Abstract
A new approach to reduce the chromium and aluminium concentrations in FeCrAl alloys without significantly impairing corrosion resistance is to alloy with 1–2 wt.% silicon. This paper investigates the “silicon effect” on oxidation by comparing the oxidation behavior and scale microstructure of two FeCrAl alloys, one alloyed with silicon and the other not, in dry and wet air at 600 °C and 800 °C. Both alloys formed thin protective oxide scales and the Cr-evaporation rates were small. In wet air at 800 °C the Si-alloyed FeCrAl formed an oxide scale containing mullite and tridymite together with α- and γ-alumina. It is suggested that the reported improvement of the corrosion resistance of Al- and Cr-lean FeCrAl’s by silicon alloying is caused by the appearance of Si-rich phases in the scale.
Highlights
Ferritic Fe-base alloys containing chromium and aluminium, commonly referred to as FeCrAl alloys are frequently used as high temperature resistant materials
FeCrAl alloys form oxide scales dominated by metastable forms of alumina which are less protective than α-alumina scales
The idea behind the present paper is to elucidate the beneficial effect of silicon on FeCrAl oxidation behaviour by characterizing the protective alumina scales formed by a Si-alloyed model alloy in humid air and in dry air and by comparing with the microstructure of oxidation of a similar FeCrAl which is not alloyed with silicon
Summary
Ferritic Fe-base alloys containing chromium and aluminium, commonly referred to as FeCrAl alloys are frequently used as high temperature resistant materials. The ability of FeCrAl alloys to resist high temperature corrosion relies on the formation of a protective alumina scale. Recently there has been an increasing interest to use FeCrAl alloys in lower temperature applications such as biomass boilers [4], nuclear [5, 6] and SOFC balance-of-plant components [7], operating in the temperature range 500–800 °C. In this temperature range, FeCrAl alloys form oxide scales dominated by metastable forms of alumina which are less protective than α-alumina scales. Using FeCrAl alloys at such relatively low temperatures gives rise to two major challenges: Firstly, the relatively slow diffusion of aluminium in the alloy and the corresponding slow supply of aluminium to the growing scale can hinder formation of a protective alumina scale at low temperature; secondly, the formation of significant amounts of brittle phases such as α′ and sigma phase has to be avoided in order to maintain ductility of the material [8, 9]
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