A2 Oxidation and Accelerated Degradation

Topic

This nanoparticle—possessing a surface-to-volume ratio far higher than that of another model in this set—oxidizes at an accelerated rate compared to the same material in its bulk form (a macroscopic block with a much lower surface-to-volume ratio). The oxidation of either form—whether the nanoparticle or the equivalent bulk material—follows the same parabolic, self-limiting oxidation law utilized for the same purpose by another model from a different set of materials. Because the nanoparticle’s surface-to-volume ratio is so much higher than that of the bulk material, a given thickness of formed oxide layer leaves a proportionally much smaller volume of unoxidized metal beneath it relative to the surface area that continues to oxidize.

The fact that this same parabolic, self-limiting oxidation law applies to the oxide layer thickness—rather than directly to the volume of metal consumed—is what causes the high surface-to-volume ratio to relatively accelerate the nanoparticle's oxidation compared to that of the bulk material: a specific oxide layer thickness consumes a much larger fraction of the nanoparticle's total volume than it does for the equivalent bulk material. The even higher surface-to-volume ratio of the nanoparticle makes this relative acceleration of oxidation—compared to the bulk material—even more pronounced. Consequently, the surface-to-volume ratio characterizing this model—and the associated accelerated oxidation—means that this acceleration (driven by specific surface area) can, if sustained long enough, lead to the consumption of the nanoparticle's entire volume; this is something the self-limiting parabolic oxidation law would never allow within a comparable timeframe for the equivalent bulk material. Whereas in bulk material the already-formed oxide layer increasingly hinders the oxidation's advance into the underlying metal, in this nanoparticle, the remaining metal can be completely consumed before such self-limitation becomes significant. The fact that this surface-to-volume ratio ultimately determines how close the nanoparticle is to total volume consumption means that two nanoparticles of the same material but different sizes will oxidize at vastly different relative rates.

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