Internal Oxidation of a Ag-1.3 at.% Se Alloy. Part II: Kinetics of Internal Oxidation

Abstract

The kinetics of internal oxidation of a two-phase Ag-1.3 at.% Se alloy were studied at 750, 800 and 830°C in pure oxygen. The parabolic rate law was followed to a reasonable extent for the first 2 hr of oxidation. The rate of internal oxidation was much faster than expected from the classical Wagner model. Three major factors affected this: (i) the non-uniform distribution of Ag2Se particles in the as-cast alloy caused non-uniform internal oxidation of the alloy; (ii) the formation of a liquid phase during in-situ oxidation of Ag2Se particles accelerated the diffusional processes and lowered the activation energy of the oxidation reactions (a sort of “catastrophic” internal oxidation); (iii) gradual oxidation of the alloying element in the form of second-phase particles caused the extension of the single internal-oxidation front (IOF) to the internal-oxidation volume (IOV) between the inner and outer IOFs. The IOV requires a much lower oxygen quantity for its advancement than the (single) IOF thereby enabling much faster internal oxidation of two-phase alloys that oxidize in the in situ (diffusionless) mode. The kinetics are, also discussed according to the treatment which was proposed for internal oxidation of two-phase alloys by Gesmundo.7