Limitations on the Use of Surface Doping for Improving High-Temperature Oxidation Resistance

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Abstract
For more than 50 years, scientists have studied the “magic dust” of high-temperature oxidation—certain oxygen active or “reactive” elements which, when added to alloys in small quantities, effect profound improvements in their oxidation resistance. In general, high-temperature oxidation resistance is achieved by the oxidation of one or more alloy components to form a dense, stable, slow-growing, external oxide layer, or ’scale” such as α-Cr2O3, α-Al2O3, or SiO2. When added properly, reactive elements have a beneficial effect on the formation and growth of both α-Cr2O3and α-Al2O3scales. A standard list of reactive element (RE) effects would include: (1) an improvement in scale adhesion or resistance to spallation, (2) a change in the scale growth mechanism, (3) a reduction in the oxidation rate, related to the change in mechanism, (4) a modification in the scale microstructure, and (5) in the case of alloys that form Cr2O3scales, an improvement in selective oxidation, meaning that a lower Cr concentration in the alloy is required to form and maintain an external Cr2O3scale.