Sea-salt-induced moderate-temperature degradation of thermally-sprayed MCrAlY bond-coats

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Metallic NiCoCrAlY bond-coats or overlay coatings are commonly used to protect superalloy components in the hot-section of shipboard gas-turbine engines. However, the accelerated degradation (or hot-corrosion) of such bond-coats at modest temperatures (~850 °C) in the presence of impurities such as sea salt from the marine environment is a serious concern. Here, the interplay between the microstructure, presence/absence of sea salt or pure NaCl, heat-treatment duration, atmosphere and pre-oxidation, and the resulting degradation of thermally-sprayed NiCoCrAlY bond-coats is studied systematically. The degradation in the presence of sea salt or pure NaCl entails the formation of a two-layer oxide, where Cr-rich oxide is on top and Al-rich oxide is at the bottom. The mechanisms behind this two-layer oxide formation are investigated, which appear to be a combination of: (i) preferential formation of Al-rich oxide first, (ii) initial fluxing of the Al-rich TGO by the molten sea salt, and (iii) upward diffusion of Cr from the bond-coat through the Al-rich oxide layer. It is also shown that a modest pre-oxidation heat-treatment can be an effective way to mitigate sea-salt-induced degradation of NiCoCrAlY bond-coats.