Abstract
Fe40Al alloys produced by casting and sintering were anodized to form nanostructured oxide layers. Microstructural characterization revealed a lower grain size for the sintered alloy compared to the cast alloy. Anodizing was performed in four electrolytes with a constant etidronic acid concentration (0.3 M) and different ethylene glycol-water ratios. Different voltages (5–400 V) and treatment times (1–4 hours) were evaluated for cast and sintered Fe40Al alloys. Among the studied anodization regimes, only four anodic films formed on sintered Fe40Al were homogeneous. These films were annealed at 900°C and analyzed by conventional characterization techniques (SEM/EDS and XRD). Results indicated a non-self-ordered morphology and different content of P, Fe, and Al species as a function of the electrolyte composition. Annealing post-treatment led to the formation of Fe/Al oxides and spinels in selected anodic films. An additional peak of FePO4 was detected only in the anodic film formed in pure ethylene glycol. The band gap values (UV-Vis reflectance spectroscopy) of the anodic film formed in ethylene glycol decreased from 2.87 eV to 2.45 eV after annealing. This band gap decrease was associated with the preferential formation of conductive phases (Fe2O3 and FeAlO3) over insulating phases (γ-Al2O3 and FeAl2O4) after annealing.
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