Deposition of Brass Coatings

Similar, but slightly weaker effect is also observed for less stable ZnL+ complexes. In contrast to Cu(II) species, formation of OH-containing ZnLOH complexes becomes obvious at pH 5. Simulations showed that the amounts of the remaining hydroxo-complexes are negligible, and they can be omitted from 

Comparison ofvoltammetric data obtained at the same RDE rotating velocities for Zn(II)-free and Zn(II)-containing Cu(II) solutions shows that in both cases, current densities are similar over a wide region ranging up to —0.6 V. As pH increases, the rate of hydrogen evolution falls. Furthermore, the Cu(II) reduction 

For coatings obtained at pH 5, this effect was weaker and Zn(II) reduction began at about -0.72 V. The latter case is in agreement with the previous thermodynamic estimation. 

The main Cu-Zn phases detected in various brass coatings are as follows  

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Laister and Benham have shown that under more arduous conditions (immersion for 6 months in sea-water) a minimum thickness of 0-025 mm of silver is required to protect steel, even when the silver is itself further protected by a thin rhodium coating. In similar circumstances brass was completely protected by 0 012 5 mm of silver. The use of an undercoating deposit of intermediate electrode potential is generally desirable when precious metal coatings are applied to more reactive base metals, e.g. steel, zinc alloys and aluminium, since otherwise corrosion at discontinuities in the coating will be accelerated by the high e.m.f. of the couple formed between the coating and the basis metal. The thickness of undercoat may have to be increased substantially above the values indicated if the basis metal is affected by special defects such as porosity. 

Another path to alloy deposition is via diffusion. In this case different coatings are deposited alternately, and then heat treatment is applied to promote mutual diffusion, thus ending up with an alloy. As a specific example, an alloy of 80% Ni and 20% Cr can be produced by the deposition of alternating layers of 19- m-thick Ni and 6- m-thick Cr. Subsequent heating to 1000°C for 4 to 5 h produces completely diffused alloys of rather high quality as far as corrosion is concerned. Brass can also be produced by interdiffusion of Cu and Zn under suitable conditions. 

Yet another positive aspect of the diffusion phenomenon is the creation of alloys by first depositing alternate layers of different coatings and then creating an alloy by heating to promote diffusion to produce an alloy. Specifically, brass deposits may be produced by first depositing copper and zinc layers alternately. Subsequent heating produces the required brass. This type of approach obviates the undesired direct method of brass deposition via cyanide process. 

Figure 24.4 depicts the change of surface electron energy level as a function of the thickness of a plasma polymer. In this case, plasma polymer of acetylene/N2 was deposited on brass and the contact current was measured against nylon 66. The result indicates the following two important aspects of the surface state First, the surface state electron energy level at a thin-coating thickness is influenced by that of the substrate material but becomes independent of the thickness above a threshold... 

Figure 4.12 (a) Deposition of iron and aluminum particles in glass and brass tubes of different sizes with coatings of various types. A goal of this set of experiments was to vary the electrical and surface characteristics of the system. The dashed curves are theoretical calculations (Friedlander and Johnstone. 1957). (b) Deposition of monodisper.se olive oil droplets with particle diameters ranging from 1.4 to 21 in a 1/2" glass pipe at two different Reynolds numbers. (After Liu and Agarwai, 1974.)... 

Kostrova, G.F. and Obozinskaya, VA. (1985) Determination of optimal conditions electrolytic deposition of decorative brass coating of pyrophosphate electrolyte. 5ov. Electrochem., 21, 352 -354. 

In hybrid coatings deposited on plastic substrates, the critical load for which failure occurred was found to pronouncedly decrease when the friction coefficient increased. Microwave oxygen plasma modification of polypropylene substrates was found to favor the adhesion more than wet-chemical modification (Blees, 2000). Hybrid sol-gel coatings deposited on brass were also found to be the best compromise in terms of tarnishing, corrosion and scratching resistance, when conpared to hard coatings (TiN, TiZrN or acrylic varnish) (Dumont, 2000). 

Deposition of TiN(gold) and Zr(CN) (gold-yellow) ZrN (brass), TiC(black), and Ti(N,C) (rose, violet, etc.) for decorative wear-resistant coatings. 

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