Effect of annealing temperature on the oxidation behavior of ferrosilicon-aluminum-coated low carbon steel by mechanical alloying

Abstract

In this study, the oxidation resistance of ferrosilicon-aluminum coating was studied for the first time via cyclic oxidation testing at 800 °C for eight cycles (160 h). The correlation between the microstructure and oxidation resistance of ferrosilicon-aluminum coating at different annealing temperatures was determined using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDX). Following the annealing treatment, three layers were found to have formed in the coating—a coating (outer) layer and two diffusion zones comprising Fe–Al–Si (intermediate) and Fe2Al5 (inner) layers. After oxidation, the diffusion zones transformed into either a single layer (following annealing at 600 °C) or three layers (following annealing at 700 and 800 °C). In all samples, vertical micro-cracking was observed from the substrate surface to the surface of the diffusion zone, and oxide scales had formed on the diffusion zone surface following oxidation. These scales were identified as Al2O3 with minor impurities (Fe and Si). However, two types of oxide scale were observed in the coating diffusion zone of the sample annealed at 600 °C. The mass gain and parabolic rate constants (Kp) of this coating following eight cycles of annealing (10.45 mg/cm2 and 1.78 × 10−4 (mg2/cm4. s), respectively) were lower than those of the other coatings. These results suggest that the residual product of the reaction in the diffusion zone during oxidation and the vertical micro-cracking contributed to the formation of the oxide scale in the coating.