Effect of austempering temperature on corrosion evolution during immersion of high carbon nano-structured bainitic steel in aqueous chloride environment

Abstract Nano-structured bainitic (NSB) steels have a dual-phase structure comprising carbon-rich retained austenite (RA) and bainitic ferrite (BF). These steels exhibit high strength as well as sufficient ductility and damage tolerance and hence can find potential application in marine areas. However, a systematic study of the corrosion behavior is required to assess the reliability and longevity of these steels for the above applications. The role of austempering temperature (250 and 350°C: specimens NB250 and NB350 respectively) and in turn the microstructural constituents on the corrosion response in aqueous chloride environment has been studied using immersion tests at different intervals of time (10, 20 and 30 days). Characterization of corrosion products has been done using microscopy and multiple spectroscopy techniques to reveal the attack mechanism and corroded layer composition. The corrosion rate, measured using weight loss experiments, decreases with a reduction in austempering temperature of the specimens and with an increase in immersion duration. Cross-sectional imaging of the corroded regions shows highly non-uniform but deeper penetration of corrosion with an increase in austempering temperature. In contrast, specimen transformed at the lowest temperature showed uniform and shallow penetration of corrosion. The corroded layer formed over NB350 after 10 days mostly comprised of oxyhydroxides but oxides took predominance after 30 days. However, NB250 showed mostly oxyhydroxides on the surface after 10 as well as 30 days. Hence, decreasing the austempering temperature while transforming austenite to nano-bainite not only provides higher strength but also greater corrosion resistance in saline environments.