Role of Surface Oxides in Corrosion of Carbon Black in Phosphoric Acid Solution at Elevated Temperature

Abstract

The effects of surface oxides on the corrosion behaviour of carbon black (Vulcan XC-72) have been investigated with and without concurrent oxygen-blowing in 96% H3PO4 solution at 145-degrees-C by using potentiodynamic and current decay transient methods in connection with Fourier transform infrared (FTIR) spectroscopy. The surface oxides were identified by FTIR spectroscopy. A cyclic voltammetry curve showed an anodic current peak at 0.8 V(RHE) and an abrupt increase in anodic current above 1.4 V(RHE). The results of FTIR spectroscopy demonstrated that the surface oxides consist of carboxyl and quinone groups and that the amount of carboxyl group decreases markedly, while that of the quinone group decreases slightly with increasing applied potential from 0.8 to 1.3 V(RHE). The current decay transients showed the two-staged variation of passivation index with time for applied anodic potentials. The second stage of the passivation index exhibited the stronger time dependence, the lower the applied anodic potential, indicating the occurrence of a promoted passivation on the carbon black surface. The raised passivation is attributable to the increase in the amount of carboxyl group with decreasing applied anodic potential and with oxygen-blowing. The abrupt increase in anodic current above 1.4 V(RHE) appears to be due to the decrease in the amount of carboxyl group. The experimental results suggest that the corrosion behaviour of carbon black with concurrent oxygen-blowing is determined by surface oxide of carboxyl group, which provides the more enhanced passivity of the carbon black, the lower the applied anodic potential.