The Role of Silicon in the Protection Against Type I Hot Corrosion

Hot corrosion dramatically lowers the life of superalloy components by inducing pitting (Type II) or an extensive homogeneous attack (Type I), hence initiating cracks and decreasing the load bearing section. This degradation phenomena may occur in the coldest areas of high pressure turbine components, but they are commonly found in the low pressure turbine parts like in the DS200 + Hf nickel-based superalloy investigated in this work. Therefore, a Si-modified aluminide slurry coating was applied to the alloy, and its hot corrosion resistance was assessed under type I conditions with a Na2SO4 deposit at various temperatures and times. The comparison against simple slurry aluminide with and without Si and out-of-pack aluminide coatings revealed for the first time in the literature that Si ties up W in the coatings thereby impeding acidic dissolution, which greatly improves the corrosion resistance of DS200 + Hf superalloy. The W-free out-of-pack aluminide coating forms a protective alumina scale that delays the initiation of hot corrosion unless the coating is damaged. In contrast, the simple slurry aluminide coating without Si does not offer any protection in spite of the Cr segregation close to the surface.