## 3 Electrodeposition
### Electrolytes
It is important to highlight a key issue concerning the composition of alloys obtained from solutions containing oxyanions. Most authors report the composition of the resulting alloys in terms of their metal content. However, these alloys may also contain oxygen, with reported levels reaching from about 4 at% up to 22 at% in the case of NiCoCuMoW alloys [57; 58]. The origin of this oxygen is not clearly explained; it may result from secondary surface oxidation, or it is likely related to the reduction mechanism of oxyanion precursors. Nevertheless, the presence of oxygen attributed to metal oxides [29; 30; 55] is observed in coatings containing elements such as molybdenum, tungsten, and chromium. An interesting behavior of the FeCoNiCuZn alloy in contact with the electrolyte (sulfate-chloride-citrate) was reported by [PERSON] et al. [41]. After the electrodeposition was completed, the alloy was left in the bath for varying periods of time (up to 60 min). A gradual increase in copper content was observed, rising from 13 at% to 58 at%, while the concentrations of the remaining metals in the alloy progressively decreased in a similar manner. This compositional shift led to a change in the phase structure, introducing metallic copper into the structure of the high-entropy alloy. These observations highlight the necessity of the immediate removal of the coatings from the plating bath to prevent unfavorable secondary reactions. The application of non-aqueous systems opens up new possibilities for the formation of multicomponent alloys, as they eliminate hydrogen evolution that typically occurs in aqueous solutions. This modifies plating conditions and broadens the electrochemical window, allowing for the codeposition of metals with significantly different reduction potentials [37]. Organic electrolytes are typically based on aprotic solvents such as dimethylformamide (DMF) and acetonitrile (AN) [16; 37; 38; 42; 49; 50], which promote uniform coating deposition and readily dissolve metal salts added in the form of chlorides. To increase the conductivity of such electrolytes, lithium perchlorate (LiClO\({}_{4}\)) is added. Notably, this type of electrolyte was used for the first electrodeposition of the high-entropy alloy BiFeCoNiMn [16]. Alternative electrolytes include mixtures of dimethylsulfoxide (DMSO) and acetonitrile [56], as well as a novel bath based on glycerin [40], both used as solvents for metal chlorides. The codeposition of FeNiCoCrMn alloys has been reported using both aqueous and non-aqueous electrolytes, although direct comparison of the results is difficult due to differences in deposition modes. Nevertheless, clear differences in alloy morphology have been observed: from compact but rough surfaces obtained from an aqueous chloride-sulfate-citrate bath (pH 1.5, galvanostatic deposition) [55] to coatings consisting of globular units with inter-globular porosity deposited from a chloride-DMF-AN bath under potentiostatic conditions [38], and finally to smooth morphologies with surface crystal clusters obtained using the same type of organic bath under pulse potential deposition [49]. Notably, changes in the plating conditions preserved the same crystal lattice of the alloy (FCC) [49; 55], although HEAs exhibited different proportions of particular elements Fe23 Ni21 Co20 Cr16 Mn20 [55], Fe(37-41)Ni(4-10)Co(27-29)Cr(24-27)Mn(0.1-0.6) [38], and Fe(24-31)Ni(6-8)Co(18-22)Cr(7-10)Mn(35-41) [49]. The replacement of DMF with DMSO [56; 38] in the organic solvent mixture can significantly modify both the composition and morphology of the alloy, although it cannot be excluded that the type of substrate, such as aluminum [38] or copper [56], may also influence nucleation and the subsequent growth of the alloy under potentiostatic conditions (Figure 3). Notably, the composition of the alloys was found to be time-dependent (30-90 min), with improved incorporation of manganese and nickel into the FeNiCoCrMn alloy over longer deposition times [56; 38].