In the first instance there is an obvious loss of ma­terial from exposed surfaces and thin sections may be wasted away or seriously weakened. The effect of oxidation on a low silicon thin-section material is amply illustrated by Fig 3.73. This shows the oxida­tion present on a jubilee clip removed from a reactor and shows the growth of oxide on all surfaces to­gether with the thinning of the material.

When oxide forms, it occupies a larger volume than the material from which it is derived. The ratio of oxide thickness to that of the original metal from which it is formed is about 2.5 so that the volume change is considerable. If the scale is formed between two bolted or welded components then these are forced apart (oxide jacking) once any initial clearance has been taken up. Similarly, the growth of oxide reduces the clearance between moving components and results in seizure. This straining of the material may lead to failure of the component. Interfacial oxide builds up slightly less quickly than that formed on a free sur­face and the rate of formation does not slow down significantly as the jacking forces increase. The effects of interfacial oxide are illustrated in Fig 3.74.