Effect of Silicon Content on High-temperature Oxidation Behavior of Hot-dip Al-Si Coatings

The effect of silicon content on the high temperature oxidation resistance of Al-Si coatings at 850 ℃ in air was investigated. The initial structure and the structure evolution of the coatings during oxidation observed and analyzed by a combination of optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The oxidation mechanism of Al-Si coatings was discussed. The results showed that the formation and growth of small voids led to the failure of coatings, these voids formed in the innermost layer contacting with the substrate. With the heating time increased, small voids gathered and finally turned into big holes. These holes caused micro cracks in the alloy layer, thus, air could get access to the inner layer and coatings lost its function. With the silicon increased, the thickness of alloy layer became thinner and thinner, which was easier to produce micro cracks in the alloy layer, and the high temperature oxidation resistance became weak at 850 ℃.