ABSTRACT
Active corrosion, as discussed earlier, usually means atmospheric degradation with the formation of nonpassivating corrosion products such as gaseous SiO and CO, which result from oxidizing SiC in water vapor:
SiC s 2H O g SiO g CO g 2H g2 2( ) ( ) ( ) ( ) ( )+ = + + (19.18)
Kinetic data on this system (Kim and Readey 1989) and on the active corrosion of SiO2 in hydrogen at temperatures above 1200°C (Readey 1991)
SiO s H g SiO g H O g2 2 2( ) ( ) ( ) ( )+ = + (19.19)
show that, at these high temperatures, diusion of the product gases away from the corroding surface controls the rate of corrosion. As a result, as discussed earlier, the kinetics of the reaction can be easily modeled, and quantitative comparisons can be made with experimental data to conrm diusion control (Readey 1991). ermodynamic data for the reaction and gas diusion coecients calculated from the kinetic theory of gases (Equation 19.13) give the necessary information to permit quantitative prediction of corrosion rates. Of course, if one of the many possible surface reactions controls the rate, then prediction of corrosion rates without experimental data is impossible. In any event, for any given material, at the highest temperatures, gas diusion will control the corrosion rate, and predictions of corrosion behavior and corrosion rates can be made (Readey 1998a,b).
