The effect of electrochemically induced annealing on the pitting resistance of metastable austenite stainless steel
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I. INTRODUCTION
SOME austenite stainless steels, such as AISI304 and AISI321 steels, are metastable in ambient temperature. Martensite will appear in these steels under the proper conditions, such as in plastic deformation, cathodic hydrogen charging, and cooling after welding. Cold working and welding are inevitable processing methods for stainless steel parts, so the martensite in such steel parts is common. The appearance of martensite changes the single-phase state of the steels, so the physical and chemical properties of the steels also change. Since martensite and austenite have different corrosion potentials, it is easy for them to become the anode and cathode, respectively, of a corrosion battery. According to this theory, the corrosion resistance of metastable austenite stainless steel will decrease when martensite appears. Some reports verify this deduction. The researchers Cigada et al. consider that deformation-induced martensite is one of the main causes of the stress-corrosion cracking of these steels in a Cl-containing medium.[1] Hidehiko et al. advance the theory that the reason that martensite is preferentially dissolved is its high-density dislocations.[2] Kwok et al. reported that the pitting potential of martensite is about 550 mV lower than that of austenite in a martensite stainless steel,[3] so that martensite tends to be dissolved preferentially. Based on these facts and theories, it is natural to assume that eliminating or decreasing martensite will promote the corrosion resistance of metastable austenite stainless steels. At a relatively high temperature, martensite will transform to an austenite state.[4,5] However, austenite stainless steels ZHILIN LI, Professor, and JUNCAI QI, Graduate Student, are with the College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China. WEI LIU, Professor, is with the College of Chemical Engineering, Beijing University of Chemical Technology. Contact e-mail: [email protected] Manuscript submitted December 23, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
are widely used in the chemical and petroleum industries. The large parts used in those industries are difficult to heat to a high temperature. On the other hand, carbides, such as chromium carbides, may precipitate during high-temperature heating; this precipitation will decrease the corrosion resistance of the steels. Therefore, high-temperature heating is not a suitable method by which to promote the corrosion resistance of metastable austenite stainless steels with martensite. In 2000, Burstein and his colleagues discovered the electrochemically induced annealing (EIA) phenomenon.[6] Through this process, after obtaining martensite in AISI304 austenite stainless steel through grinding or rolling, the steel is treated with a series of anodic/cathodic pulses in an 8-M aqueous solution of sodium nitrite at 80 °C. After the treatment, the martensite in the steel surface decreases or disappears, but the steel still keeps t
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