Chemical conversion treatment of Mg−Al alloy (AZ91) using colloidal silica as an alternative to chromate conversion was investigated as a function of solution pH, temperature, solution conditions, and treatment time. The solution used for the colloidal silica coating consisted of colloidal silica, titanium sulfate, and cobalt ions to maintain good anti-corrosion and adhesion properties. Adding CoSO4 to the colloidal silica solution enhanced the adhesion force between the silica film and magnesium substrate. The optimum conditions for the chemical conversion treatment solution were pH 2, 90-sec treatment, and 25°C.