ON BEHAVIOR OF SILICON IN ANNEALING PROCESS OF ALUMINIUM SHEET (PART 1)

Abstract

When we wanted to produce a transparent silver anodic oxide film by sulphuric acid anodizing process, we have frequently encountered with the fact that the oxide film has happened to have yellowish brown color. Having examined this coloring-defect, we found that it was caused by the precipitation of fine particles out of the supersaturated silicon in commercially pure aluminium sheets when they were annealed at 300-400°C.Then, we investigated the behavior of silicon in the alloy of highly pure aluminium and silicon in order to eliminate the effects of other impurities. (See Tables 1 and 2). These specimens were annealed at 300, 350, 400, 450, 500, and 550°C and analysed.As the results, the content of the precipitated silicon reached their maximum in the annealed specimens at 300-350°C and the amount of fine particles of the precipitated silicon also reached maximum in the same specimens. (See Fig. 1 and Fig. 3). The color of anodic oxide film of the same specimens was also yellowish brown. (See Table. 3).As shown in Table. 4, the coloring range of anodic oxide film corresponds to the range of the precipitation of fine silicon particles. Therefore, the coloring-defect of anodic oxide film is eliminated by annealing the aluminium sheets at the temperature above the solidus line corresponding to their silicon content.The effects of the total content of precipitated silicon on the properties of aluminium sheet are shown in Figs. 5-9. Among these properties, the corrosion resistance only was appearently influenced by the size of precipitated silicon particles. (See Fig. 6).