Corrosion behaviour of Fe-Al(-Ti) alloys in steam

(Parts of this page are published: Vogel et al.,Intermetallics 18 (2010), 1375)

D. Vogel¹, A. Hotař^1,2, A.Vogel¹, M. Palm^{, 1}, F. U. Renner¹*

The application of metallic structures relies in many cases on the formation of a protective surface film formed in a reaction with the environment. In contact with oxygen such films usually consist of either chromia, silica, or alumina. While chromia- or silica-forming stainless steels suffer from evaporation at elevated temperatures especially in the presence of water vapour, alumina passive films are chemically stable up to very high temperatures [1]. Under these conditions alumina-forming stainless steels [2] and also iron-aluminides are a cheaper alternative to expensive Ni-based superalloys [3-4]. Alumina shows a good corrosion resistance also in aggressive environments, e.g. in chlorine-containing vapours [5]. However, the growth of the passive alumina films is complex. Non-protective, metastable γ-, δ- or θ-Al₂O₃ phases often proceed the formation of an adherent α-Al₂O₃ layer in oxidising environments [6-10]. Due to differences in specific volume and porosity within the initial metastable Al₂O₃ layers the final α-Al₂O₃ layer is often rich in structural defects. Also, a larger difference in the thermal expansion coefficients between alumina and iron-aluminide substrates leads more readily to spallation, primarily during thermal cycling [11,12]. In addition the literature suggests an influence of the dew point of the reaction gas atmospheres on the corrosion resistance but the role of water vapour or humidity has been rarely addressed. The effect of water on novel materials used at high temperatures is especially important for turbine applications. Nevertheless, to our knowledge literature about the corrosion behaviour of iron-aluminides in defined humid atmospheres or pure water vapour is scarce. Here we present first results of corrosion tests in water vapour at 700°C using binary as well as selected ternary iron-aluminide samples without pre-oxidation treatment at higher temperatures. The ternary alloys investigated here were selected because their high creep resistance, which surpasses that of other Fe₃Al-based alloys [13], qualifies them for applications at and above 600 °C.

The corrosion behaviour of binary Fe-Al alloys with Al contents ranging from 10 to 45 at.% Al and of ternary Fe-Al-Ti alloys with Al contents of about 25 at.% Al has been investigated in steam. No pre-oxidation has been applied. Saturated water vapour produced by a “steam generator” was passed over the samples in a quartz tube. The tests were conducted at 700 °C for 672 h without employing a carrier gas. The weight changes of the samples were determined after the experiments, and the formed scales were characterised by light optical microscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results for the different alloys vary quite markedly. Corrosion resistance for the binary alloys improves with increasing Al content, while the corrosion attack of the Fe-Al-Ti alloys is more severe.