Effects of a Reactive Element on isothermal and cyclic oxidation of chromia-forming alloys: SEM/EDX, TEM and SIMS investigations.

Scanning electron microscopy (SEM), transmission electron microscopy (TEM), secondary ion mass spectrometry (SIMS), secondary neutral mass spectrometry (SNMS) and X-ray diffraction (XRD) were used to study the reactive element effect on chromia-forming alloys. The reactive element, neodymium, was introduced as an oxide film at the surface of the alloys. The analyses were performed during the early stages of oxidation at 1,273 K. Uncoated and Nd2O3-coated alloys have been oxidised for 1, 5, 30, 60, 120 minutes and 50 hours in air at atmospheric pressure. Chromia growth mechanisms were studied by two-stage O-16(2)/O-18(2) oxidation exposures followed by SIMS and SNMS analyses. Chromia grains quickly grew on uncoated samples, whereas they slowly developed on Nd2O3-coated specimens. A neodymium-containing phase rapidly evolved from Nd2O3, to NdCrO3 and then NdTi21O38. Indeed, the main phase evolution appeared during the first 60 minutes of the oxidation process. Chromia growth mechanism was not changed after 1 hour of oxidation because Nd was not yet incorporated into Cr2O3 scales. During the early stages of oxidation, Nd was mainly concentrated in the outer part of the scale composed of a spinel phase, Mn1.5Cr1.5O4. After two hours of oxidation, Nd was incorporated inside the chromia scale, leading to inward diffusion of oxygen. These results clearly demonstrated that the incorporation of the reactive element in the chromia scale as grain boundary segregant is the main explanation of the reactive element effect in the case of chromia-forming alloys.