Macroscopic Size Effect in the Composition and Properties of Alloys of Iron Group Metals with Tungsten Prepared by Induced Codeposition: Alloy Deposition Mechanism and Its Implications

By the example of electrodeposition of Co-W alloys, this work shows that observed peculiarities of induced codeposition, including the macroscopic size effect in the composition and properties of deposited layers and their nanocrystallinity, are a consequence of the fact that the deposition-inducing species (a complex of the deposition-inducing metal) has the form of a high-molecular-weight polymer. Under the conditions of (relatively) high current loading on a plating electrolyte (high volume current density), this results in involvement of water molecules in the electrochemical process, formation of oxy-hydroxide layers, hydrogenation, an increase in the alloy tungsten content as a result of the side reaction of hydrogen evolution, alkalization of near-electrode region, and polymerization of the deposition-inducing metal species. Because of the presence of macroscopic size effect (i.e., the dependences of composition and properties of deposited coatings on the electrodeposition surface area), industrial scaling up of this electrodeposition technology will require maintaining the current loading on a plating bath at a constant level, along with other parameters traditional for electrochemical materials science.