## 4 Electroless Deposition  
Electroless deposition is a chemical method for forming metallic coatings without the use of an external electrical current, enabling uniform coatings even on complex-shaped surfaces. This technique relies on controlled redox reactions between metal ions and reducing agents in solution [70] via a displacement reaction with more active metal (aluminum) [2]. The choice of reducing agent (e.g., sodium hypophosphite, dimethyl borane, sodium borohydride, glucose) must be appropriate for the specific metal (typically nickel, cobalt, copper, noble metals) being deposited, as the metal itself acts as a catalyst for the oxidation of the reducer, ensuring the continuity of the deposition process [70]. As such, the process is inherently autocatalytic, with the growing metal layer sustaining the reaction once initiated. Electroless deposition allows for the production of single-metal coatings, as well as binary, ternary, and even quaternary alloys, often containing small amounts of nonmetals such as phosphorus or boron originating from the reducing agent. The successful deposition of multicomponent alloys requires that at least one of the metal components exhibits catalytic activity toward the oxidation of the reducing agent, which is essential for sustaining the autocatalytic process. Although autocatalytically deposited multicomponent alloys have already been discussed in the literature [70; 71], these were not considered high-entropy alloys because they typically did not meet the fundamental criteria of HEAs, such as containing at least five principal elements in near-equimolar ratios and achieving high configurational entropy. Instead, many of these coatings were designed for specific functional purposes (e.g., corrosion protection, hydrogen evolution catalyst) without regard to entropy-based alloy design principles. In fact, only a very limited number of studies [72; 73; 74; 17] have successfully demonstrated the electroless deposition of true high- or medium-entropy alloy coatings (Table 5). [PERSON] et al. 's paper [17] appears to be the first study reporting on an electrolessly deposited high-entropy alloy matrix reinforced with graphene oxide, specifically CuNiFeCrMo-GO. The alloy was synthesized from an acid chloride-hypophosphite bath, with lactic acid serving as a stabilizing agent. The HEA matrix exhibited a cell-like morphology with particles growing laterally toward the smooth substrate surface. The resulting nanocomposite coating formed a solid solution comprising a mixture of FCC and BCC phases. Although only metallic elements were identified in the alloy, no detailed compositional ratios were provided. The deposited coating showed a significant enhancement of hardness (325 HV) exceeding that of the mild steel substrate by approximately 2.5 times and a 27% reduction in wear rate. [PERSON] and [PERSON] [73] investigated the formation of a medium-entropy Ni69 Co19 WZr0.8P9 alloy using an alkaline chloride-citrate-hypophosphite bath, supplemented with lactic acid as a stabilizer and a zwitterionic surfactant to enhance adhesion. The resulting nongrained coating, with an average crystallite size of 0.4 nm, demonstrated improved corrosion resistance compared to mild steel and a NiZrP coating, although it was inferior to that of conventional NiP alloys. Nevertheless, the quaternary alloy exhibited superior adhesion and wear resistance relative to its binary and ternary counterparts. [PERSON] et al. [74] developed a method for forming high-entropy alloys on aluminum substrates using immersion plating. Equimolar layers of copper, nickel, and cobalt were sequentially deposited, followed by heat treatment to induce thermal diffusion of the elements. This process transformed the initially multilayered structure, composed of distinct phases (HCP-Co, FCC-Cu, FCC-Ni), into a single-phase (FCC) ternary alloy with a homogeneous elemental distribution across both the surface and cross-section. The resulting alloy exhibited superior corrosion resistance compared to single-metal coatings.