The influence of ion-implanted yttrium on the selective oxidation of chromium in Co-25 wt.% Cr

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Specimens of Co-25 wt.% Cr, Co-25 wt.% Cr-1 wt.% Y, and yttriumimplanted Co-25 wt.% Cr alloy were oxidized at 1000°C in 1 atm O2. The implantation dosage ranged between 1016 to 1018 ions/cm2. The unimplanted binary alloy oxidized to a duplex Co-rich scale, but the Y-containing ternary alloy formed a continuous Cr2O3 layer. When the implantation dosages were lower than a nominal 1018 ions/cm2, the alloy failed to develop a similar continuous Cr2O3 layer as that observed with the Y-containing alloy. A temporarily stable external Cr2O3 scale was formed on the most heavily implanted specimen (1×1018 Y+/cm2). This Cr2O3 scale consisted of very fine-grained oxide, which is permeable to the outward transport of Cr and Co. Internal oxidation pretreatment of the ion-implanted specimens converting the Y metal to its oxide prior to the oxidation experiment, can enhance the development of an external Cr2O3 scale, but this scale is also unstable. Results suggest that the selective oxidation of chromium in an ordinarily non-Cr2O3 -forming alloy can be due to the reactive element oxides acting as preferential nucleation sites on the alloy surfaces, but the subsequent growth of these scales may require a continuous supply of reactive elements in the alloy.