Alloy deposition hydrogen evolution

Figure 3.21 Hydrogen evolution after each stage of BiPt surface alloy synthesis, (a) Pt film after deposition and anneal (b) immediately after Bi-UPD (c) after second anneal to form the BiPt surface alloy. Adapted from [Greeley et al., 2006a] see this reference for more details.

The composition of the codeposition bath is defined not only by the concentration and type of electrolyte used for depositing the matrix metal, but also by the particle loading in suspension, the pH, the temperature, and the additives used. A variety of electrolytes have been used for the electrocodeposition process including simple metal sulfate or acidic metal sulfate baths to form a metal matrix of copper, iron, nickel, cobalt, or chromium, or their alloys. Deposition of a nickel matrix has also been conducted using a Watts bath which consists of nickel sulfate, nickel chloride and boric acid, and electrolyte baths based on nickel fluoborate or nickel sulfamate. Although many of the bath chemistries used provide high current efficiency, the effect of hydrogen evolution on electrocodeposition is not discussed in the literature. 

Figure 11.2. Alloy deposition with hydrogen evolution. (From Science and Technology of Surface Coating, a NATO Advanced Study Institute publication, 1974, with permission from Academic Press.)...

A hydroxide suppression model first proposed by Dahms and Croll (2) explains anomalous codeposition behavior of zinc-iron group alloys. This explanation was later supported by a number of workers (3) who measured a rise in pH near the cathode surface during the deposition of Zn-Co alloy. In this model it was assumed that the Zn(OH)2 was formed during deposition as a consequence of hydrogen evolution, thus raising pH in the vicinity of the cathode. Zinc would deposit via the Zn(OH)2 layer, while cobalt deposition took place by discharge of Co2+ ions... 

Palladium is employed in a number of industrial applications and fundamental studies because of its high catalytic activity for many chemical reactions, e.g. its ability to absorb hydrogen [41], On the other hand, due to hydrogen absorption, only brittle Pd deposits can be obtained in aqueous solutions. The advantage of performing electrodeposition of Pd in ionic liquids is that hydrogen evolution does not occur. Sun et al. demonstrated that Pd and some of its alloys, namely Pd-Ag [42], Pd-Au [43] and Pd-In [44], can be obtained from the basic l-ethyl-3-methylimidazolium chloride/tetrafluoroborate ionic liquid. Compact alloy deposits were obtained and the Pd content in the deposits increased with the increase in Pd mole fraction in the plating bath. 

The influence of side reactions (such as hydrogen evolution) on the deposition rate uniformity is tmother important consideration [70] both for alloy and pure metal deposition. 

Considerations of the mechanism of charge transfer discussed for metal deposition apply also to alloys, but there are some differences. First, it must be realized that alloy deposition is a complex process, in which at least two parallel reactions take place simultaneously (i.e., the deposition of the two metals constituting the alloy), and in many cases hydrogen evolution constitutes a third parallel reaction. 

Crousier et al. examined the role of hydrogen evolution in the process of deposition of Mo-Ni alloys on different substrates (glassy carbon, Ni and Pd). It was found that on carbon and Ni substrates, bright and smooth deposits were formed, while on Pd no alloy was formed. This observation was related to easy absorption and diffusion of atomic hydrogen into Pd, which prevented its availability for the alloy codeposition process. Hence, it was concluded that hydrogen plays an important role in the codeposition of the alloy. This conclusion of the authors is, however, not convincing. Firstly, it is known that hydrogen atoms can also permeate into Ni to some extent. Secondly, unsuccessful attempt to deposit Mo-Ni alloys on Pd may also be attributed, for example, to kinetic limitations. 

Sectrodeposition of nickel and cobalt has been investigated intensively in aqueous solutions. Both metals are interesting for nanotechnology as magnetic nanostructures can be formed in aqueous solutions [47]. Hovrever, the bulk electrodeposition is accompanied by a massive hydrogen evolution. Both elements can also be deposited from acidic chloroaluminate liquids [48,49]. Cobalt and zinc-cobalt alloys... 

The contaminants may be deposited on the surfaces of the materials in the form of anhydrous or hydrated species. Some pollutants, like CO2, SO, NO, and HCl, are typical of urban and industrial areas, give rise to acid rains, and might contribute to the cathodic processes, while others, such as chlorides, are typical but not exclusive of marine and coastal areas and give rise to hygroscopic salts that increase the duration of wetting of surfaces, increase the conductivity of solutions, and make less protective the corrosion products. Some others, such as the sulfides, which can result from microbiological activity, alter the composition of the corrosion products, their protective capability, and the nobility of the metal often they are semiconductors, depolarize the cathodic process of hydrogen evolution, and may be oxidized to sulfuric acid by bacteria. Ammonia alters the composition of corrosion products and the solubility of metal ions it has particularly drastic effects on copper alloys and their corrosion forms. In the transport of these contaminants toward the surfaces, an important role is exerted by the wind and by the orientation of the surfaces, which can promote or hinder the washout by the rains. 

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