Effect of bainitic and retained austenite's length scale on the aqueous corrosion behaviour of nanostructured bainite in 3.5 wt% NaCl

Abstract

Microstructural refinement offers immense advantages in terms of enhancement of strength, hardness and wear resistance in carbide-free bainitic steels. Lath thickness in bainite can be changed while maintaining the similar volume fractions of bainitic ferrite (BF) and retained austenite (RA) by modifying the grain size of austenite by changing the austenitizing temperature. However, the effect of this microstructural refinement on the corrosion response is difficult to predict and has not been studied. Hence, the current study aims to study the corrosion response of carbide-free bainitic steels formed by austenitization at 900°C (NSB_900) and 1000°C (NSB_1000) respectively and austempered at 250°C. Finer BF lath thickness and RA was obtained in NSB_1000 while the volume fractions of both BF and RA were the same in both the blocks of steels. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) technique have been used to evaluate the corrosion behaviour of the two steel blocks. Lower corrosion current density was found for NSB_1000 which had a finer microstructure although corrosion potential was similar for both the specimens due to similar bainitic volume fraction. The side-section SEM of NSB_1000 showed a thinner corroded layer and the top surface showed a better connected and adhered corrosion film as compared to NSB_900 which showed the corroded layer as disconnected patches leading to a significant part of the sample still being exposed to subsequent corrosion. This manifested in NSB_1000 showing higher polarisation resistance, solution resistance, charge transfer resistance and higher tendency towards an ideal capacitive behaviour.