A Study into the Localized Corrosion of Magnesium Alloy Magnox Al-80

Magnesium (Mg) nonoxidizing alloy, known as Magnox, was historically used as a fuel cladding material for the first generation of carbon dioxide gas-cooled nuclear reactors in the United Kingdom. Waste Magnox is currently stored in cooling ponds, pending final disposal. The corrosion resistance of Mg and its alloys is relatively poor compared to modern cladding materials, such as zirconium alloys, so it is important to have a knowledge of the chloride concentration/pH dependence on breakdown and localized corrosion characteristics prior to waste retrievals taking place. These results show that Magnox exhibits passivity in high-pH solutions, with charge transfer resistance and passive film thicknesses showing an increase with immersion time. When chloride is added to the system, the higher pH maintains Magnox passivity, as shown through a combination of potentiodynamic and time-lapse/post-corrosion imaging experiments. Potentiodynamic polarization of Magnox reveals a −229 mV/decade linear dependence of breakdown potential with chloride ion concentration. The use of the scanning vibrating electrode technique enabled the localized corrosion characteristics to be followed. At high pH where Magnox is passive, at low chloride concentrations, the anodes that form predominantly couple to the visually intact surface in the vicinity of the anode. The high pH, however, means that visually intact Magnox in the vicinity of the anode is less prone to breakdown, restricting anode propagation such that the anodes remain largely static. In high-chloride concentrations, the higher conductivity means that the anode and cathode can couple over greater distances, and so propagation along the surface can occur at a much faster rate, with the visually intact surface acting as a distributed cathode. In addition, the chloride anion itself, when present at high concentration, will play a role in rapid passive film dissolution, enabling rapid anode propagation.