ASSESSMENT OF THE LIMITS TO THE PROTECTIVE EFFECT OF OXIDE SCALES IN HIGH-TEMPERATURE TECHNOLOGY, SET BY DEFORMATION OF THE SUBSTRATE MATERIAL

At high temperatures materials have to stand corrosive attack by respective environments. The prerequisite for good high temperature corrosion resistance is the formation of dense protective oxide scales on the alloy surfaces which prevent increased corrosive attack. Due to their brittle nature, however, even low strains may crack the protective scales so that considerations on the limits of the protective effect become necessary when the substrate alloy is subjected to strains. These considerations are performed in the paper including aspects of scale cracking and healing and in the end a scale failure diagram is developed which shows the limits of the protective effect of oxide scales as a function of (substrate) strain and strain rate. It is also proposed to extend this diagram by the parameters of physical defect size in the scale and of scale thickness, which would lead to increased precision when characterizing the limits of the protective effect. The use of the scale failure diagram in design and service helps to avoid component damage resulting from the interaction of high temperature corrosion and mechanical stresses.