Selective oxidation of Fe-Al (8wt.%) binary alloy during continuous annealing

Aluminum is one of the alloying elements whose proportion in advanced high-strength steels (AHSS) can be high. This study focuses on the reaction mechanisms occurring during continuous annealing of AHSS, with emphasis on the behavior of aluminum. To provide a better understanding of these mechanisms, we used a binary Fe-Al (8 wt.%) model alloy. The samples were annealed in a laboratory furnace with a temperature profile relevant to the galvanizing line practice. Samples were heated to 800°C and held at this temperature for 60 s before being cooled to room temperature. The rapid cooling of the samples, that occurs when the annealing furnace is stopped, acts as a quench for reactions. The gas atmosphere consisted of a mixture of N2 – 5 vol.% H2 with a dew point of 0 °C. With the chosen annealing conditions, native iron oxides are reduced, selective internal and external oxidation of aluminum occurs, and metallic iron particles are formed on the alloy surface. The annealed samples were characterized using several complementary analysis techniques. The surface of annealed samples was observed in a Field Emission Gun Scanning Electron Microscope to obtain high resolution images. Thin cross-sections were extracted from the oxidized samples using a Focused Ion Beam Microscope and characterized in a Transmission Electron Microscope. X-ray Photoelectron Spectroscopy was used to analyse the extreme surface of the samples. An external oxide layer was found between the iron particles. Internal oxidation is visible on cross-sections. The composition, morphology and depth of both internal and external oxidations was studied. Elementary mechanisms of the oxidation reaction are proposed and discussed.