Stresses and decohesion of oxide scales

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Abstract
Stresses in oxide scale–metal systems can lead to decohesion of the scales when critical values are exceeded, which manifests itself in stable growth of interfacial cracks and separation along the scale/metal interface or in sudden spalling of the scale or of parts of it as a result of unstable cracking processes. The discussions in this paper are focused on decohesion of scales resulting from stable interfacial crack growth. It is attempted to summarise the critical conditions for separation at the scale/metal interface, the effect of stresses on the scale/metal interface behaviour, and the effect of decohesion on the protective effect. With the help of experimental results, the location of the plane of decohesion in multilayer scales and the decohesion of oxide scales on heat exchanger tubes as components used in practice are discussed. Finally, some considerations on the measurement of adhesion strength of scales are carried out leading to the recommendation of a fracture mechanics approach to this problem. MST/858