THE INFLUENCE OF ION-IMPLANTED YTTRIUM ON THE SELECTIVE OXIDATION OF CHROMIUM IN CO-25 WT PERCENT CR

Specimens of Co-25 wt. % Cr, Co-25 wt. % Cr-1 wt. % Y, and yttrium-implanted Co-25 wt. % Cr alloy were oxidized at 1000 degree C in 1 atm O//2. The implantation dosage ranged between 10**1**6 to 10**1**8 ions/cm**2. The unimplanted binary alloy oxidized to a duplex Co-rich scale but the Y-containing ternary alloy formed a continuous Cr//2O//3 layer. When the implantation dosages were lower than a nominal 10**1**8 ions/cm**2, the alloy failed to develop the continuous Cr//2O//3 layer observed with the Y-containing alloy. A temporarily stable external Cr//2O//3 scale was formed on the most heavily implanted specimen (1 multiplied by 10**1**8 Y** plus /cm**2). This Cr//2O//3 scale consisted of 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 Cr//2O//3 scale but this scale is also unstable. Results suggest that the selective oxidation of chromium in an ordinarily non-Cr//2O//3-forming alloy can be due to reactive element oxides acting as preferential nucleation sites on the alloy surfaces.