Under the condition of long-time high-temperature oxidation, oxidation in Al-Si coating will lead to degradation of the coating. To solve this problem, the Al-Si coating was treated before the oxidation experiment at different pre-oxidation temperatures and times. The structure, morphology and element distribution of the oxide film were characterized by XRD, SEM and EPMA, and the oxidation kinetics curves were drawn. The results show that the oxidation resistance of the pre-oxidized samples is improved in the process of constant temperature oxidation at 1000 °C. When the pre-oxidation temperature is set at 950 °C, the mass gain of the samples is the lowest, and the accelerated decomposition of Cr2O3 caused by high pre-oxidation temperature is avoided. After oxidation for 300 h, the oxide film on the surface is still continuous and dense without peeling. The samples pre-oxidized at 950 °C for 7 h show the best oxidation resistance in a 1000 °C constant temperature oxidation process, more α-Al2O3 oxides are generated on the coating surface and the element diffusion between the matrix and the coating caused by overly long oxidation time is avoided.
In this study, effects of pre-oxidation on the hot corrosion behaviour of Al–Si coatings in a Na2SO4 and 25 w/% NaCl mixture at 900 °C were investigated. The results showed that after 120 h of corrosion, a large amount of β-NiAl residual was found with no obvious sulphides or oxidation in the pre-oxidised Al–Si coating. The pre-formed oxide film on the pre-oxidised coating prevented external O2– and species such as S2– and Cl– from the molten salts from directly contacting the coating and reduced the rate of TiO2 formation in the oxide layer, thus forming an Al2O3-based oxide film and reducing the growth rate of the oxide film on the coating surface. Cracking and peeling of the oxide film, caused by the internal stress owing to the difference in the thermal expansion coefficient between the oxides in the oxide layer, are suppressed; thus, the hot corrosion resistance of the coating is significantly improved. .
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