Pre-oxidation of an iron aluminide coating using electrolytic oxidation methods: A feasibility study

In this research, probability of formation of a two-layer coating consisted of iron aluminide and aluminum oxide on 9Cr1Mo steel has been investigated. To produce FeAl layers, 9Cr1Mo steelwas hot-dip aluminized and then laser-surface alloyed. Formation of second layer was conductedby hot-dip aluminizing of laser-alloyed layer to form aluminum layer followed by anodizing and plasma electrolytic oxidation to oxidize it. The microstructure and phase identification of thelayers were examined by optical microscopy, scanning electron microscopy and X-ray diffractionanalysis. Microstructural examinations showed that Fe2Al5 intermetallic phase with a serrated morphology was the main feature of hot-dip aluminized layer. By applying laser surface alloyingthis layer changed to an FeAl aluminide layer with semi-smooth morphology. Evaluations alsoindicated that hot-dip aluminizing of FeAl layer led to the formation of FeAl3 and aluminumlayers, respectively. Results of hard anodizing process showed that anodizing of top aluminumlayer was impossible. This may be attributed to the current leakage due to the existence of cracks or low amount of iron impurities in aluminum layer. However, by applying of plasma electrolytic oxidation method a non-uniform layer consisted of aluminum oxide and aluminosilicate formed. The non-uniformity stems from the geometry of the samples which leads to non-uniform sparking.