Localised electrochemical processes on laser powder bed fused 316 stainless steel with various heat treatments in high-temperature water

Laser powder bed fusion (LPBF) is an advanced additive manufacturing technology for stainless steel components fabrication, and a comprehensive understanding of electrochemical behaviour of the LPBF stainless steels is critical for expanding their applications in nuclear and other high-temperature water environments. In the present study, comparison of microstructure and high-temperature electrochemical behaviours between LPBF 316 and wrought 316 in simulated pressurized water reactor environment was made. Three heat treatments, stress relieving (SR), solution annealing (SA) and hot isostatic pressing (HIP), were used to heat treat the LPBF 316. The results showed that both the yield strength and impact energy of the SA and HIP treated LPBF 316 were lower than that of the SR treated sample, whilst the tensile elongation of the SA and HIP treated samples was higher than SR treated sample. These changes were found to be due to the disappearance of the cellular structures, decrease in the dislocation density and the occurrence of recrystallization during the treatments. Electrochemical impedance spectroscopy measurements at 288 °C and their interpretation with the Mixed-Conduction Model indicated that corrosion rate of the heat-treated LPBF 316 variants is significantly lower than that of the wrought 316. In addition, SR material exhibits marginally lower corrosion rates than SA and HIP ones. Microstructure examination after high-temperature water exposure revealed an inhomogeneous inner oxide layer on LPBF 316 in contrast to the thicker and more uniform inner oxide layer on wrought 316. The localised nature of electrochemical processes is suggested to be induced by the nano-precipitates in the LPBF samples.