THE STEADY-STATE CORROSION KINETICS OF 2-PHASE BINARY-ALLOYS FORMING THE MOST-STABLE OXIDE

The steady-state kinetics in the high-temperature oxidation of binary A-B alloys containing a mixture of the conjugated solid solutions of B in A (alpha phase) and A in B (beta phase) with exclusive formation o f the most-stable oxide BOv have been examined, assuming that the exte rnal scale grows on top of a subsurface layer of alpha phase. The resu lts obtained are compared with the corresponding behavior of alloys wh ich are single phase in the whole range of composition. Under identica l values of all the parameters involved the concentration of B at the alloy-scale interface is smaller for two-phase than for single-phase a lloys under the same concentration of B in the alloy as a result of th e restricted flux of B through the alpha-phase layer. As a consequence of this, the two-phase alloys corrode more slowly than single-phase a lloys and this difference increases as the solubility of B in the alph a phase decreases. Finally, the simultaneous formation of BOv both ext ernally and as internal oxide is more likely for two-phase than for si ngle-phase materials.