ELECTROCHEMICAL CORROSION OF COMPOSITE CERAMICS AND THERMAL SPRAY COATINGS IN THE ZrB₂–SiC–AlN SYSTEM

Abstract

Polarization studies of the ZrB₂–SiC–AlN compact ceramic material and thermal spray coatings of the same composition were conducted in a 3% NaCl solid solution to analyze their cathodic and anodic behavior. The compact ceramic material was produced by hot pressing, the plasma coating 240 µm thick was deposited onto a graphite С/С–SiC substrate, and the detonation coating 340 µm thick was applied to 12Kh18N9T stainless steel. The microstructure and phase composition of the compact sample and coatings were examined. The microstructure was heterophase in all cases. The compact sample and plasma-sprayed coating contained SiC, AlN, and ZrB₂ as the main phases, and the detonation-sprayed coating additionally had a small amount of nickel and zirconium oxide. Electron microprobe analysis showed that the plasma-sprayed coating had 20 wt.% oxygen; i.e., the coating contained oxide phases in the amount not revealed by X-rays. The compact ceramic sample showed exceptionally high resistance to electrochemical oxidation: electrochemical potential, E_cor, at which corrosion current occurs is very high and constitutes +1.51 eV. For the detonation- and plasma-sprayed coatings, E_cor = –0.25 and –0.05 eV, respectively. The high resistance of the compact ceramic material to electrochemical oxidation correlates with its resistance to high-temperature oxidation above 1700 °С. This is due to the formation of a Al₂O₃–SiO₂ mullite film on the surface. The lower resistance of the coatings to electrochemical oxidation compared to the compact material is associated with their increased porosity.