Low Temperature Hot Corrosion of Cobalt-Base Alloys: PartI. Morphology of the Reaction Product
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I.
INTRODUCTION
THE detrimental
effects of liquid Na2SO4 deposits on the lives of gas turbine components have been known for over twenty-five years. The mechanism of this corrosion reaction, commonly known as hot corrosion, has been extensively studied. This form of attack normally results in sulfidation with an aluminum and/or chromium depletion zone at the alloy (or coating)/scale interface. A few years ago, it was noted that gas turbines operating in marine environments exhibited rapid degradation of first stage CoCrAIY coated blades under low power conditions where the metal temperatures ( - 6 0 0 to 750 ~ were considerably below the melting point of Na2SO4 (884 ~ Also, the corrosion product had an unusual pitting morphology with little or no depletion zone of Cr or A1. Initially, this low power or low temperature hot corrosion was ascribed to the NaC1 particles ingested with the air, and a number of studies, 2-5 in our laboratory and at other places, were concerned with the effect of NaC1 on the Na2SOainduced corrosion. However, the morphology of attack caused by Na2SOa-NaC1 salts in laboratory tests is quite different from that found on the engine hardware. It should also be noted that because of the presence of SO3 in combustion gases NaCI should be converted to Na2SO4, and the residence time of NaC1 particles on the blade surface should at most be of the order of tens of milliseconds. 6'7 In view of these arguments, it seems unlikely that NaC1 can introduce the kind of attack observed on CoCrA1Y coatings under low power conditions. It is now believed that this corrosion occurs by formation of an Na2SO4-CoSO4 liquid which results from sulfation of cobalt oxides present on the surface of coatings. Indeed, water soluble cobalt was observed in the salt deposits found on the first stage blades, l Also, burner rig tests have been able to reproduce the pitting attack only when Na2SOa-containing deposits were used in conjunction with sufficient SO3 in the gas to stabilize a n N a 2 S O 4 - C o S O 4 liquid, s The importance of the sulfate-induced attack has also been confirmed in laboratory tests. 9'~~ Jones and Gadomski observed rapid attack of four cobalt- and nickel-
K. L. LUTHRA, Metallurgist, is with Corporate Research and Development, General Electric Company, P.O. Box 8, Schenectady, NY 12301. Manuscript submitted January 18, 1982.
METALLURGICALTRANSACTIONS A
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based alloys coated with mixtures of Na2SO4 and sulfates of Fe, Ni, Co, and Cu. 9 In another paper, we have outlined in considerable detail the conditions of temperature and Pso3 leading to the formation of Na2SOa-CoSO4 liquids. 10 Also, rapid attack of cobalt-base alloys was observed only when a liquid phase formed from an initially pure Na2SO4 deposit. In O 2 - 5 0 2 - S O 3 environments containing (SO2 + SO3) comparable to that in normal gas turbine conditions, Co-30Cr showed a maximum in corrosion at about 650 to 700 ~ which coincides with the temperature at which rapid corrosion rates have been observed in gas turbine conditions. Although
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