Abstract
The oxidation of mild steel crevices in pressurized carbon dioxide containing small amounts of carbon monoxide, hydrogen and water is analysed mathematically. For bulk gas which is hydrogenβrich with respect to the waterβgas shift reaction equilibrium, conversion of hydrogen to water on magnetite surfaces is rapid. This, and the differential diffusion of hydrogen and water, lead to crevice gas compositions very different from the bulk gas.
In restricted crevices, protective oxidation causes enhanced carbon monoxide concentration. In less restricted crevices, enhanced water concentrations are possible. Both situations lead to shorter times to breakaway oxidation than in the bulk gas.
Crevice breakaway oxidation rates may be enhanced by a factor of 1.7 for suitable crevice geometry, provided that the bulk gas is not equilibrated at the crevice mouth. In more restricted crevices the rate is depressed.
These predictions of the analysis are substantiated by experimental oxidation of mild steel specimens within inert glass capsules having a leak path. Such capsules provide a convenient method of exposing material to a wide range of gas compositions within the same autoclave.