Effect of pH on Semiconducting Property of Passive Film Formed on Ultra-High-Strength Corrosion-Resistant Steel in Sulfu
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ULTRA-HIGH-STRENGTH corrosion-resistant steels are generally developed for a combination of ultra-high-strength and high corrosion resistance, at the base of ultra-high-strength steels [i.e., 300M, AerMet 100 (Carpenter Technology Corporation, Wyomissing, PA), and AISI 4340 steel].[1,2] With yield strengths higher than 1380 MPa, these steels are usually used in aircraft components requiring high strength, high fracture toughness, and exceptional resistance to stress corrosion cracking and fatigue.[3,4] As a martensitic corrosion-resistant steel, the microstructure consists of martensite, carbides, and d-ferrite. The properties of the martensitic steel are strongly influenced by heat treatment conditions (austenitizing temperatures and times). According to Choi et al.[5] and Isfahanya et al.,[6] the corrosion resistance of the martensitic steels mainly depends on the extent of solution of carbides and alloying elements in the austenite. The better corrosion resistance could be attributed to the decrease of Cr-rich M23C6 carbide at the higher austenitizing temperature, which acted as preferential sites for pitting corrosion. MIN SUN, Postdoctor, is with the School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, P.R. China, and also with the Department of Materials Science, Fudan University, Shanghai 200433, P.R. China. Contact e-mail: [email protected] KUI XIAO, Associate Research Fellow, CHAOFANG DONG and XIAOGANG LI, Professors, are with the School of Materials Science and Engineering, University of Science and Technology Beijing. PING ZHONG, Research Fellow, is with the Beijing Institute of Aeronautical Materials, Beijing 100095, P.R. China. Manuscript submitted September 10, 2012. Article published online July 12, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
Previous research[7,8] has suggested that passive films formed on stainless steels can effectively protect materials from corrosion in solutions. Corrosion resistance and semiconducting properties of passive films, composed of an iron-rich outer layer and a chromium-rich inner layer, are strongly affected by chemical composition and film thickness. Wallinder et al.[9] studied the electrochemical behavior and chemical composition of the passive film formed on 316L stainless steel by passivation treatment using EIS and XPS. The results showed that protective property of the oxide film increased the corrosion resistance due to a high Cr content in the passive film and increased film thickness. Haupt and Strehblow[10] confirmed that passivation of Fe-Cr alloys in 0.5 M H2SO4 leads to a pronounced increase of Cr within the passive film. The Cr stabilized the passive layer and reduced the passive current density, as observed by its slow dissolution rate. Duplex stainless steels generally have better resistance to localized corrosion and stress corrosion cracking than many austenitic stainless steels. The properties of the duplex stainless steels originate from a mixture of austenitic and ferritic phases.[11] However,
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