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 (Epit) and the Degree Of Reactivation (DOR) at each ageing temperature, respectively. The scatter in the measured Epit 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.