A Study of Adherent Oxide Scales,
UNITED TECHNOLOGIES RESEARCH CENTER EAST HARTFORD CT
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The effects whereby minor element additions yttrium improve oxide scale adherence were investigated. These studies have led to the proposal of a new and different mechanism for oxide adherence. This new mechanism involves the effects of interfacial segregation of indigenous alloy impurities such as sulfur. A new mechanism to account for the beneficial effects that small additions have on the adherence of oxide scales is proposed. Sulfur is known to be present at tramp levels less than 100 ppm within nickel and nickel-based alloys and it can segregate to metal surfaces. However, it has been disclosed here that such sulfur segregation can also markedly affect the adherence of the protective oxide scale. In the absence of elements like yttrium, such segregation effects weaken the bond between the protective scale and the substrate metal. The role of the yttrium is to interact with sulfur to form a refractory sulfide. This interaction lessens the amount of sulfur available to segregate to and concentrate at the critical scale-metal interface. The results of experiments involving Auger spectroscopy, optical, scanning electron microscopy, scanning electron microprobe and scanning transmission electron microscopy techniques in conjunction with isothermal and cyclic oxidation testing which have led us to propose this mechanism are presented. Originator supplied keywords include Oxidation, Coatings, Oxide Scale Adherence, Auger Electron Spectroscopy, Oxide Scale Formation, Laser-Processing, Minor Element Effects.
- Metallurgy and Metallography