Corrosion Performance of 700 Series Stellite Alloys in Various Media
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JMEPEG https://doi.org/10.1007/s11665-019-04303-9
Corrosion Performance of 700 Series Stellite Alloys in Various Media K. Kamal, Y.P. Ding, R. Liu, J.H. Yao, and M.X. Yao (Submitted January 22, 2019; in revised form June 4, 2019) The corrosion performance of 700 series Stellite alloys including Stellite 706, Stellite 712 and Stellite 720, which are high-carbon high-molybdenum Stellite alloys, in various media such as Green Death solution, 3.5 wt.% NaCl solution and morpholine solution with pH 9.5, is studied using electrochemical and immersion methods. The obtained polarization curves are analyzed along with the corroded surfaces examined using SEM/EDS to investigate the corrosion mechanisms of these alloys. The experimental results reveal that the 700 series Stellite alloys possess excellent corrosion resistance in the tested media, even in the severe corrosive condition—Green Death solution at 60 °C where no noticeable pitting is found on the surfaces of the alloys. It is shown that the corrosion resistance of these Stellite alloys depends strongly on the behavior of the passivation film formed on the alloy surfaces. Additional Mo content in these alloys benefits the stabilization of the passivation film thus enhances the corrosion resistance of the alloys. Both carbon and molybdenum contents affect the passivation film formation and stabilization. Keywords
electrochemical corrosion, molybdenum, oxide film, pitting corrosion, polarization, stellite alloy
1. Introduction A family of cobalt-based alloys, designated as Stellite alloys, display exceptional performance in resisting severe wear, corrosion and high-temperature degradation (Ref 1). The main alloying elements of these alloys include chromium (Cr), tungsten (W) or molybdenum (Mo) as well as a certain amount of carbon (C). The chief difference among the individual Stellite alloys is their carbon content, resulting in different carbide volume fraction within the alloys. Stellite alloys can conventionally be categorized as high-carbon alloys (> 1.2 wt.%) designed for wear service; low-carbon alloys (< 0.5 wt.%) for high temperature service and to combat corrosion or simultaneous corrosion and wear (Ref 2). Chromium has multiple functions in Stellite alloys. It is the predominant carbide former, provides oxidation resistance and strengthens the matrix as a solute (Ref 3). Tungsten and molybdenum in Stellite alloys have similar function present in the matrix to provide additional strength due to their large atomic size. Also, when present in large quantities, they participate in the formation of carbides during alloy solidification. Molybdenum can improve general and localized corrosion resistance of Stellite alloys, particularly in non-oxidized acids (Ref 4).
K. Kamal and R. Liu, Department of Mechanical and Aerospace Engineering, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada; Y.P. Ding and J.H. Yao, Institute of Laser Advanced Manufacturing, Zhejiang University of Technology, No. 288 Liuhe Road, Hangzhou 310023, China; and M.X. Yao, Ken
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