ABSTRACT
The paper briefly summarizes fundamental aspects of transport processes and scale properties in growth of continuous scales. While early interpretations were based on the Wagner model and scale growth by lattice diffusion, it has been increasingly recognized that transport along grain boundaries and other easy diffusion paths often predominate in growth of highly protective scales. This, in turn, means that knowledge of the composition, structure and properties of grain boundary and interfacial regions are of fundamental importance in understanding high temperature corrosion. These aspects are illustrated by examples involving the effects of oxygen active elements and the growth of duplex oxidesulphide scales. The influence of water vapour on the oxidation behaviour of metals and alloys – and particularly the loss of protective behaviour of technical steels – is also briefly discussed. There are no satisfactory interpretations of these phenomena, but as a possible correlation water vapour may significantly affect defect structures and defect-dependent properties of some oxide systems by serving as a source for proton defects in the oxides.
