Corrosion And Corrosion-Fatigue Properties Of AISI 304L Austenitic Stainless Steel Modified By Plasma-Based Low-Energy Nitrogen Ion Implantation

Austenitic stainless steels exhibit excellent resistance to corrosion, however, low hardness and poor wear properties have impeded their choice for diverse range of applications, including critical components in nuclear reactors. A high nitrogen face-centered-cubic phase ??N? with a max nitrogen contents of 25 at.% which was characterized by excellent corrosion properties and high hardness was obtained on the nitrided surface of austenitic stainless steel by plasma-based low-energy nitrogen ion implantation. In order to meet the urgent needs of application in nuclear reactors, the corrosion properties, especially corrosion-fatigue properties in the borate buffer solution with pH=8.4 should be further study, and the corrosion and corrosion-fatigue mechanisms of ?N should be further discussed.AISI 304L austenitic stainless steel was modified by using the plasma-based low-energy nitrogen ion implantation ?PBLEII? at a process temperature of 400? for a processing time of 4 h in order to improve the corrosion and corrosion-fatigue resistance of the austenitic stainless steel. A single high-nitrogen face-centered-cubic phase ??N? layer is about 12?m with a maximal nitrogen concentration of about 25 at.%was formed on the nitrogen-modified austenitic stainless steel. Passive films of ?N phase layer were characterized by using anodic polarization, electrochemical impedance spectroscopy, Mott-Schottky analysis in a borate buffer solutionwith pH= 8.4. The protective passive films on the ?N phase layer were obtained as n-type and p-type semiconductors in the potential range above and below the flat band potential, respectively. The semiconducting properties were corresponding to the two regions structure:the iron hydroxide/oxides in the outer region and the chromium hydroxide/oxides and iron oxides accompanying the chromium and iron nitrides in the inner region and with NH₃ were detected at the outmost layer of ?N phase layer and the metal bonds were detected at the outmost layer of original stainless steel by Auger Electron Spectroscopy and x-Ray Photoelectron Spectroscopy ?AES/XPS?. The ?N phase layer has a maximal phase angle of 83°, the passive film thickness of ?N phase layer and original stainless steel calculated by power law distribution relation were 2.82±0.67 nm and 2.97±0.34 nm, respectively. The donor and acceptor densities of the ?N phase layer decreased approximately from 6.6×10²¹ cm^-3 to 3.2x 10²⁰ cm^-3 and from 6.9 ×10²¹ cm^-3 to 4.6 X 10²⁰ cm-3, respectively. The flat band potential decreased approximately from-480 mV?SCE? to-640 mV?SCE?, relative to that of the original austenitic stainless steel. According AES analysis, the concentration of O in passive film formed on ?N phase is higher than that formed on original stainless steel. The passivation mechanism that the effect of N atoms in ?N phase layer on corrosion properties were proposed in borate buffer solution with pH value of 8.4.The corrosion-fatigue properties of the ?N phase layer on the austenitic stainless steel were examined by the push-pull fatigue experiments with a ratio R of tensile and compression of-1 in the borate buffer solution with pH value of 8.4. The ?N phase layer has an increased corrosion-fatigue strength up to 230 MPa from 180 MPa of the original austenitic stainless steel with an apparent increase of about 28%. The corrosion-fatigue crack initiation in the ?N phase layer was found as a controllable stage in the fracture process at the interface between the ?N phase layer and the austenitic stainless steel matrix with the arc corrosion-fatigue source. Some tiny corrosion-fatigue striations were obtained on the corrosion-fracture surfaces of the ?N phase layer. The high density of slip bands and dislocations in the ?N phase layer was able to prevent the crack initiation and propagation, leading to improvement of the corrosion-fatigue properties in the borate buffer solution. The fracture process in the ?N phase layer formed on the austenitic stainless steel was described as four stages:crack initiation, crack propagation earlier stage, ?N phase layer fracture, crack propagation later stage and ultimate fracture.