Stability of External α-Al2O3 Scales on Alloy 602 CA at 1100–1200 °C
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An external ultrathin α-Al2O3 scale grown on the Ni-base alloy 602 CA during air oxidation at 800 °C was characterized by means of high-resolution TEM/EDX and electron diffraction. Alloy samples pre-oxidized at 800 °C were subsequently exposed at 1100, 1150 and 1200 °C for up to 100 h. Whereas the external alumina remained stable at 1100 °C, with the increasing exposure temperature, the pre-grown alumina scale tended to break down resulting in an external chromia scale accompanied by internal alumina precipitation. The transition from external to internal Al oxidation was investigated using SEM/EDX/EBSD. The critical Al depletion at the scale-alloy interface during the post-exposure at 1100–1200 °C was modeled using the CALPHAD-based thermodynamic-kinetic approach.
KeywordsAlloy 602 CA Internal oxidation Alumina scales Depletion Breakaway Pre-oxidation
The authors are grateful to Dr. H. Hattendorf from VDM Metals (Werdohl, Germany) for providing a batch of 602 CA. Mr. H. Cosler is kindly acknowledged for carrying out oxidation tests.
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