Kinetics of chlorination of Co and Co-10 At. Pct Pt alloy by reaction with HCI gas
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I.
INTRODUCTION
LEACHING,dealloying, or more specifically, the preferential removal of one or more components from an alloy, readily occurs in gaseous environments at elevated temperatures just as it does in aqueous solutions. The phenomenon is a necessary step in several extractive metallurgical processes and in the production of catalysts and porous bearings. The loss of Cr from iron, nickel, or cobalt base alloys, which are designed for elevated temperature performance, is also well known. The aggressive "environments" include vacuum, chlorine and other gases, molten salts, and liquid metals. ''2'3 Fruehan I emphasizes the importance of the formation of volatile chlorides in several commercial processes, including the Kroll 4 process for the production of titanium; the use of chlorine as the chlorinating agent in the extraction of Ta, Cb, and W from complex ores, s in the treatment of manganiferous iron ores, 6 and in the detinning of tin plate; 7 and the use of HCI gas in the treatment of low grade iron ores s and in segregation roasting process for Cu 9 and nickel recovery from laterite ores. ~0 In spite of this widespread importance, there are relatively few fundamental studies which investigate the kinetics of the chlorination of pure metals l'~'-15 or alloys. ~6.17Of these, the detailed investigations by Fruehan are the most illuminating for the purposes of this paper. He investigated the rates of chlorination of Fe (528 to 921 K), Ni (890 to 1249 K), and Sn (340 to 396 K) in C12-He and CI2-Ar gas mixtures.~ In these investigations the overall reaction products were gaseous (FeC13)2, NiC12, and SnC14, respectively. Mass transport of C12 through the gas boundary layer largely determined the rate of the reaction for iron (>680 K), nickel (>993 K), and tin (340 to 396 K) in CI~-He and C12-Ar mixtures. At lower temperatures transport of NiCl2(g) in the gas boundary layer was rate deterY, S. KIM, formerly Research Associate, Department of Materials Science and Engineering, The Pennsylvania State University, University
Park, PA 16802, is now with AVCO Corporation-Lycoming Division, Williamsport, PA 17701. H.W. PICKERING is Professor of Metallurgy, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802. Manuscript submitted January 28, 1982. METALLURGICALTRANSACTIONS B
mining in the case of the Ni-C12 reaction, and a slow surface reaction between C12(g) and a surface layer of solid FeC12 controlled the rate in the case of the Fe-CI2 reaction. In a later work, t~ Fruehan and Martonik measured the rates of chlorination of iron (810 to 1175 K) and nickel (850 to 1340 K) in HCI(g) and Ar(g) or He(g) gas mixtures. They also found that the rates are controlled by transport through the gas boundary layer at the surface of the sample. At low temperatures (< 1000 K) and high contents of HCI in the gas mixture the steady state weight loss is controlled by diffusion of metal chlorides through the gas boundary layer, and the temperature dependence of the rates is esse
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