The Role of Reactive Elements on Scale Growth in High‐Temperature Oxidation of Pure Nickel, Iron, Cobalt, and Copper: I . Oxidation Kinetics and Scale Morphology

Abstract

Recently, Pieraggi and Rapp proposed the “poisoned interface” model, as a novel interpretation for the reactive‐element effect (REE) in chromia‐forming alloys; the mechanism should also be applicable to any other scaling reaction involving a cation‐diffusing scale and a suitable impurity segregant. The premise of the current study was that the relative size of the ions involved is an important factor in producing the required interfacial segregation by the reactive element. The high‐temperature oxidation kinetics and morphologies were studied for pure Ni, Co, Fe, and Cu coupons coated with thin superficial films (100 to 500 Å) of Ca, Sr, or Ba introduced by vacuum evaporation. In every case, the modified coupons showed a reduction in scaling rate compared to unmodified coupons, and there was a noticeable change in oxide morphology At 850°C, 250 Å of initially deposited Ca on pure Ni effected a reduction in the parabolic rate constant for Ni by a factor of 20. These results provide preliminary evidence which is consistent with the “poisoned interface” interpretation for the REE.