Investigations for the Validation of the Defect Based Scale Failure Diagrams—Part II: Extension of the Concept and Application to Nickel Oxide, Titanium Oxide and Iron Oxide

Abstract

The chemical stability of oxide scales and the oxide growth kinetics are important factors to consider when choosing a material for high temperature application. Low oxide growth rates and good chemical stability are, however, not the only aspects to be taken into account. The mechanical stability of the oxide scale formed can also play a significant role, especially when external loads or fast heating or cooling rates come into play. In this work, experimental data on oxide scale failure and a defect based scale failure model are used to calculate mechanical stability diagrams for titanium oxide and iron oxide. For these diagrams the original η-c-approach is extended by a term characterizing the level of residual strains in the scale. In addition to titanium and iron oxide this extended approach is also applied to former measurement data on nickel oxide. With the stability diagrams developed it is possible to estimate the maximum tolerable strain for the oxide scale as a function of the physical defect situation in the scale. Metallographic inspection and 4-point bending tests are used to derive the mechanical stability parameter η and the parameter ε_r for the residual strain. Once these parameters are known, metallographic inspection alone is sufficient to estimate the remaining tolerable load or strain limit after a certain oxidation period.