In-service degradation and life prediction of coatings for advanced land-based gas turbine buckets

Abstract

In-service degradation mechanisms of over-aluminized MCrAlY duplex coatings have been determined by evaluating two long-term service-run blades. Cyclic oxidation tests were also conducted to characterize the degradation of the duplex coatings, GT29+ and GT33+. The results show that degradation of duplex coatings is manifested by the formation of oxide scale, the transformation of β-phase into γ, the coarsening of β-phase particles in the MCrAlY coating, and the enlargement of the interdiffusion zone. Considering all of the degradation mechanisms observed in these service-run coated buckets, a coating life prediction model, developed under an EPRI-funded program, was used to estimate the life of GT29+ and GT33+ coatings. A comparison of the model calculations and experimental data indicated that, conservatively, the useful life of duplex coatings can be predicted solely based on the Al content in the top aluminide coating. The predicted value of the coating life is increased by a factor of 1.4 to 2 if MCrAlY is included in the lifetime estimation.