Invited Review Paper in Commemoration of Over 50 Years of Oxidation of Metals: Alumina Scale Adhesion Mechanisms: A Retrospective Assessment

Key factors related to protective alumina scale adhesion are reviewed in an historical context. Critical experiments that solidified the framework of new understanding are highlighted. These include scanning and transmission electron microscopy (SEM/STEM), scanning and hot stage Auger, X-ray photoelectron spectroscopy, analytical electron microscopy and focused ion beam (FIB-STEM) analyses of reactive element (RE) segregation, photo-luminescence spectroscopy, creep tests of RE-doped Al2O3, imaging secondary ion mass spectrometry, S-RE or Hf/Y co-doped alloys, desulfurization, and density functional theory models of doped interfaces. The popular mechanisms (pegging, growth stress, scale plasticity, vacancy sink, chemical bond) are each addressed by simple logic tests. In some instances, the opposite of what was initially proposed is actually the case, thus discounting that mechanism. In contrast, chemical bond explanations survive these tests and are discussed in more detail. Control of sulfur segregation is presented as both necessary and sufficient for scale adhesion. Reactive elements also segregate and strengthen the scale-metal interface, while, at the same time and more importantly, they prevent sulfur interfacial segregation and bond weakening. Kinetic effects (Al2O3 grain boundary diffusion) are of fundamental oxidative interest, but cannot account for first-order effects on adhesion. These precepts are consistent with historical and recent industrial successes, for example, RE-doped FeCrAl heater/structural alloys, Hf-doped Ni(Pt)Al aluminide bond coats, and melt-desulfurized (0.1 ppmw S) Ni-base superalloys. Outstanding issues for which further investigation may prove insightful or advantageous are noted.Graphical Abstract Pegging, vacancy sink, scale plasticity, growth stress, graded seal theories refuted. Chemical bond (segregation) mechanisms supported. Low S prevents segregation, interface voids, spallation. RE doping prevents S segregation; increases adhesion. REIII(Y) dopants decrease Al outward, REIV(Hf) decrease O inward g.b. diffusion and growth.