The Criterion for the Onset of Internal Oxidation Beneath the External Scales on Binary Alloys

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Abstract
Ternary diffusion theory is used to establish a criterion for the onset of an internal oxidation zone beneath the external scale when oxidizing conditions favor formation of the oxide of the less noble metal in a binary alloy. The problem is posed in terms of ternary solution thermodynamics and diffusion and is correlated to the locus of the virtual diffusion path on the ternary alloy‐oxygen isotherm. External scale formation alone occurs when the virtual diffusion path in the alloy phase contacts the oxygen solubility curve for the alloy. External scale formation accompanied by internal oxidation occurs when the virtual diffusion path for the alloy leads to oxygen supersaturation and oxide precipitation by cutting into and across the two‐phase alloy/oxide field. These assertions can be used to define a limiting bulk atom fraction of the selectively oxidized solute, , in the alloy above which internal oxidation does not occur: satisfieswhere is the Wagner interaction coefficient between oxygen and in the alloy, and are the direct diffusion coefficients of and oxygen in the alloy, ν is the atomic ratio of oxygen to in the oxide , is an auxiliary functionand replaces with as the corrosion constant for the parabolic rate of recession of the alloy/surface scale interface. Typical ranges of parameters applicable to a variety of alloy systems are used to show the effects of the diffusion coefficients, scaling constants, and interaction parameters on the limiting value .

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