Browsing by Author "Chang L."
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Item Corrosion of 316L stainless steel produced by laser powder bed fusion and powder metallurgy in pressurized water reactor primary coolant(2024-05-01) Bojinov M.; Chang L.; Saario T.; Que Z.Corrosion mechanism of 316 L stainless steel produced by laser powder bed fusion-hot isostatic pressing (LPBF-HIP) and powder metallurgy-hot isostatic pressing (PM-HIP) is studied with in-situ electrochemical impedance measurements coupled to detailed oxide film characterization. Quantitative analysis of impedance spectra using the Mixed-Conduction Model and estimation of local kinetic and transport parameters by interpretation of in-depth elemental composition profiles indicated lower corrosion and oxidation rates of LPBF-HIP and PM-HIP materials in comparison to conventional wrought 316 L. This owes to a higher fraction of low-angle grain boundaries, smaller grain size, the presence of nano-sized oxide particles and elevated Cr and Ni contents.Item Effect of hydrogen on electrochemical behavior of additively manufactured 316L in pressurized water reactor primary water(2023-11-01) Bojinov M.; Saario T.; Ge Y.; Chang L.; Que Z.The electrochemical behavior of laser powder bed fusion (LPBF) 316 L stainless steel subject to different heat-treatments (solution annealing and hot isostatic pressing) is compared to nuclear-grade wrought 316 L in pressurized water reactor primary water at 288 °C (with and without dissolved hydrogen) using current-time transients, cyclic voltammetry and electrochemical impedance spectroscopy (EIS). Analysis of spectra by the Mixed-Conduction Model revealed slower corrosion rates of LPBF 316 L than wrought 316 L, the effect being more pronounced in the presence of dissolved hydrogen. The characteristics of the barrier layer and the oxide film/coolant interface were irreversibly altered upon removal of dissolved hydrogen.Item Localised electrochemical processes on laser powder bed fused 316 stainless steel with various heat treatments in high-temperature water(2022-12-01) Que Z.; Chang L.; Saario T.; Bojinov M.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.Item Mechanistic understanding of the localized corrosion behavior of laser powder bed fused 316L stainless steel in pressurized water reactor primary water(2024-01-01) Ge Y.; Chang L.; Bojinov M.; Saario T.; Que Z.The laser powder bed fused (LPBFed) stainless steels showed anomalous and localized corrosion behavior in the nuclear reactor high-temperature water compared to their wrought counterparts, which affects their performance during plant operation. In this study, advanced microstructural characterization was performed on LPBFed 316 L sample along with wrought 316 L sample after corrosion tests to understand the underlying mechanisms. The results showed that an inhomogeneous/discontinuous inner oxide layer formed on LPBFed 316 L, in contrast to the continuous inner oxide layer on the wrought 316 L specimen. This discontinuous inner oxide layer was identified to consist of Cr-enriched nano-sized spinel oxide and the barrier layer features a Ni-enriched hexagonal close-packed Laves phase. Localized/preferential oxidation was found to occur along the cellular walls which were tangled with high density dislocations and decorated with Mn and Si-enriched nano-sized precipitates, and the nano-precipitates were observed in the core of dispersed Cr-enriched inner oxide crystals.