Electrochemical corrosion of solid and liquid phase sintered silicon carbide in acidic and alkaline environments

Solid and liquid phase sintered silicon carbide (SiC) ceramics are used in aggressive environments, e.g. as seals and linings in chemical plant equipments. There exist data concerning corrosion of solid phase sintered SiC (SSiC), but there are only few data concerning their electrochemical corrosion behaviour. The corrosion of liquid phase sintered SiC ceramics (LPS SiC) containing yttria aluminium oxide grain boundary phases has been investigated by standard methods that have shown the decisive influence of the oxide grain boundary on the corrosion stability of these materials. But no electrochemical investigations are known. In this study therefore, potentiodynamic polarisation measurements have been used to determine the corrosion mechanisms of SSiC and LPS SiC ceramics at room temperature in acidic and alkaline environments. The investigation has shown a pronounced electrochemical corrosion in acids and alkaline solutions for both types of materials. In HCl and HNO3 pseudo-passivity features due to the formation of a thin layer of SiO2 on the surface were observed, whereas in NaOH soluble silicate ions were observed resulting in more pronounced corrosion. Microstructural observations of initial and corroded samples revealed that the residual carbon found in the microstructure of SSiC did not dissolve preferentially. The corrosion current densities of the LPS SiC materials were caused by the dissolution of SiC and not by the corrosion of the oxide grain boundary phase. The corrosion current densities of the LPS SiC materials investigated were lower than those of the SSiC materials.