Low Temperature Hot Corrosion of Cobalt-Base Alloys: Part II. Reaction Mechanism
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I.
INTRODUCTION
SoDIUM-sulfate-induced corrosion, commonly known as hot corrosion, is a major factor limiting the lives of many parts exposed to hot combustion gases in gas turbines. Until recently, it was widely believed that severe corrosion problems occurred only at high temperatures where liquid salt mixtures containing Na2SO4 (mp = 884 ~ deposited on the surface. However, this belief was destroyed when turbines operating in marine environments showed severe degradation of cobalt-base CoCrA1Y coatings at lower temperatures ( - 6 0 0 to 750 ~ where Na2SO4-containing salts depositing on the surface of blades would be expected to be solid. This corrosion behavior, frequently called lowtemperature hot corrosion, has been attributed to the formation of a low-melting (mp = 565 ~ Na2SO4-CoSO4 eutectic. 1.2The mechanism of this corrosion is the subject of this paper; the relevant experimental studies were described in Part 1. 3 The mechanism of hot corrosion has been the subject of a number of studies, 4-17 which have been extensively reviewed recently by Stringer. 18However, most of these studies have been concerned with corrosion at temperatures above the melting point of Na2SO4. The most frequently cited theory is a fluxing model, proposed initially by Bornstein and DeCrescente 5 and further extended and developed by Goebel and Pettit. 6,7According to this theory, the rapid degradation of alloys and coatings results from an acidic or basic dissolution of protective oxide films on the surface, which for most industrial alloys and coatings consist of CrzO3 and/or A1203. These oxides may subsequently precipitate in the liquid salt, forming porous oxide scales. Basic fluxing or dissolution was proposed to result from a local increase in the oxide ion concentration in Na2SO4 as a result of formation of internal sulfides near the scale/salt interface. Acidic fluxing was proposed to result from a local increase in the salt acidity by removal of oxide ions in the salt through the formation of basic complexes (e.g., VO~ or MoO 2-) with other components in the substrate or salt. K. L. LUTHRA, Metallurgist, is with Corporate Research and Developmeat, General Electric Company, P.O. Box 8, Schenectady, NY 12301. Manuscript submitted January 18, 1982.
METALLURGICAL TRANSACTIONS A
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Recently, Rapp and Goto extended the concept of fluxing further and suggested that for hot corrosion to occur the gradient in the solubility of protective oxides should be negative at the oxide/salt interface. Then, the oxide can dissolve at the oxide/salt interface and precipitate some distance away from it in a region of lower solubility, and corrosion can occur indefinitely. Although the fluxing theory, as proposed, is consistent with the corrosion behavior of many materials at high temperatures, it does not appear to be applicable to low temperature hot corrosion of cobalt-base alloys. 37 We have recently shown that a minimum Ps03 of the order of 10 -5 atm is required to stabilize an Na2SO4-CoSO4 liquid salt below the melting point of Na2SO4.
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