Chlorine-induced high temperature corrosion of HVAF-sprayed Ni-based alumina and chromia forming coatings

Highlights: • HVAF-sprayed coatings were exposed to HCl atmosphere with and without KCl deposit. • Corrosion kinetics and mechanisms were studied. • The corrosion of NiCr was based on "electrochemical mechanism" at the beginning. • The corrosion mechanism was followed by "chlorine-active corrosion" process. • The NiAl coating showed much higher corrosion resistance than the NiCr coating. - Abstract: Chlorine-induced corrosion of HVAF-sprayed Ni21Cr and Ni5Al coatings was investigated in 5 vol.% O₂ + 500vppm HCl + N₂ with and without KCl at 600 °C up to 168 h. Both coatings were protective in the absence of KCl. With KCl, Ni21Cr degraded through a two-stage mechanism: 1) formation of K₂CrO₄ followed by diffusion of Cl⁻ through the oxide grain boundaries to yield chlorine and a non-protective oxide, and 2) inward diffusion of chlorine though defects in the non-protective oxide, leading to breakaway oxidation. Cl⁻/Cl₂ could not diffuse through the protective alumina scale formed on Ni5Al, hence the corrosion resistance increased.