Aqueous corrosion of magnesium and magnox

The early stages of film growth on magnesium and Magnox alloy have been investigated in situ using an automatic nulling ellipsometer coupled with potential control of the specimens. SIMS and scanning Auger spectroscopy in conjunction with ion beam milling have been used where possible to confirm the ellipsometric results. Initial results obtained in 0.01 mol/dm3 NaOH indicated that substrate dissolution was leading to a roughening of the film/substrate interface. This was attributed to carbon dioxide absorption in the air-saturated solutions causing a reduction in pH to values below 11.5 with a consequent breakdown in passivity. This led to further experiments under various conditions of solution composition and pH. The proposal of a rough surface was investigated initially by computer simulations whereby the rough surface was treated as an effective film containing both the substrate and the overlying film. Confirmation of these theoretical models was sought by the use of ellipsometry at a different angle of incidence and by scanning electron microscopy. Maintaining the pH near 12 prevented the breakdown of passivity in Magnox. This enabled a more detailed study of the corrosion processes to be made. The behaviour of Mg under these conditions was still anomalous and was attributed to the more porous film present on Mg compared to Magnox. This could cause a channelling of the corrosion attack leading to a roughened substrate. The results obtained for Magnox in neutral and strongly alkaline (1 mol/dm3 NaOH) could not be explained either by a single layer or rough layer model. The results could, however, be explained if it was assumed that the optical constants of the substrate were changing. This could occur if there was segregation of the alloy components near the metal surface.