Segregation of neodymium in chromia grain-boundaries during high-temperature oxidation of neodymium oxide-coated chromia-forming alloys

The influence of MOCVD reactive-element-oxide (REO) coatings (Nd2O3) on high-temperature, chromia-forming alloy oxidation was investigated REO coatings decreased steel oxidation rates and greatly enhanced oxide scale adherence. Uncoated and coated F17Ti samples were oxidized over the temperature range 700-1050 degrees C in air at atmospheric pressure. SIMS experiments were performed on oxidized-coated samples in order to deter mine the RE distribution through the oxide scale. Nd was distributed across the oxide layers with a higher concentration in the outer part of the scale. Transmission-electron microscopy (TEM) investigations were performed to move precisely locate the RE through the scale. Transverse cross sections, prepared on oxidized Nd2O3-coated Fe-30Cr (model system); showed that Nd, associated with Cr and O, segregated at chromia grain boundaries. It is thought that this is the main cause of the beneficial effects usually ascribed to the RE in chromia-forming allays. The effect of chromia grain-boundary segregation on chromia growth mechanism and its influence on the reactive-element effect (REE) are discussed.