High temperature corrosion of superalloys in an environment containing both oxygen and chlorine
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INTRODUCTION
H I G H temperature environments containing chlorine may be encountered in a number of important industrial processes. These include the preparation of vinyl chloride monomers, calcining and chlorination of metallurgical ores, the disposal of toxic and municipal wastes by incineration, and waste heat recuperation from industrial processes in which chlorine is used. The effect of chlorine as an accelerator of high temperature corrosion is well known, ~ and severe corrosion problems have been encountered in a number of these processes. In many cases, the economics of the process is dependent on the availability of suitably corrosion resistant structural materials. Selection of appropriate materials is difficult because very little design data are available on the corrosion behavior of metals in chlorine containing environments at temperatures above 500 ~ The superalloys are a class of highly alloyed metals which have been developed for use in severely corrosive and high temperature environments. In most industrial environments, superalloys are found to have better corrosion resistance than ordinary metals such as stainless steels. Superalloys have not been extensively tested in chlorine containing environments, however, and the unusual characteristics of corrosion by chlorine make it unreasonable to presume that materials which are resistant to corrosion in other environments will necessarily be corrosion resistant in environments which contain chlorine. In chlorinating environments, the corrosion products consist of chlorides, which generally have lower melting points and boiling points than the oxides of the same metals. 2 Metals which form protective scales in oxidizing environments often do not do so in chlorinating environments. When both oxygen and chlorine are present, a protective scale may form which reduces the rate of the chlorination attack, 3 a form of mixed corrosion may be observed in J. M. OH, formerly Graduate Student, Department of Civil Engineering, Mechanics, and Metallurgy, University of Illinois at Chicago, is Metallurgist, United States Bureau of Mines, Albany Research Center, Albany, OR 97321. M.J. McNALLAN is Associate Professor of Metallurgy, Department of Civil Engineering, Mechanics, and Metallurgy, University of Illinois at Chicago, P.O. Box 4348, Chicago, IL 60680. G. Y. LAI, Group Leader, High Temperature Alloys Group, Kokomo Technology, and M. F. ROTHMAN, Product Applications Manager, Heat Resistant Alloys, are with Cabot Corporation, 1020 West Park Avenue, Kokomo, IN 46901. Manuscript submitted October 7, 1984. METALLURGICALTRANSACTIONS A
which the rate of attack is higher than would be observed in either pure reactant, 45'6 or the corrosion may proceed at the same rate as it does in uncontaminated chlorine. 7 During mixed corrosion in oxygen-chlorine mixtures, the morphology of the oxide corrosion product often exerts a critical influence on the kinetics of the corrosion process, with rapid corrosion occurring when the oxide scale is porous or nonadherent. 8 Both c
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