Ellipsometric characterization of surface films on AZ31 magnesium alloy exposed to a Na2SO4 solution

Magnesium is an attractive structural material due to its very low density. Among the magnesium alloys, the AZ series containing aluminum and zinc as main alloying elements presents good mechanical properties. However, the great challenge to be overcome by these materials is the poor corrosion resistance, consequence of their high surface reactivity with the formation of oxide/hydroxide films, which generally offer low protection. Therefore, to study the surface characteristics of these alloys is very important. In this paper, the surface of AZ31 magnesium alloy is characterized by ellipsometry after exposure to a 0.1 M Na2SO4 solution for 3 min, 5 min, 15 min, and 30 min. Through ellipsometric measurements and proper ellipsometric modeling, the thickness of the surface films grown for all cited immersion times could be known. The modeling acknowledges the three-layer surface film is formed for an inner layer composed initially of mixing of MgO and Al2O3. As the immersion time progresses until 5 min, the MgO is successively replaced by magnesium hydroxide (Mg(OH)2), thickening the intermediate layer of the film, which is composed of dense Mg(OH)2. On the other hand, the outer layer is composed of porous Mg(OH)2. The total thickness of the film grows in two stages: in the first 5 min, the film experiments a disruptive thickening until 339 nm; between 5 and 30 min, the total thickness evolves from 339 nm to 386 nm.