General corrosion behavior and mechanism of low-temperature plasma nitrided 17-4PH stainless steel in high temperature water

The general corrosion behavior and mechanism of low-temperature plasma nitrided 17-4PH stainless steel with and without a work-hardened layer in high temperature water were investigated. The general corrosion test was conducted under high temperature (280 C-degrees) and high pressure (13.3 MPa) circulating water conditions. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy were employed to reveal the underlying mechanism. The results showed that, for the non-nitrided specimens, high density of grain boundaries and dislocations in the work-hardened layer accelerated the formation of the inner oxide and the deposition of the outer oxide, which reduced the corrosion resistance. Besides, the increased surface roughness also enhanced the deposition of the outer oxide layer. For the nitrided specimen without a work-hardened layer, the nitrided layer was composed of expanded martensite. The compressive residual stress in the expanded martensite could inhibit element diffusion, thus reducing the thickness of the inner oxide layer and mitigating corrosion. While for the nitrided specimen with a work-hardened layer, the nitrided layer was mainly composed of gamma'-Fe4N twins, which induced intergranular corrosion and reduced the corrosion resistance. Besides, the diffusion of nitrogen from the nitrided layer into the solution during the corrosion process influenced the local H+ content in the solution, subsequently impacting the deposition of the outer oxide particles. In conclusion, the presence of a work-hardened layer is undesirable for plasma nitriding in improving corrosion resistance in high temperature water. These findings can provide valuable insights for the extension of the material's service life.