Influence of calcium hydroxide and anodic solution temperature on corrosion property of anodising coatings formed on AZ31 Mg alloys

Anodisation of AZ31 Mg alloy (3 mass%Al, 1 mass%Zn) was performed in 1M NaOH solution, with and without adding 35 mM of calcium hydroxide at various temperatures. The anodising process was carried out at constant potential 3 V; the change in current density during the first period of anodising revealed the mechanisms of anodic film formation. Adding 35 mM Ca(OH)₂ to the solution improved the corrosion resistance, e.g. in 17 mM NaCl and 0·1M Na₂SO₄ solution at 298 K from anodic polarisation measurements. The anodic layer thickness increases with the amount of Ca(OH)₂ added. The temperature of the solution also affects the anodising process; i.e. increasing the solution temperature improved the corrosion resistance of the anodised film.