The Influence of SO₂ on the Corrosion of Mg and Mg-Al Alloys

The SO₂-induced atmospheric corrosion of some magnesium-aluminum (Mg-Al) alloys, including Mg alloy AZ91D, and commercially pure Mg (CP Mg) was investigated using well-controlled laboratory exposures and included real-time measurements of SO₂ deposition. The influence of SO₂ concentration, alloy composition, humidity, and ppb level additions of O3 or NO₂ on the rate of SO₂ deposition was investigated. SO₂ accelerates the corrosion of Mg and Mg alloys causing localized corrosion, MgSO36H2O being the dominant corrosion product. At 60% RH, traces of O3 or NO₂ strongly increased both the SO₂ deposition and the corrosion rate. The rate of SO₂ deposition was strongly dependent on humidity; at 70% RH and higher the SO₂ deposition rate was very rapid and constant in time while it was transient below 50% RH. At 60% RH, a change from transient to rapid, steady-state, SO₂ deposition occurred. The sudden activation is explained by the onset of electrochemical corrosion. The activation behavior was shown to depend on SO₂ concentration, the thickness of the surface film and by the presence of ambient O₂.