Suitability of aluminizing for protection of nichel superalloys turbine blades in oxidative and corrosive environments

Three kinds of turbine blades (made by casting Renè 77, Renè 80 and CMSX-4 nickel based alloys) were submitted to an above pack aluminizing treatment. The suitability of this surface coating for preventing high temperature oxidation and hot salt corrosion was investigated. Oxidation tests were performed in air at 1050°C for 500 and 1000 h; salt corrosion at 950°C was studied by using a NaCl+Na2SO4 mixture. The microstructure of aluminized blades before and after these tests was compared by X-ray diffraction, scanning electron microscopy and electron probe microanalysis. The composition of both coating and metal substrate was assessed in terms of phases present and concentration profiles of chemical elements. The oxidation treatments merely changed the structure of the more external part of the aluminide diffusion coating: a thin alumina layer (4-5μm thick) formed and the consequent aluminium depletion in the β (Al-Ni type) phase resulted in the formation of the intermetallic γ' (Ni3Al type). As oxidation treatments involved long-term exposition at high temperature the metal substrates underwent variations in microstructure too. Grain growth of γ matrix and grain boundaries segregation of precipitates with a high content of Cr, Ti, W and Mo were observed to occur in Renè 77 and Renè 80 alloys. Coarsening and coalescence of γ' were always observed. The aluminide diffusion coating suffered of strong degradation due to hot salt corrosion: reaction zones randomly placed and cracks were observed in all the parts of surface layers after the treatment with molten salts. Moreover, detrimental reactions affected the metal substrate too in the case of Renè 77 and Renè 80 alloys, while the CMSX-4 blade matrix was not damaged.