극저온 황산에서 양극산화된 A356 합금의 피막 경도 및 내식성

Abstract

Cast aluminum alloy A356 exhibits microstructural heterogeneity associated with Si particles, which undermines the uniform growth of anodic oxide and limits long-term corrosion resistance. Generally, corrosion resistance of aluminum alloys can be significantly enhanced by an anodizing, which creates nanoporous oxide layer on surface. However, the anodizing of casting aluminum alloys is not widely explored compared to wrought aluminum alloys. This study evaluated the anodization behavior and performance of A356 alloy under constant current conditions, with or without ethylene glycol (EG), and in low and sub-zero temperature environments. With a mixing the sulfuric acid with EG, the temperature of electrolyte can be decreased to –15 ℃. The required electric field during the anodizing increased with a decrease of electrolyte temperature. Some voids in anodic oxide were observed around the Si particles. Anodic oxide created at sub-zero temperatures (-15 ℃) shows a lower content of sulfuric acid, and higher hardness compared to the anodic oxides formed at 0 ℃ and 15 ℃. Moreover, anodized A356 alloy at –15 ℃ had a better corrosion resistance than other samples. Therefore, the anodizing process combining EG mixing and sub-zero temperature reducing the sulfate ions in the anodic oxide can provide a new solution for surface treatment of casting aluminum alloys to enhance the surface hardness and corrosion resistance.