Corrosion mechanism of a NiCoCrAlTaY coated Mar-M247 superalloy in molten salt vapour

A mechanism of corrosion is proposed relating to a NiCoCrAlTaY coated Mar-M247 superalloy exposure in molten Na2SO4 salt vapour at 900 °C. The corrosion process indicates the parabolic “diffusion controlled” region I at rate of 4.6 × 10−6 mg2 cm−4 s−1, the combination of parabolic and linear “reaction–diffusion controlled” region II at rate of 2.28 × 10−5 mg2 cm−4 s−1 and 4.35 × 10−5 mg cm−2 s−1, and finally the complete “reaction controlled” region III at the same rate of 3.92 × 10−4 mg cm−2 s−1 as the uncoated superalloy. The corrosion kinetics of each region and of the whole process are formulated rationally associated with the corrosion mechanism.

► A novel three-stage corrosion mechanism is proposed. ► The corrosion kinetic mechanisms are formulated as a function of time. ► Life of the coating can be predicted related to the thickness and the corrosion rates.