The precise effect of grain size on corrosion at a fundamental level is not widely understood. In this work we will focus solely on the corrosion of magnesium of 99.9 % purity, in order to address a critical fundamental knowledge gap in the assessment of Mg corrosion as a function of grain size. This is of a critical importance as corrosion susceptibility for a given environment is commonly interpreted in terms of Pourbaix diagrams, [1] which do not account for the effect of any microstructure parameters, such as grain size. Furthermore they tell us nothing about the rate at which Mg may corrode in the active region. With regards to Mg, the oxide layer is not stable in aqueous solutions owing to high compression stresses within the oxide layer (geometrical mismatch with respect to the hexagonal Mg lattice), which cause cracks. [2] One possible way of compensating for this effect is by deliberately introducing a large volume fraction of grain boundaries in the bulk material and diminishing the mismatch. However, it is unknown if the oxide that forms on such boundary regions in Mg will have better coherency and if this will be manifest in the corrosion response. Also, the development of advanced casting methods for Mg alloys leading to thin sections below 1 mm translates to an urgent need to study the effect of grain size on corrosion to ensure the integrity of such components or at least understand the factors which control it. We would like to emphasise that there is a lack of literature on purely microstructural effects on corrosion, particularly in the absence of alloying elements. If the effects of chemistry could be isolated and the grain structure varied, then potentially the effect of grain size on corrosion could be determined. Some relevant prior works however have indicated that a decrease in the grain size improves corrosion resistance for c-stainless steel, and was found to reduce the localized effects of corrosion such that both intergranular corrosion rate and pitting were reduced. [3] Corrosion resistance also increased with decreasing grain size in zirconium at high temperature when nanocrystalline grains were compared to coarse grains. [4] It was further found that corrosion morphology was altered as polycrystalline Cu was grain refined by equal channel angular pressing, [5] whilst corrosion reduction was also demonstrated for IF steel. [6] In the present study a cast ingot of pure magnesium (99.9 %) was used as the initial material for the work. Specimens were then prepared for the study using a variety of mechanical or thermo-mechanical processes and annealed at 250°C in a potassium sulphate salt-bath with specimens packaged in Al foil, in order to obtain a variation in grain size. A range of grain sizes was produced by processing the as-cast specimens using Surface Mechanical Attrition Treatment (SMAT) and the Equal Channel Angular Pressing (ECAP). ECAP is a severe plastic deformation technique that results in grain refinement of the bulk of a material by pressing it through an angular die (ty...