Low-temperature oxidation behavior and mechanism of hot-dip Al and AlSi coatings on Mo substrate at 600 °C in static air

The hot-dip Al and AlSi coatings are synthesized on Mo substrate at different temperatures. The microstructure and low-temperature oxidation behavior both of the coatings are investigated. The results indicate that hot-dip Al coating is mainly made up of an Al₄Mo outer layer and an Al₈Mo₃ interface layer, while the hot-dip AlSi coating is consists of a Mo(Si, Al)₂ inner layer and AlSi alloy outer layer containing Mo(Si, Al)₂ grains. After exposed at 600 °C for 40 h, a large number of cracks initiate on surface and inside of the hot-dip Al coating, its mass gain per unit area (Δm) reach to 10.32 mg/cm², and the RSa (average surface roughness) and Sdr (surface area growth rate) values increase from 0.216 μm and 6.602% before oxidation to 1.214 μm and 35.297%, respectively. During the oxidation process, the structure of the hot-dip AlSi coating is preserved intact, and almost no cracks are observed in the coating. After oxidized at 600 °C for 150 h, the Δm of hot-dip AlSi coating is only 1.56 mg/cm², and the RSa and Sdr values increase from 0.458 μm and 7.083% to 1.509 μm and 43.586%, respectively. Besides, the oxidation rate constant (K_p) of hot-dip Al and AlSi coatings are 7.42 × 10⁻⁴ and 4.47 × 10⁻⁶ mg²·cm⁻⁴·s⁻¹, respectively, and the latter is only 6.17 × 10⁻³ times than that of the former. The excellent oxidation resistance of hot-dip AlSi coating is attributed to its small grain size, stable and dense coating structure, and complex oxide layer composition.