Electrochemical Corrosion of Composite Ceramics and Thermal Spray Coatings in the ZrB2-SiC-AlN System

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-sprayed coating 240 mu m thick was deposited onto a C/C-SiC graphite substrate, and the detonation-sprayed coating 340 mu m thick was applied to 12Kh18N9T stainless steel. The microstructure and phase composition of the compact sample and thermal spray 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-corr, at which corrosion current occurs is very high and constitutes +1.51 eV. For the detonation- and plasma-sprayed coatings, E-corr = -0.25 and -0.05 eV, respectively. The great resistance of the compact ceramic material to electrochemical oxidation correlates with its resistance to high-temperature oxidation above 1700 degrees C. This is due to the formation of an 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.