THE ROLE OF REACTIVE ELEMENTS ON SCALE GROWTH IN HIGH-TEMPERATURE OXIDATION OF PURE NICKEL, IRON, COBALT, AND COPPER .2. SECONDARY-ION MASS-SPECTROMETRY

The high-temperature oxidation kinetics and scale morphologies for pure Ni, Co, Fe, and Cu coupons coated with a 250 Angstrom deposit of vacuum-evaporated Ca, Ba, or Sr were previously presented and interpreted.(1) In every case, the coated coupons showed a reduction in scaling rate Compared with unmodified coupons. The greatest effect was shown by Ni/Ca oxidized at 850 degrees C (a factor of 20 reduction in scaling rate), and a change in growth mechanism of the NiO scale was observed. In this study, oxidized coupons, consisting of the most effective deposit for each metal substrate (Ni/Ca, Co/Sr, Fe/Sr, and Cu/Ba) were chosen for chemical analysis by imaging secondary ion mass spectrometry (SIMS). The distribution of the alkaline earths in the oxide scales was revealed to be slightly different in each case, due to the competing tendencies for solubility and segregation in the respective oxides. Segregation of alkaline earths at the metal/scale interface was observed for Co/Sr, Fe/Sr, and Cu/Ba. For the Ni/Ca coupons, however, no interfacial enrichment of calcium was detected by this technique. These results are discussed in relation to the poisoned interface model of Pieraggi and Rapp,(2) which has been put forward as an interpretation of the reactive element effect for chromia-forming alloys.