Diffusion inter-granular

A small leak grows by a combination of corrosion and erosion. Although a high velocity jet of steam or water blows out of the leak, there is back-diffusion of sodium round the jet into the hole, where it reacts with water and corrodes the metal. Grains are loosened by inter-granular corrosion and swept away by the jet. 

To ensure that this last effect is not a rate-determining step and to reach the tme reaction speed, we can carry out the experiments either on very thin powder beds or out of agitated engine (fluidized bed, rotary furnaces). Note that many authors observe an influence of the mass of the sample on the thermogravimetric speed. This influence is normal because this speed is an extensive property. If we consider the rate, brought back to the unit of initial amount of matter, in a fixed-bed, this influence of the initial amormt and the influence of the thickness of the bed are generally the proof of the intervention of the inter-granular diffusion in the speed. 

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