Effect of experimental procedures on the cyclic, hot-corrosion behavior ofNiCoCrAlY-type bondcoat alloys

A simplified test procedure was established to assess the hot-corrosion beh avior of MCrAlY-type nickel-base alloys under the influence of molten sodiu m sulfate as well as sodium sulfate/potassium sulfate salt blends. Salt-coa ted specimens were exposed to 1-hr thermal cycles at 950 degrees C in flowi ng oxygen for rip to 500 cycles. Mass-change data of the specimens revealed a significant dependence of the corrosion attack not only on the average c ontaminant flux: rate, as expected but also on the initial amount of salt d eposited during each recoating cycle. Furthermore, deposit removal before s alt recoating markedly influenced the corrosion attack of the alloys. This was apparently related to changes in salt chemistry by the dissolution of e lements such as Cr from the alloy, which can shift the basicity of the salt and thus affect the extent of attack. Substituting Na for K in sodium sulf ate/potassium sulfate salt blends generally resulted in decreased attack. F or K-containing salt deposits, increasing the gross amount of alkali compar ed to sulfur resulted in increased sample Ir,eight losses due to scale spal lation. In contrast, decreasing the amount of sulfur in such deposits which contained exclusively Na as the alkali resulted in a significantly reduced corrosion attack compared to stoichiometric sodium sulfate.