**1. Introduction**

Co-based superalloys are promising materials for high temperature structural applications because of their high melting points and favorable mechanical properties [1–3]. Applications of these alloys include gas turbines, aircraft engines, and chemical reactors [4–6]. The Co-based superalloys are often alloyed with chromium to provide oxidation resistance [7,8]. The superalloys alloyed with Cr form a compact chromia scale (Cr2O3) on their surface. Nevertheless, at high temperatures and high oxygen partial pressures, the Cr2O3 scale is prone to degradation. During long-term oxidation, volatile high-valent oxides of Cr, such as CrO2 and CrO3, start to form at the expense of Cr2O3. This effect is called "chromia evaporation" and is often pronounced in humid atmospheres [9]. The loss of protective chromia scale leads to a reduced life span of the Co-based superalloys. Several authors have, therefore, investigated the possibility of improving the high temperature oxidation stability of Co superalloys by alloying with Al [10–12]. Al-based alloys form a protective oxide scale composed of alumina (Al2O3). Al2O3 has a lower growth rate compared to Cr2O3 and is non-volatile. Furthermore, Al is a non-transition element. It has a smaller tendency to form complex oxides with transition metals compared to Cr, thereby reducing the risk of scale spallation over time. The formation of alumina scales may be achieved by pack aluminizing the alloy's surface [13–15]. The application of Al-Co coatings could significantly extend the alloy's lifetime [16–22].

Aluminides are aluminum-based intermetallic compounds with transition metals. Cobalt aluminides are interesting for high temperature applications since they possess a combination of high melting points and good corrosion resistance. At ~18–30 at.% Co, different structurally complex

aluminides in the Al-Co binary system have been observed (Figure 1, [23,24]). These include Al9Co2 (P21/C), Al5Co2 (P63/mmc), Z-Al3Co (P2/m) and family of Al13Co4 phases containing m-Al13Co4 (C2/m), O-Al13Co4 (Pmn21), O'-Al13Co4 (Pnma), Y1-Al13Co4 (C2/m) and Y2-Al13Co4 (Immm) [25–34]. Although the individual cobalt aluminides are brittle [35], the Al-Co precipitates may significantly strengthen the Al alloys [36,37]. The presence of Al5Co2 and Al13Co4 intermetallic compounds (IMCs) may also be beneficial in Co-based alloys as they may greatly improve the alloy's wear resistance [38]. Most aluminides form protective alumina scales with large resistance against corrosion [39].

**Figure 1.** Phase diagram of the Al-Co binary system, redrawn from [23,24].

High temperature corrosion studies of cobalt aluminides are limited. Metal oxidation is a heterogeneous reaction taking place in several elementary steps [40]. In the first step, a gaseous oxygen molecule is transported to the metal surface. Upon approaching the solid phase, the adsorption of oxygen molecules to the metal substrate occurs. Subsequently, the oxygen is dissociated to atoms and later reduced to the O2<sup>−</sup> anions. In parallel, oxidation of metal atoms takes place at the metal-oxide interface. Recently, M. Wardé et al. studied the adsorption of oxygen on the Al9Co2 (001) and Al13Co4 (100) surfaces at high temperatures and reduced oxygen pressures [41]. At the surfaces, only Al–O bonding was observed. Al–O distances were also calculated from first principles [41,42]. The Al–O lengths were shorter in comparison with Co–O distances. The obtained Al–O distances were in agreement with the typical distances of oxygen adsorption on the Al (111) surface, as well as with the Al–O distances in Al2O3.

Al-rich Al-Co alloys belong to a relatively new group of complex metallic alloys (CMA, [43,44]). These materials contain structurally complex phases including quasicrystals. The structurally complex phases have non-periodically ordered atomic arrangements [45,46]. Consequently, the properties of CMA are different from those observed in traditional materials [43]. The quasicrystalline surfaces have a good adhesion and low coefficient of friction [47]. Owing to their high hardness and good oxidation resistance, the Al-TM alloys (TM = transition metal) are suitable for high temperature coatings [48–51]. The Al-Co CMAs are also interesting for catalytic and hydrogen generation applications [52–55]. The corrosion studies of Al-Co alloys are limited. Lekatou et al. investigated the

corrosion behavior of an Al82Co18 (metal concentrations are given in at.%) alloy in saline solution [56]. Three methods of alloy preparation were investigated: casting, arc-melting and free sintering. The alloy prepared by arc-melting was found to be the most corrosion-resistant. The Al82Co18 alloy was composed of (Al), Al9Co2 and m-Al13Co4. The complex intermetallic m-Al13Co4 in the alloy was found to have the highest corrosion resistance. Recently, we have investigated the corrosion behavior of as-solidified and near equilibrium Al-Co alloys in various environments [24,57,58]. The alloys were composed of various intermetallic phases. In HCl and NaCl solutions, a pitting corrosion occurred. A higher corrosion resistance of structurally complex Z-Al3Co phase in Cl-containing electrolytes was observed. The difference in the corrosion behavior could be ascribed to the strong covalent character of metallic bonds in the structurally complex Al-Co phases which prevents aluminum diffusion. The studies also suggest that the existence of an electrical contact between different alloy phases play an important role in the overall alloy's corrosion behavior.

High temperature oxidation studies of Al-Co alloys have been limited to Co-rich alloys only. Zhang et al. investigated the oxidation behavior of Co-5 at.% Al and Co-10 at.% Al alloys at 973 and 1073 K [59,60]. The oxide scales were primarily composed of cobalt oxide and cobalt-aluminum oxide. The oxides grown on these alloys were relatively thick (>10 μm after 24 h). Only a limited amount of Al2O3 was found at the inner side of the scales. Irving et al. studied the oxidation behavior of Co-xAl alloys (0 < x < 32.4 at.%) at 1073–1273 K [61]. The authors found that 20–25 at.% Al is necessary to form a continuous alumina scale. As such, larger Al concentrations are needed to improve the corrosion resistance of Al-Co alloys.

To our best knowledge, oxidation studies of Al-rich Al-Co alloys have not been reported yet. In the present work, we aim to study the oxidation behavior of Al-rich Al-Co alloys in air at 773–1173 K. Two alloys, Al71Co29 and Al76Co24 (composition in at.%) were prepared by arc-melting. The composition of the Al71Co29 alloy was chosen close to Al5Co2 (71.4 at.%). The composition of the Al76Co24 alloy was close to Al13Co4 (76.5 at.%). The high temperature oxidation of the alloys was studied with the aim to identify the role of the alloy's chemical composition and microstructure on the overall corrosion behavior.
