Effect of ageing temperature on the microstructure and localised corrosion of 15–5PH stainless steel

The effect of microstructure on the localised corrosion resistance of 15–5PH stainless steel was investigated as a function of ageing temperature (450°C, 540°C and 650°C). Microstructural characterisation revealed an increase in reverted austenite content for samples aged at 650°C. Cu precipitates increased in size with ageing temperature. Cr carbides were observed in samples aged at 540°C, and in samples aged at 650°C with a greater size and number density. Potentiodynamic Polarisation (PDP) and Double-Loop Electrochemical Potentiokinetic Reactivation (DL-EPR) were used to determine the Pitting Potential (E pit ) and the Degree Of Reactivation (DOR) at each ageing temperature, respectively. The scatter in the measured E pit values was too large to confidently rank pitting susceptibility as a function of ageing temperature. However, it was found that the DOR increased with ageing temperature; this increase correlated with an increase in density of Cr carbides in the microstructure at an ageing temperatures of 540°C and the formation of reverted austenite at 650 °C. These heat treatment induced changes resulted in the distribution of Cr in the microstructure becoming less homogeneous as the ageing temperature increased, leading to a likely increase in the number of susceptible sites for localised corrosion by disruption of the integrity of the passive film. • The microstructure of 15–5PH was characterized as a function of ageing temperature. • Cr carbides form during ageing at 650°C and at 540°C but not at 450°C. • Higher temperature ageing promotes more austenite reversion and less localized corrosion resistance. • The integrity of the passive film was investigated with a modified DL-EPR test. • Lower Cr content in austenite and Cr carbides are responsible for reactivation.