Localised atmospheric corrosion of magnesium-aluminum alloys produced by semi solid casting - A 2D and 3D investigation

The localised corrosion of the two magnesium-aluminium alloys AM50 and AZ91produced by a semi-solid casting method was investigated under atmospheric corrosion conditions, in the presence of NaCl. Gravimetry confirmed the beneficial effect from ambient levels of carbon dioxide (CO2) in suppressing corrosion through the formation magnesium hydroxy carbonates. Both 2D and 3D characterisation revealed that ambient CO2 has a strong influence on the morphology and rate of localised corrosion. In CO2-free atmosphere, the alloys suffered localised corrosion resulting in clustered and interconnected cavities that extended deep into the material. In contrast, corrosion in the presence of ambient levels of CO2 (400 ppm) resulted in evenly distributed shallow corrosion sites. The results indicate that CO2 neutralises the cathodic areas resulting in diminished cathodic activity and inhibiting NaCl (aq) droplet coalescence.