Low-Temperature Oxy-Nitriding of AISI 304 Austenitic Stainless Steel for Combat Corrosion and Wear in HCl Medium
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TRODUCTION
AUSTENITIC stainless steels are widely used because of their high corrosion resistance, which results from a continuous and protective surface passive layer on the surface.[1] It has been reported that the passive film is only a few nanometers thick with iron and chromium hydroxide in the outer layer iron, compact chromium oxide in the inner layer.[2–5] The corrosion resistance of austenitic stainless steels is often restricted by its sensibility to local breakdown and pit nucleation.[6] Pitting is a form of corrosion that focuses on a small range of metal surfaces and penetrates the interior of the metal. The pits will nucleate and grow after some damage of the passive layer. Pitting occurs mostly on the surface of stainless steel with a narrow and deep inner diameter. Once in the stable pitting growth stage, it will automatically accelerate to the inside of the substrate, thereby causing local stress concentration or perforation of the material. Besides, the low hardness and poor wear resistance values of austenitic stainless steel impede their
LONGYI LI, JUN WANG, HONGYUAN FAN, and BO ZENG are with the School of Mechanical Engineering, Sichuan University, Chengdu 610065, P.R. China. Contact e-mail: [email protected] JING YAN is with the Research Institute of Natural Gas Technology, PetroChina Southwest Oil and Gas Field Company, Chengdu 610213, P.R. China. XIAOYING LI and HANSHAN DONG are with the School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. Manuscript submitted June 23, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
widespread application in conditions involving both mechanical and chemical attacks.[7,8] In the refining and petrochemical industries, low-alloy steel has high strength and is commonly used for bolts[9]; however, its low corrosion resistance limits its use. Austenitic stainless steel to which Cr and Ni are added has a markedly improved corrosion resistance. Duplex stainless steel has an austenitic-ferritic two-phase structure, and its strength and corrosion resistance of the acid-resistant medium is higher than that of austenitic stainless steel.[10] However, both of the above stainless steels have problems of low hardness and weak wear resistance,[1] which limits their further application. An efficient method to prevent steel from corrosion and consequently extend their life is the use of low-temperature nitriding/carburizing treatment. S-phase was originally derived from the experimental results of Ichii et al.[11] In the XRD results, in addition to the austenite peaks, there are five adjacent peaks with lower angles, which are not recorded in the ASTM standard, so they named it ‘‘S-phase.’’ In the process of LTON treatment, active N and C atoms were driven into austenitic stainless steel substrate under high chemical potentials, thereby forming a significant supersaturation of N(C) in its surface, which is known as S-phase. It is built by introducing interstitial atoms into the matrix of the face-centered cubic structure under low-t
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