Haiping Lai

Microstructural Investigation of the Initial Oxidation of the FeCrAlRE Alloy Kanthal AF in Dry and Wet O2 at 600 and 800 ° C

The FeCrAlRE (where RE is reactive element) alloy Kanthal AF was exposed isothermally at 600 and 800 degrees C for 72 h in dry O-2 and in O-2 with 10 vol % H2O. The mass gains were 3-5 times higher at the higher temperature. The presence of water vapor increased the oxidation rate at 800 degrees C, while no significant effect was observed at 600 degrees C. A thin two-layered oxide formed at 600 degrees C: an outer (Fe, Cr)(2)O-3 corundum-type oxide, containing some Al, and an inner, probably amorphous, Al-rich oxide. At 800 degrees C a two-layered oxide formed in both environments. The inner layer consisted of inward grown alpha-Al2O3. In dry O-2 the originally formed outward grown gamma-Al2O3 had transformed to alpha-Al2O3 after 72 h. Water vapor stabilized the outward grown gamma-Al2O3 and hence no transformation occurred after 72 h in humid environment. RE-rich oxide particles with varying composition (Y, Zr, and Ti) were distributed in the base oxide at both temperatures and in both environments. The RE-rich particles were separated from the alloy substrate by a layer of Al-rich oxide. At 800 degrees C the Y-rich RE particles were surrounded by thick oxide patches in both dry and humid O-2

The influence of platinum on the oxidation and sodium sulfate induced hot corrosion of NiAl diffusion coatings

Abstract Oxidation and corrosion properties at 900°C of two coatings, one containing platinum and the other platinum-free, on IN792 were investigated. During the corrosion exposures, sodium sulfate salt was situated in the furnace together with the coated specimens. The temperature of the salt was kept above its melting point but lower than the temperature of the coating i.e. 900°C. The exposure times ranged from 100 to 1000 h. The formed oxide scales were studied by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy and scanning transmission electron microscopy techniques. It is shown that the presence of Pt improves the protective properties of the coating against corrosion. The propagation stage of corrosion on the platinum-free coating is reached already after 100 h of exposure. On the other hand the scale formed on the platinum-rich samples still appeared to be protective even after 500 h. The details concerning structure and chemistry of the scales formed are presented and discussed.

Cyclic oxidation of two FeCrAlRE foils at 1100 oC - The influence of the concentration of minor alloying elements on scale microstructure

Two FeCrAlRE alloys, a commercial, 0C404, and a model alloy in the form of thin foils, with different Mn, Nb, Mo and Ti concentrations were subjected to cyclic oxidation in lab air at 1100°C. The oxidized samples were studied by gravimetry, Grazing-Incidence X-ray Diffraction (GI-XRD), Scanning Transmission Electron Microscopy (STEM), and Energy Dispersive X-ray (EDX) analysis. The two FeCrAl alloys exhibit different oxidation kinetics; however, both alloys have the same weight gain after 500 hours exposure. During the early stages the scale consists mainly of α-Al2O3 together with some oxide particles containing Mn, Al, Fe and Cr formed on the alloys. After 500 hours the 0C404 scale locally also consists of larger polycrystalline regions of Mn-Cr-Al spinel. In addition, Si-rich oxide, chromia and Al-Cr oxide could be observed at the metal/oxide interface.