377continue either by diffusion of dichromate ions through remaining discontinuities in the oxide film or by diffusion of magnesium ions through the film itself. Such a mechanism would account for the relatively rapid initial increase in chromium content, followed by a much slower rate as shown by the linear portion of the curves in Figs. 2 and 3.
ConclusionsOn the assumption that the passivating film formed on magnesium, after immersion in an aqueous solution of potassium dichromate, is independent of the state of sub-division of the metal, a simple method, requiring only a limited amount of equipment, is available to measure both the absolute and relative surface areas of coarse magnesium powder in a range which is not easily determined by conventional techniques. The results are consistent to within about &lo%. The passive film which is formed is considered to arise from the 'precipitation' of insoluble chromium oxides on the metal surface.