MICROSTRUCTURAL EVIDENCE FOR SHORT-CIRCUIT OXYGEN DIFFUSION PATHS IN THE OXIDATION OF A DILUTE NI-CR ALLOY

A comparative study of high-temperature oxidation of Ni containing I a t.% Cr and pure Ni was carried out. Instead of the conventional kineti cs study using thermogravimetry, a microlithographic marker experiment was designed. Observation of the markers using cross-sectional TEM an d SEM has revealed striking differences in the scale morphology, micro structures, and oxidation mechanisms between pure Ni and the Cr-doped Ni substrates. In particular, the results suggest that a small additio n of Cr promotes significant inward transport of oxygen. Marker experi ments revealed that NiO grown on pure Ni is wholly attributable to out ward-cation diffusion. In contrast, NiO grown on Ni-1 at.% Cr exhibite d formation of a substantial inner layer having a submicron grain size , established by the markers lo have formed from oxygen ingress. For p ure Ni, voids were observed to be distributed only within oxide grains . In contrast, for Ni containing 1 at.% Cr, elongated pores formed ext ensively along oxide-grain boundaries. Formation of new fine-grain oxi de in these pores was observed to have sometimes completely resealed t he void. It is, therefore, proposed that the transport of oxygen in th e case of oxide scale grown on Ni-1 at.% Cr occurs via voids (pores) f ormed by vacancy coalescence at the grain boundaries.