Conditions for the initiation of oxide-scale cracking and spallation

The processes leading to oxide spallation are discussed in terms of two routes depending on the relative strengths of the surface oxide and the oxide-metal interface. Assuming that the major source of stress is the differential contraction strains produced during a temperature decrease, simple equations are developed which predict the critical temperature decrease to initiate spallation in terms of the oxide thickness and its elastic properties. The concepts are checked using a 20Cr-25Ni-Nb-stabilized stainless steel oxidized in sensitive thermobalances. It is shown that this material exhibits cracking and spallation behaviour commensurate with a relatively strong oxide-metal interface. An upper-limit estimate is made of the energy required to produce decohesion at this interface.