Abstract
This chapter is dedicated to the description of high temperature oxidation of both chromia and alumina forming alloys. The defect structures of iron and chromium are firstly reviewed. The effects of elements on stainless steel oxidation behaviour are further addressed. For the chromia-forming stainless steel, the oxidation rate is reduced with the increased silicon content but not in a monotonic manner. Titanium and niobium can reduce breakaway oxidation of Fe–18Cr–10Ni austenitic stainless steel. Titanium can enhance the adhesion of scale to the Fe–18Cr by mechanical keying effect of TiO2 formed at the steel/scale interface. For the alumina-forming stainless steel, the formation of alumina and its transformation during oxidation are reviewed. Chromium can be added to reduce the critical aluminium content in the steels in order to form alumina at high temperatures. The addition of reactive elements with appropriate level can improve scale adhesion and reduce the steel oxidation rate. Refractory element like molybdenum can increase strength of material but also accelerate the oxidation rate of the steels containing reactive elements. The development of new alumina-forming austenitic alloy grades is finally described.
Highlights
The typical thermal oxides formed on iron at temperatures higher than the eutectoid one consist of the oxygen-deficient haematite sitting on the metal-deficient magnetite which sits on the major metal-deficient wüstite
The second part of this chapter reviews the effects of alloying elements on the high temperature oxidation of the chromia-forming stainless steels
Titanium and niobium can help suppress the breakaway oxidation for the austenitic stainless steel since it significantly increases the ratio between the diffusivity of chromium in steel which represents the chromium supply from metal to metal/scale interface and the oxidation parabolic rate constant which represents the chromium consumption rate for oxidation
Summary
4.1.1 Oxidation Mechanism of Iron The formation of thermal oxide on pure iron is well known, complex, and is briefly described . At temperatures higher than the eutectoid temperature of the Fe–O system i.e. 570 oC, the formed thermal oxide scale typically consists of the thickest wüstite, with magnetite sitting on the wüstite layer and the outermost haematite sitting on magnetite [1]. It was reported that the predominant defects in wüstite and magnetite are iron vacancy, while the dominant defect in haematite is oxygen vacancy [4].
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