**1. Introduction**

Aluminum alloys are frequently used materials in many applications due to their low specific mass, good mechanical properties, formability, high recycling potential and superior corrosion resistance [1,2]. Two principal classifications of Al alloys exist: wrought and casting alloys, both of which are heat-treatable and non-heat-treatable. Alloying elements, including transition metals (TM), are often added to Al alloys to improve their physical and mechanical properties [3]. Complex metallic alloys (CMAs) are materials composed of structurally complex intermetallic phases (SCIPs). In SCIPs a long-range order is observed. Well-defined atomic clusters are found in the structure, typically of icosahedral symmetry. Such phases also include quasicrystals (QCs) characterized by an infinitely large unit cell (often with icosahedral symmetry) and quasicrystalline approximants, whose unit cell can be described by "classical" crystallography but contains hundreds to thousands of atoms [4–8]. Aluminum-based CMAs contain approximately 70 at.% Al. Most Albased CMAs are alloyed with TM. However, non-transition metals like Mg, can also be used. Depending on their chemical composition, the structural complexity may vary from dozens of atoms per unit cell up to thousands of atoms. This group of materials has been receiving a significant attention since the discovery of quasicrystals in melt-spun Al–Mn

**Citation:** Duriška, L.; ˇ Cerniˇ ˇ cková, I.; Priputen, P.; Palcut, M. Aqueous Corrosion of Aluminum-Transition Metal Alloys Composed of Structurally Complex Phases: A Review. *Materials* **2021**, *14*, 5418. https://doi.org/10.3390/ma14185418

Academic Editor: Daniel de la Fuente

Received: 15 July 2021 Accepted: 14 September 2021 Published: 19 September 2021

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alloy by Shechtman et al. [4]. The discovery of quasicrystals was awarded by Nobel prize in Chemistry in 2011.

In 1982, D. Shechtman discovered a quasiperiodic arrangement of atoms in an Al– Mn alloy showing an icosahedral symmetry but no unit cell [4]. Many such compounds correspond to stable or metastable states in various phase diagrams [5]. The icosahedral phase, as the first quasicrystalline structure discovered in the Al–Mn system, has a long-distance arrangement without translational symmetry [4–7]. Many elements in thermodynamically stable QCs observed yet belong to alkali, alkali-earth, transition, or rare-earth metals (Figure 1). Typical examples of QCs and quasicrystalline approximants are Al–TM alloys, where the TM is formed by one or more transition metals (TM = Cu, Co, Ni, Fe, etc.) [9]. Thus, the quasicrystalline materials are often low-cost materials that are easy to produce in large amounts [5–7,9]. Examples of quasicrystalline phases are summarized in Table 1 [7,8,10–24].


**Figure 1.** Chemical elements forming thermodynamically stable QCs. Main forming elements (Al, Ti, Zn, Cd) are marked by orange color, alloying elements are denoted by green.



The atomic structure of QCs offers an interesting combination of properties, such as low thermal conductivity combined with high electrical resistivity [25], low coefficient of friction [26,27] and/or high hardness [28–30]. The CMAs have a large potential for technical applications as the combination of the properties is unique and not observed in conventional materials. Potential applications in thermoelectrics, coatings, composites, and catalysts have been reported [5,9,31]. It has been shown that quasicrystalline materials may be more efficient catalysts compared to their crystalline counterparts. In 1994, a superior catalytic activity of quasicrystalline Al–Pd alloy relative to crystalline Al–Pd, pure Pd, and pure Cu was described [32]. Surface energies of icosahedral Al70Pd21Mn9 and Al65Cu23Fe12 phases are comparable to polytetrafluoroethylene, and yet quite different from pure aluminum and related crystalline materials. Studies revealed that surface energy is decreasing by increasing structural perfection [33]. Considering the above fact, a direct application of the quasicrystalline coatings as scratch resistant films is already on the market, offering a lowered adhesion to some polymers or food [5].

Other possible applications of QCs are found in energy saving, namely thermal insulation, light absorption, power generation, and hydrogen storage [34]. Thermal barrier demonstrators could be assessed in real conditions during the aircraft engine test on the ground. The design of selective quasicrystalline light absorbers takes advantage of the specific optical properties, e.g., the Al65Cu23Fe12 alloy has solar absorbance of 90% [9,34].

The corrosion of Al-based CMAs is a relatively new field, with first investigations emerging 28 years ago. Initial studies were focused on a small family of alloys. First, corrosion parameters were reported for quasicrystalline Al–Cr–Cu–Fe and Al–Cu–Fe alloys in aqueous Na2SO4 (0.5 mol dm−3) and in solutions of different pH [35]. The corrosion behavior of the quasicrystalline Al–Pd–Mn alloy was later studied in aqueous NaCl [36]. Nevertheless, the experimental polarization curves have not been analyzed in terms of electrochemical reactions. In later years, a more systematic approach to corrosion of Al–based CMAs has been adapted. Alloys with carefully chosen chemical composition and phase constitution have been prepared and investigated [37]. Furthermore, different electrolytes were studied [37,38]. A good thermodynamic stability of the materials at pH between 4 and pH 9 has been observed [37]. Several authors found that the electrochemical properties of the materials are determined by their chemical composition rather than by their complex crystal structure [37,39]. A high temperature oxidation of several Albased CMAs was also studied [40–42]. The presence of high Al concentration improves the corrosion resistance of SCIPs [43,44]. Despite their practical potential, however, the corrosion behavior of Al–TM SCIPs has not been systematically reviewed yet. In the present work, we aim to provide a systematic review of aqueous corrosion behavior of bulk Al-based CMAs and compare them with traditional alloys.
