## 3 Electrodeposition
### Current/Potential Regimes  
Control over the elemental and phase composition of the alloys can be achieved by adjusting current density, electrolysis voltage, or cathode potential, not only in terms of their absolute values but also the mode of their application like direct or pulsed (Figure 4). Figure 3: Surface morphology of FeNiCoCrMn alloys electrodeposited at constant potential from organic-chloride electrolytes: (**a**) DMF–AN bath, 2.5 V, aluminum substrate, Fe41 Ni4 Co27 Cr27 Mn0.3 [38]; (**b**) DMSO–AN bath, \(-2.5\) V, copper substrate, Fe20 Ni12 Co33 Cr3 Mn32 [56]. Reproduced from references under license CC BY 4.0. Galvanostatic deposition (GD, or direct current deposition, DC) is the most common and simplest mode of high-entropy alloy codeposition [21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 51]. It maintains a constant current density throughout the deposition. This causes continuous metal ion consumption at the cathode, which can lead to concentration polarization and more intense hydrogen coevolution. Consequently, the method offers limited control over nucleation and crystal growth, often resulting in compositional variations through the coating thickness, non-uniform grain sizes, and uneven current distribution across the cathode surface (edge effect). Potentiostatic deposition (PD), in turn, involves maintaining a constant electrode potential versus a reference electrode throughout the deposition process. This approach allows for more accurate control over the thermodynamic conditions of metal ion reduction, which can result in improved uniformity in both HEA composition and microstructure [16; 37; 38; 39; 40; 52; 53; 59]. Electrodeposition at constant electrolysis voltage (CV) is a much less common method used in HEA codeposition [41]. It involves maintaining a fixed voltage between the working and counter electrodes during the process. Unlike galvanostatic deposition, this method can result in fluctuations in current density with time, potentially leading to less reproducible alloy compositions and less controlled microstructures. Pulsed electrodeposition offers advantages by modulating nucleation and growth kinetics, ensuring better metal ion replenishment at the cathode surface and suppressing unwanted side reactions. This provides a powerful tool for tailoring the microstructure, morphology, and uniformity of the resulting alloy coatings. Pulsed electrodeposition can be carried out either by switching the cathode potential between two different values of PP [42; 43] or by rapidly alternating the current PC [44; 45; 46; 47; 48; 49; 50; 54; 67; 68] between a set value (\"on cycle\") and zero (\"off cycle\"), each time maintaining a fixed potential or current density for a defined duration. [PERSON] et al. [44] compared two electrodeposition modes (DC, PC) for CoNiWReP alloys produced from aqueous sulfate-citrate baths with tungsten and rhenium as oxygen-type salts and phosphorus as hypophosphite. All HEA coatings were silver-white, amorphous, and adhered well to a copper substrate. Coatings deposited by direct current showed cracks, likely due to internal stress or hydrogen embrittlement. In contrast, pulsed current improved coating quality, reducing or eliminating cracks. Moreover, pulse current plating enhanced the codeposition of cobalt and tungsten while somewhat reducing the codeposition of nickel and rhenium compared to direct current (DC: Co(28-29)Ni(24-25)W(7-8)Re(31-32)P(8), PC: Co(32-37)Ni(17-23)W(10)Re(27)P(7-9)). [PERSON] et al. [49] investigated the effect of potential pulses (\"on\" duty cycle, frequency) on the codeposition of a CoCrFeMnNi alloy from an organic chloride bath. Using switchable potentials (\(-9\) V/1 V), they obtained single-phase (FCC) HEAs with inhomogeneous morphology and surface crystal clusters. The applied potential conditions did not  
Figure 4: Current-potential modes of HEAs’ electrodeposition: (a) galvanostatic, potentiostatic, or constant voltage; (b) pulsed potential; and (c) pulsed current.