## 3 Electrodeposition
### Current/Potential Regimes
show a consistent trend in alloy composition changes. Nevertheless, increasing the duty cycle (from 50% to 60%) led to a reduction in crystallite size, while the effect of changing frequency (2.5 kHz or 5.0 kHz) depended on the duty cycle. [PERSON] et al. [45] applied pulse electrodeposition at different current densities (20-100 mA/cm\({}^{2}\), 50% duty cycle) and used a slightly acidic chloride-sulfate-citrate-oxyanion bath. They observed that increasing the current density did not affect the order of element contents in the FeCoNiMoW alloys, although the individual concentrations of the metals changed in three distinct ways. These trends included the following: (i) increased contents of Fe and Co, (ii) a slight increase followed by stabilization in Ni content, and (iii) decreased contents of Mo and W. Such changes were attributed to the formation of ternary complexes between iron-group metals, tungsten or molybdenum, and citrate. Since the reduction of tungsten or molybdenum appeared as diffusion-controlled at higher current densities, and due to the difficulty of transporting these heavy ternary complexes to the cathode surface, their incorporation into the coating decreased as the current density increased. With increasing current density, the initially less compact, dome-shaped deposits transformed into a very rough and coarse surface with a characteristic cauliflower-like morphology. Simultaneously, microcracks observed in the cross-sections of FeCoNiMoW alloys were attributed to differences in the atomic radii of the constituent metals and/or hydrogen coevolution during deposition. In turn, [PERSON] et al. [28] deposited the same alloys from a similar bath type but using a direct current. They found that increasing the current density (20-80 A/cm\({}^{2}\)) did not affect the order of the element content in the FeCoNiMoW alloys, similar to the case of pulse currents, but the individual element contents changed in different ways: (i) a decrease in Fe content, (ii) stable Co content, (iii) an increase in Ni content, and (iv) an increase in followed by stabilization of W and Mo contents. The authors stated that iron deposition was controlled by ion diffusion and concentration polarization, which was inhibited at higher current densities. In contrast, the deposition of nickel, molybdenum, and tungsten was governed by electrochemical polarization, which was enhanced as the current density increased, leading to better incorporation of these elements into the coating. These studies show that pulsed current favors the incorporation of Fe and Co, while direct current promotes the deposition of Ni, Mo, and W in FeCoNiMoW alloys due to different rate-limiting steps in metal reduction from citrate-type baths. Notably, although the coatings were homogeneous, surface cracks appeared and their density increased with rising current density, a phenomenon attributed to hydrogen embrittlement. [PERSON] et al. [59] investigated the effect of deposition potential (-1.5 V to -2.7 V) on the codeposition of AlCrFeMnNi and AlCrFeMnNiCu alloys from organic chloride baths (DMF-AN). They found that more negative potentials only slightly affected the incorporation of aluminum, chromium, iron, and manganese (by about 1 at%) but significantly altered the nickel and copper contents (by about 10 at%) in variable ways. All of the deposited alloys were amorphous, exhibiting spherical surface growths and increasing cracking at more negative deposition potentials. [PERSON] et al. [68] investigated the codeposition of CoFeNiZn alloys from a chloride-sulfate bath using a pulsed current (PC) mode and a horizontally positioned cathode with an unconventional pentagonal geometry (Figure 5a). This setup enabled the formation of gradient coatings due to uneven mass transport along the cathode surface. The increased concentration of zinc ions in the plating baths resulted in faster transport toward the apex of the cathode compared to the bottom area.