Kinetics and Mechanism of Columbium Alloy Corrosion In Superheated Steam

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Abstract
Screening tests of numerous binary, ternary, and quaternary columbium alloys in superheated steam, up to 704 C, showed that Nb-V binary alloys had the greatest corrosion resistance.1 The binary alloys were studied in detail and were observed to follow initially low power rate laws varying from quartic to parabolic behavior. A rate transition occurred after longer exposures to a linear rate law which was associated with spalling and cracks in the oxide films. The corrosion product was shown to be different than that formed in air or oxygen for Nb-3.5V and was a complex unknown phase. An increase in vanadium content to 10 atomic percent (a/o) enabled beta-Nb2O5 to form with the unknown phase, and Nb02 also formed as a third phase on Nb-15V. A change in oxide morphology was noted as the vanadium increased from 3.5 to 8.9 a/o, the latter forming oxide “fingers” which grew into the substrate. The activation energy for corrosion, 13 kcal/mol, was the same for binary Nb-V alloys which formed either the unknown oxide phase or NbO2 at temperatures below 482 C, and was considerably less than that for oxygen diffusion in Nb2O5 containing 1 mol percent V2O5. The apparent diffusion-controlled process, according to the kinetics, and low corrosion activation energy strongly suggest oxide grain boundary diffusion as a rate-limiting step during the pretransition period. Oxygen contamination of the substrate occurred readily, the tolerance for oxygen decreasing with increasing vanadium. Cracks which formed in the oxide due to volume mismatch at the oxide-metal interface were observed to have propagated through the embrittled substrate. Spoiling and contamination characteristics appear to be limiting properties for corrosion limetime.

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