Abstract
Structural changes in coatings are caused by the selective removal of elements at the outer face and by element transfer to and from the base alloy. The physical metallurgy of the diffusion processes involved in these changes is summarized. It is shown that complex situations can be modelled by using the concept of a diffusion path on the isothermal section of a ternary phase diagram and this concept is applied to the production of cementation coatings and to the behaviour of cementation and overlay coatings in service. Attempts to devise diffusion barriers to the element transfer are also discussed. MST/287