The concentration of nitrogen in typical steels tends to be small, typically 10-30 parts per million, so much of the information about nitrides in steels comes from studies of nitrogen-enriched surfaces. There are significant structural differences between these two nitrides, neither of their compositions are exactly stoichiometric. The kinetic theory is incomplete because there is no treatment of the nucleation of the nitrides, nor of the time taken for the dissociation reactions that must occur at the surface of the iron. A layer of wuestite on the surface of the iron can promote the nucleation of particular nitrides but the reasons for this are not clear. High nitrogen stainless steels, although difficult to produce, have significant applications including impellers and high-performance bearings. They contain up to about 0.6wt% of nitrogen so are prone to the precipitation of chromium nitrides following heat treatment.