**7. Comparison of Al–TM Alloys with Different Chemical Composition**

Corrosion parameters of previously studied Al–based CMAs are given in Table 5. The corrosion behavior of Al–Co, Al–Pd and Al–Pd–Co alloys in aqueous NaCl (0.6 mol L−1) is compared in Figure 19. Corrosion potentials of Al–Co alloys decrease with increasing Al concentration. The Al–Pd alloys have lower corrosion potentials. The values are smaller than the corrosion potentials of the remaining two alloy groups. Furthermore, the corrosion currents of Al–Pd alloys are higher compared to the Al–Pd–Co and Al–Co alloys (Figure 19b). These observations indicate that Al–Pd alloys are more susceptible to corrosion attack compared to the remaining two alloy groups.




**Table 5.** *Cont.*


**Table 5.** *Cont.*

**Figure 19.** Corrosion parameters of Al–Pd, Al–Co and Al–Pd–Co alloys in aqueous NaCl (0.6 mol L<sup>−</sup>1). Data is compiled from references [81,82,84,105,107].

The corrosion behavior of the Al–Pd–Co alloys is closer to Al–Co alloys (Figure 19). This observation is unexpected, since Al–Co–Pd and Al–Co alloys have different phase constitutions. Moreover, the preferentially corroding phase is ε<sup>n</sup> in the Al–Pd–Co alloys. ε<sup>n</sup> is absent in the Al–Co alloys. It can be noted that Co substitution for Pd significantly improves the corrosion resistance of εn. The positive influence of Co on the corrosion resistance of Al–TM alloys has also been observed by Sukhova and Polonskyy [108]. It is therefore the chemical composition and not the crystal structure of the phase that plays a dominant role in the corrosion resistance.

To further probe the role of chemical composition, we have compared the corrosion parameters of the previously discussed Al–TM alloys. The data compilation is plotted in Figure 20. The parameters are relatively scattered due to large differences in the overall alloy chemical compositions (Table 5). Nevertheless, some general trends can be noted. The as-solidified Al–Pd–Co alloys have corrosion current densities comparable to Al–Cu–Fe alloys. The corrosion potentials of the Al–Pd–Co and Al–Cu–Fe alloys are close to −650 mV (vs. Ag/AgCl)). The Al–Cr–Fe alloy is also included in Figure 20. This alloy has a lower corrosion potential compared to the remainder of the alloys. This is related to the absence of noble metals, such as Pd, in the alloy. Furthermore, the Al–Cr–Fe alloy has a low corrosion current due to the presence of Cr [90]. This element is responsible for a rapid passive layer formation on the alloy surface.

**Figure 20.** Corrosion parameters of ternary Al–TM alloys in aqueous NaCl (0.6 mol L−1). Data is compiled from references [90,107,109,110].

The corrosion parameters of Al–Co–Ti alloys [110] are also included in the same figure. The corrosion potentials of these alloys are comparable to Al–Pd–Co alloys (Figure 20). The concentration of Ti in the Al–Co–Ti alloys was constant (2 at.%). The atomic fraction of Co was varied between 5–30 at.%. Due to small and constant Ti concentration, the

microstructural features of the Al–Co–Ti alloys were comparable to Al–Co alloys [81,84]. The corrosion current densities of Al–Pd–Co alloys, however, were smaller compared to Al–Co–Ti alloys. The Al–15Co–2Ti alloy was an exception as the alloy demonstrated a lower corrosion current compared to the remainder of the alloys. The difference was related to different intermetallic particles contained in the alloy (Al13Co4, Al9Co2, and Al3Ti). They had different volume fractions and morphologies compared to the remaining Al–Ti–Co alloys [110]. The observations show that specific Co atomic fractions may significantly increase the corrosion resistance of the bulk Al–TM alloys. The ε<sup>n</sup> phase of the Al–Pd–Co alloys had a high concentration of Co. The Co additions significantly contributed to the superior corrosion performance of the bi-phasic Al70Pd25Co5 alloy.

The corrosion parameters of the structurally complex Al–TM phases are comparable to previously studied Al–TM intermetallic phases with simpler structures [111]. Therefore, it is the chemical composition of the SCIP and not the crystal structure that influences the corrosion behavior. The electrochemical activity of the SCIPs may also vary with time. Zhu et al. investigated the corrosion performance of Al–TM intermetallic phases over time [78]. At early stages of exposure, a de–alloying was the primary corrosion mechanism. The de–alloying led to an ennoblement of intermetallic particles over time due to preferential Al leaching [78]. The ennoblement speeded up an anodic dissolution of the adjacent matrix and worsened the corrosion behavior. A long-term annealing may also influence the corrosion performance of the alloy constituent phases. It reduces internal stresses generated during casting and contributes to a more uniform element redistribution in the SCIPs.

#### **8. Conclusions**

In this paper the electrochemical corrosion behavior of Al–TM alloys composed of SCIPs has been reviewed. The following conclusions can be drawn:

1. The Al–TM alloys have a capability of forming passive layers because of high Al concentration. The Al–Cr alloys, for example, can form protective passive layers of considerable thickness in different electrolytes.

2. In halide-containing solutions the Al–TM alloys are prone to pitting corrosion. Galvanic microcells between different SCIPs form which may further accelerate the localized corrosion attack.

3. The electrochemical activity of aluminum–transition-metal SCIPs is primarily determined by electrode potential of the alloying element(s). The electrochemical nobility of individual SCIPs increases with increasing concentration of noble elements. The SCIPs with less noble elements tend to dissolve in contact with nobler particles. The SCIPs with noble metals are prone to selective de-alloying (de-aluminification). The electrochemical activity of SCIPs may change over time.

4. The chemical composition of the SCIPs has a primary influence on their corrosion properties. The structural complexity is secondary. It becomes important when phases with similar chemical composition come into close physical contact. The phase with higher structural complexity tends to be cathodic and can be retained during corrosion.

**Author Contributions:** Conceptualization, M.P. and L.D.; methodology, M.P., L. ˇ D. and I. ˇ C.; formal ˇ analysis, M.P., L.D., I. ˇ C. and P.P.; investigation, M.P., L. ˇ D., I. ˇ C. and P.P.; resources, M.P. and P.P.; ˇ data curation, M.P.; writing—original draft preparation, M.P., L.D. and I. ˇ C.; writing—review and ˇ editing, M.P., L.D., I. ˇ C. and P.P.; visualization, M.P., L. ˇ D., I. ˇ C. and P.P.; supervision, M.P.; project ˇ administration, M.P., and P.P.; funding acquisition, M.P. and P.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Grant Agency VEGA of the Ministry of Education, Science, Research and Sport of the Slovak Republic and Slovak Academy of Sciences, grant number 1/0330/18, the Slovak Research and Development Agency project number APVV-20-0124 and the European Regional Development Fund, project No. ITMS2014+: 313011W085.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** This is a review article. The data used in this paper are publicly available in cited references.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

#### **References**

