| The high magnesium alloy Magnox Al80 is used as a clad for nuclear fuel employed in the UK gas-cooled, graphite moderated power station reactors of the same name. Following irradiation, spent fuel elements are stored in aqueous environments. Historical corrosion studies and plant experience have identified suitable chemistry regimes to ensure passivity of the material, by maintaining high pH and very low concentrations of aggressive anions, notably chloride. Despite this large body of work, certain aspects of the corrosion mechanism are not well characterised, notably, the growth kinetics of surfaces with thick passive films, the manner in which chloride initiates film breakdown and the underlying mechanism of localised corrosion in alkaline environments. A variety of corrosion investigation techniques have been applied here to address these knowledge gaps.Extensive electrochemical work has been supported by characterisation of the corrosion morphology with in situ observations including correlated videomicroscopy, quantitative image analysis and micro-tomography. These results have allowed targeting of quantum mechanical atomistic calculations on ionic adsorption and substitution at a simulated passive film lattice interface. Production of magnesium microelectrodes and application for corrosion study have been reported for the first time. Finite element modelling has been used for interpretation and reconciliation of these results, allowing comparison between techniques, previous observations and with plant experience.The findings of these studies have provided clarity on a number of aspects of Magnox, and magnesium, corrosion behaviour. An extension of the high field film growth model has been proposed, incorporating the effect of an outer hydroxide layer which stabilises the very thin, dense inner oxide at the metal surface. The outcomes of atomistic simulations relating to anions on the passive film lattice interface have been related semi-quantitatively to macroscopic results. Magnesium has been clearly shown to undergo salt film corrosion in conditions not substantially different from those in a fuel storage environment.Consideration of these findings in the context of previous mechanistic work has led to the proposal of a reaction scheme which reconciles the very different behaviours of these materials with a small number of underlying reactions. These describe corrosion according to reactions across a very thin surface film (whether oxide or salt film) with the rate, evolution and morphology being determined largely by the specific mass transport processes at work. |
