A diffusion problem

When two polished metal spheres in contact with each other are immersed in a corrosive solution, their surfaces gradually corrode, leaving, however, a non-corroded zone around the point of contact. The size of this zone is a function of the size of the spheres, concentration of the corrosive, and of the time. The phenomenon has been first observed by F. C. Besic on human teeth immersed in hydrochloric acid, and studied on metal balls. In the present paper it is shown that such a phenomenon may be due to gradients of concentration of the corrosive in the neighborhood of the point of contact, caused by the chemical reaction which consumes the corrosive. Approximate expressions for the size of the uncorroded zones as a function of the size of the balls and as a function of time are derived and found to be in fair agreement with F. C. Besic's data.