Summary

Gas turbine engines undergo a form of high temperature corrosion, called type 2 hot corrosion, when used in a marine environment. The parts most severely attacked are the metallic coatings used on the hot-section turbine blades. These coatings are often a mixture of cobalt, chromium, aluminum, and yttrium and, as a class, are identified by the acronym CoCrAlY. In several research efforts it has been found that increasing the chromium content of the coating to 30 or 40 wt% from the originally used 20 wt% dramatically improved the hot corrosion resistance of these coatings. It had been speculated that in the 20 wt% chromium coatings the oxide scales that form on these coatings are a mixture of cobalt oxide, chromium oxide, and aluminum oxide,,while in the AO wt% chromium coatings, the oxide scale was predominantly chromium oxide. This continuous chromia scale was thought to impede the hot corrosion initiation. In order to study the validity of this hypothesis, the goal of this research effort was to examine the critical protective oxide scales that form on actual coatings, as part of determining how chromium improves hot corrosion resistance. Using x-ray photo-electron spectroscopy, it was found that the protective oxide scales for CoCrAlY coatings containing nominal levels of 20, 29, and 35 wt% chromium were essentially the same. They were predominantly alumina and a yttrium rich phase tentatively identified as ytrium aluminum garnet•

The yttrium in the oxide scale can provide initiation sites common to all these coatings at which the hot corrosion-process begins. This corrosion of the yttrium could serve to mechanically disrupt the remaining alumina and thereby expose the bulk of the coating to the hot corrosion process. In any case, due to the similarity of the starting scale in all of these coatings, a logical conclusion is that the chromium provides its hot corrosion benefit in this coating system by slowing the propagation phase of the bulk coating attack.

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