Oxidation performance of repaired aluminide coatings on austenitic steel substrates

The repairability of slurry aluminide coatings on austenitic stainless steels (ASS) and their oxidation behaviour were investigated in this work. Slurry aluminide coatings were deposited on IN-800HT and HR3C substrates to obtain a B2-(Fe,Ni)Al phase. Chemical stripping of the coatings was thereafter conducted with diluted nitric acid. Dissolution occured at the interface between the diffusion and interdiffusion layers of the coating due to the penetration of the nitric solution through the tensile cracks of the coatings. The greater the NiAl content in such interface the greater the dissolution. This was proved by thermodynamic calculations and experimentally with model NiAl and FeAl. Realuminization of the stripped samples resulted in somewhat thicker coatings with similar microstructures as the fresh ones. The oxidation behaviour of the fresh and the repaired coatings was compared under isothermal conditions (650 °C and 700 °C for, respectively, the IN-800HT and HR3C alloys) in air for 2000 h. The loss of Cr from the surface of the coatings and the delayed transformation of the aluminas suggest that there was some chromium evaporation. However, the oxidation behaviour was similar on the new and the repaired coatings. The oxidation kinetics appeared to follow a parabolic law for the HR3C samples. The coatings grew very thin Al2O3 oxide scales at the coating/gas interface but some spallation occurred in IN-800HT. It thus appears that the slurry aluminide coatings can be repaired and supply protection to ASS in air at 650 and 700 °C.