed ferrite/bainite duplex steel rebar. Electrochemical tests revealed the studied steel rebar to have satisfactory corrosion resistance in an NaCl medium, similar to 430 stainless steel and better than 403 stainless steel. The formation of a Cr-dominated bilayer-structured oxidation passive film endows the steel with excellent passivity. During long-term exposure, the Cr–Mo alloyed steel presented no tendency of pitting corrosion. Meanwhile, the formation of a Cr–Mo enriched rust layer inhibited penetration of the corrosive medium, efficiently delaying corrosion propagation in the steel matrix. Given its excellent anticorrosion performance, this novel alloyed steel rebar has great application potential in marine concrete structures. Electron probe microanalyzer, transmission electron microscope, and electron backscattered diffraction analysis of the rebar matrix revealed the bainite phase to have lower Cr–Mo content but more crystal defects than the ferrite phase, leading to the lower potential and higher corrosion tendency of the bainite phase. Microgalvanic corrosion between the bainite phase (anode) and ferrite phase (cathode) caused prior breakdown of the passive film on the bainite phase, stimulating corrosion initiation of this Cr–Mo alloyed duplex steel.