Hot corrosion and electrochemical behavior of NiCrAlY, NiCoCrAlY and NiCoCrAlYTa coatings in molten NaCl-Na2SO4 at 800 °C

The NiCrAlY, NiCoCrAlY and NiCoCrAlYTa coatings were prepared on a Ni16Cr13Co4Mo alloy by the D-gun spray technique. The hot corrosion behaviors of the coatings were investigated in molten NaCl-Na2SO4 at 800 °C through electrochemical measurements including potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and Mott-Schottky plots. The corrosion layers of the coatings were characterized by electron probe micro-analysis (EPMA), auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The results showed that all the corrosion layers of the three coatings displayed the electronic structure of n-p heterojunction. However, the NiCoCrAlY coating had the slowest corrosion rate because Co reduced the carrier density of the corrosion layer and improved the corrosion layer resistance (Rl). The Co sulfides effectively inhibited the internal sulfidation. Ta ion could occupy the Al2O3 lattice and increase the carrier density of the corrosion layer. The hot corrosion of NiCrAlY coating was the most severe since the (Cr, Al) sulfides along the Al2O3 stringers promoted the formation of black bumps on the coating.