Effect of oxygen doping on the corrosion behavior of nanocrystalline coating under the synergy of solid NaCl deposit and water vapor

Abstract

Nanocrystalline coating with the same chemical composition as the nickel-based superalloys was proved to possess high resistance to oxidation and scale spallation, meanwhile avoid notable elements interdiffusion. It opens up a new direction for the development of protective coatings. However, in a worse environment containing chlorine and water vapor, the ultrafine grains act like a double-edged sword, which can promote the formation of a protective scale of Al2O3 or Cr2O3, but also favor the inward diffusion of Cl and H2O to accelerate corrosion. In order to solve this problem, moderate amount of oxygen was doped into nanocrystalline coating during magnetron sputtering. Then, corrosion behavior of the two nanocrystalline coatings with or without oxygen doping was studied in the O2 + 38% H2O environment with solid NaCl deposit. Results indicated that the doped oxygen can attract active elements of Ti and Al to form TiO2 and θ-Al2O3, which pinned at grain boundaries to keep the ultrafine grains stable at high temperature while prevent inward diffusion of the corrosive media. Accordingly, a dense and pure chromia scale was formed quickly at surface, ensuring that the oxygen-doped nanocrystalline coating provided the highest corrosion resistance under the synergy of solid NaCl deposit and water vapor. In contrast, the coating without oxygen doping suffered from severe internal corrosion.