Hot corrosion behavior of low pressure plasma sprayed NiCoCrAlY+Ta coatings on nickel base superalloys

The hot corrosion behavior of NiCoCrAlY+Ta coatings obtained by low-pressure plasma spraying has been investigated (type I hot corrosion with T = 8500C). These coatings have been deposited on two nickel-base superalloys and on a cast alloy of the same composition as the coating. Comparison of the cyclic oxidation behavior at 8500C between the sprayed coating and the cast alloy shows that the sprayed material exhibits a satisfactory and higher resistance than the cast alloy. In the latter case, repeated spalling of the oxide scale results from fractures often associated with Y-rich oxide protrusions. Hot corrosion experiments (salt-spraying test with T = 8500C and thermal cycling in air every hour) have been conducted on two cast alloys, NiCoCrAl+Ta and NiCoCrAlY+Ta. The Y-containing alloy has a better corrosion resistance than the other alloy which is subject to extensive oxide spallation. In both cases, morphologies typical of basic fluxing processes, involving Cr, Al, Y, and Ta can be observed in highly attacked zones. Finally, tests on plasma sprayed coatings show a high resistance to hot corrosion of these coatings, regardless of the substrate used: a fine adherent alumina layer is formed, and only a limited fluxing of Cr, Al, and Y takes place. The quite different results obtained on cast and sprayed alloys imply that data relative to cast alloys can be unreliable when one wants to predict high temperature behavior and especially lifetimes of overlay coatings