High-temperature oxidation behavior of hot-dipped aluminide mild steel with various silicon contents

Mild steel was coated by hot-dipping into molten baths containing pure aluminum, Al–2.5Si, Al–5Si and Al–10Si (wt.%) at 700 °C for 180 s. Isothermal and cyclic oxidations were carried out at 750 °C in static air to study the oxidation behavior of the hot-dipped aluminide steel with various silicon contents. The results of isothermal oxidation show the weight gains of the aluminide steel followed a parabolic law. The isothermal oxidation rates of the aluminide steel specimens were directly proportional to the silicon content in the aluminide layers. The reason for the aluminide steel with high silicon possessing a high isothermal oxidation rate is that the silicon addition in the molten bath caused a reduction in the aluminide layer thickness and the formation of phase transformation induced voids in the aluminide layer. The results, after cyclic oxidation, show the weight gains of the aluminide steel specimens were larger than those after isothermal oxidation. Because the aluminide layer with low silicon was mainly composed of a thick brittle Fe2Al5 phase, thermal stress was easily generated in the aluminide layer and caused the formation of vertical cracks when the aluminide steel underwent cyclic oxidation. Once cracks appeared, the weight gains of the aluminide steel specimens were accelerated. Thus, mild steel after hot-dipping in pure aluminum, which had the thickest Fe2Al5 layer, possessed the worst resistance to cyclic oxidation.