It can't grow indefinitely, because the reaction with the carbon consumes the reactant in the material itself. I suspect the surface coating is a bit like the l

It can't grow indefinitely, because the reaction with the carbon consumes the reactant in the material itself. I suspect the surface coating is a bit like the layer of aluminium oxide that forms on the surface of aluminium in that it seals off the mass under it from reacting with the air, and when scratched, the material underneath reacts with the carbon in the air to form a new coating.