Formation and breakdown of a protective layer of chromium sesquioxide on L-605 alloy at 1100° C
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The formation and breakdown of a protective layer of Cr2O3 on L-605 during oxidation at 1100° C was investigated. The effects of surface deformation, pressure, and water vapor on the breakdown time were evaluated. It was found that increasing surface deformation, increased the time to breakdown. Decreasing pressure below 1 atm to 0.13 N/m2 increased time of breakdown as did decreasing water content from 25,000 to 2.5 ppm. By metallographic and microprobe examinations of samples during breakdown a model was deduced. Surface deformation promotes Cr2O3 formation, while increasing pressure and moisture increases the volatility of Cr2O3. Thus, the Cr2O3 grows for a time determined by these three factors. At the end of this time the growth stresses cause the oxide to crack, exposing a chromium-depleted metal surface to the oxidizing gas. The resultant rapid oxidation of this surface lifts the remaining Cr2O3, exposing more depleted metal. When the depletion zone is consumed and a very thick oxide has formed, the rate of oxidation slows and no further disruption is noted.
KeywordsOxidation Physical Chemistry Chromium Water Vapor Metal Surface
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