Composition modifications and heat treatment procedures for increasing the emissivity of alumina surface scales on FeCrAl alloys

Abstract

The oxidation limited lifetime of thin-walled FeCrAlY heating elements is governed by the depletion of the aluminium reservoir from the bulk alloy as a result of scale growth, whereby the lifetime is known to strongly decrease with increasing temperature. The present investigation aims at obtaining a heating element lifetime extension by increasing the emissivity of the growing alumina scales because this would allow using a lower heating element temperature without decreasing the usable heat output. For this purpose, the oxidation mechanisms of a Zr doped FeCrAlY alloy as function of time, temperature, composition and component thickness were investigated using a combination of experimental techniques such as thermogravimetry, optical microscopy, scanning electron microscopy, transmission electron microscopy, focused ion beam technique, sputtered neutrals mass spectrometry and glow discharge optical emission spectroscopy. The emissivity of the oxide scales was determined by measuring the reflectivity of oxidised specimens as a function of wavelength and correlated with the oxide scale morphologies as a function of initial alloy composition and pretreatment procedure. The results showed that an increased emissivity of the alumina scales as a result of a change in oxide morphology (i.e. wavy scaleyalloy interface and incorporation of the Zr-oxide precipitates across the oxide scale) can be obtained by suitable Zr-doping of the alloy. The emissivity value during long term exposure at high temperatures is, however, adversely affected by depletion of Zr in the bulk alloy, especially in thin components. Pretreatment of the alloys in a H2–H2O containing atmosphere (low pO2) resulted in an increased emissivity, probably due to an increased Fe (and Cr) content in the outer part of the alumina scale.