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 pur e Ni, Co, Fe, and Cu coupons coated with a 250 Angstrom deposit of vac uum-evaporated Ca, Ba, or Sr were previously presented and interpreted .(1) In every case, the coated coupons showed a reduction in scaling r ate 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 dep osit for each metal substrate (Ni/Ca, Co/Sr, Fe/Sr, and Cu/Ba) were ch osen for chemical analysis by imaging secondary ion mass spectrometry (SIMS). The distribution of the alkaline earths in the oxide scales wa s revealed to be slightly different in each case, due to the competing tendencies for solubility and segregation in the respective oxides. S egregation of alkaline earths at the metal/scale interface was observe d for Co/Sr, Fe/Sr, and Cu/Ba. For the Ni/Ca coupons, however, no inte rfacial enrichment of calcium was detected by this technique. These re sults are discussed in relation to the poisoned interface model of Pie raggi and Rapp,(2) which has been put forward as an interpretation of the reactive element effect for chromia-forming alloys.