## 3 Results and Discussion
### Corrosion Performance  
Figure 6 illustrates the potentiodynamic polarisation curves for monolithic Al and the produced composites in aerated 3.5% NaCl solution (pH 7). Polarisation curves for the composites with the lower HEA particulate content appear to be similar to the polarisation curve of Al. However, Al-5 vol% HEA exhibits somewhat different polarisation behaviour. More specifically, corrosion potential (Ecorr) appear to be slightly increased while the cathodic current densities correspond to higher values compared to Al and lower HEA containing composites. The increased cathodic current density values can be explained by the increased volume of the cathodic HEA particulates [15]. Nonetheless, the curves exhibit typical polarisation behaviour for Al and Al alloys in NaCl solutions; pitting potentials (Epit) are a few decades of mV higher than the corrosion potentials (Ecorr). The anodic part of the polarisation curves is divided into areas with a decreased rate of current density increase and areas with a sustained flat gradient; in the sustained flat gradient current density increases by several orders of magnitude, an indication of susceptibility to localised forms of corrosion [52; 53; 54]. It is thus concluded that the corrosion behaviour of the composites in 3.5% NaCl solution appears to be mainly controlled by the corrosion of matrix, in agreement with other studies [14; 28]. Figure 5: Hardness values for monolithic Al and Al-HEA composites. Figure 7 presents the surface of the monolithic alloy and the composites under SEM after potentiodynamic polarisation in 3.5% NaCl solution. The surface appears to be degraded with the formation of pits. A most interesting observation is that Al on the matrix-reinforcement interface (Figure 7b-d) appears to be subject to preferential dissolution. This can be attributed to the high mobility of the reinforcing phase compared to the Al matrix. The galvanic effect between the matrix and the reinforcement can be further postulated by the presence of high amounts of oxygen on areas of the matrix close to the reinforcement (Figure 7e). It should be mentioned that the reinforcement appears to be free of any corrosion signs. The main corrosion mechanism of MoTaNbVW in 3.5% NaCl solution is uniform corrosion and intergranular corrosion of inter-dendrites at the dendritic/inter-dendritic boundaries due to the electrochemical potential difference between the dendrites (rich in Mo, Ta, W) and inter-dendrites (rich in Nb, V). Mo, W, and Nb are beneficial in decreasing the occurrence of pit corrosion in the MoTaNbVW system [55]. Corrosion behaviour of the composites appears to be the sum of counterbalancing mechanisms: HEA particulates may improve corrosion performance of the composites by stabilising the oxide film. Indeed, [PERSON] et al. reported that the introduction of CuZr-NiAlTiW HEA particulates in Al led to improved corrosion performance due to the stabilising effect of Ni and W in the transition layer [47]. In addition, the introduction of HEA particulates in Al leads to a decrease of the available Al area that may be subject to degradation. On the other hand, the introduction of HEA particulates might lead to the formation of discontinuities on the Al oxide film; the interface of Al/reinforcement may act as a corrosion initiation site. Increasing the volume of the reinforcement might lead to a more pronounced galvanic effect that may accelerate the dissolution of Al on the Al/ reinforcement interface. Another consideration is the galvanic effect between areas richer in HEA particulates and areas with a lower concentration of particulates. However, the intensity of the galvanic effect might be lessened by the good homogeneity/particle distribution and the lack of formation of secondary intermetallic phases or layers with different compositions. Other studies highlighted the importance of the high sintering quality and relative density on enhancing the corrosion resistance of AMCs reinforced by HEA particulates in a seawater solution [47].