Mechanisms in the simultaneous erosion-oxidation attack of nickel and cobalt at high temperature
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I.
INTRODUCTION
IN some equipment or machines such as gas turbines, pressurized fluidized bed combustors, and incinerators, materials can be subjected to both erosive and corrosive processes concomitantly. The erosion of metallic systems and ceramics at ambient temperatures has been studied extensively ~-4 and, while many questions concerning the correlation of erosion properties and physical parameters remain unanswered, a very substantial and meaningful body of knowledge is available. 5'6 An enormous amount of work has also been performed on the high temperature corrosion of metals and alloys 7'8'9 and a smaller, yet still significant, amount of research into the corrosion of ceramic materials has been performed. 10., A large amount of data and substantial understanding therefore exists for the high temperature corrosion of materials at elevated temperatures. While significant research has been performed and understanding exists for room temperature erosion and high temperature corrosion processes, very little understanding is available for combined erosion-corrosion processes. Some studies have been performed 12-~9 and, although not greatly concerned with mechanism development, the results do show that when erosion and corrosion processes occur together there is a strong interaction. Examples are available that show: (a) nonprotective scales replace protective scales when erosive particles are present in the gas stream; (b) an oxide scale formed at elevated temperatures can cause metallic surfaces to become more erosion resistant; and (c) combined hot corrosion-erosion can proceed at rates greater than the sum of the rates for these two processes acting independently. A need now exists to consider different erosion-corrosion processes and to begin to describe, in a systematic way, the various interactions that may occur. The alloys or coating systems that are used in the erosivecorrosive environments that arise during operation of different machinery or equipment usually have rather complex compositions. It is therefore appropriate first to consider
C.T. KANG is Project Engineer, Union Carbide Coating Service, 1500 Polco Street, Box 24184, Indianapolis, IN 46224. E S. PETTIT, Professor and Chairman, and N. BIRKS, Professor, are with the Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh, PA 15261. Manuscript submitted March 25, 1986. METALLURGICALTRANSACTIONS A
pure metals and then to extend the investigation to related but more practical alloy compositions. This paper will present results obtained from experiments performed on nickel and cobalt as well as nickel oxide.
II.
EXPERIMENTAL
A schematic of the apparatus that was used to study the combined erosion-corrosion of metals in the present investigation is shown in Figure 1. The gas is cleaned and dried to remove traces of oil, water, and dust. The gas then flows through a fluid heater which preheats it to about 700 ~ and, at this point, particles are added to the gas stream. The gas for the particle laden stream is a
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