Electroplating The deposition

If high currents are used, hydrogen may form in small amounts in the metal deposit. Even a few hundredths of a percent make the deposit spongy, and particles are broken off easily. In addition, for electroplating, the deposit is veiy brittle and must be baked to remove the hydrogen. 

As in electroplating, the deposition bath depends on the deposit characteristics necessary in the product and typical baths for three metals are shown in Table 8.1. Solution purity is particularly essential for electroforming. 

Sonoelectrochemistry has been employed in a number of fields such as in electroplating for the achievement of deposits and films of higher density and superior quality, in the deposition of conducting polymers, in the generation of highly active metal particles and in electroanalysis. Furtlienuore, the sonolysis of water to produce hydroxyl radicals can be exploited to initiate radical reactions in aqueous solutions coupled to electrode reactions. 

Indium chemicals and electroplated metal deposits ate replacing mercury (qv) in the manufacture of alkaline batteries (qv). Indium, like mercury, functions to reduce outgassing within the battery and promotes the uniform corrosion of the anode and cathode while the battery is under electrical load. Indium inorganic chemicals also find use as catalysts in various chemical processes. 

In a similar procedure, the atomizer test, which depends on the behavior of an advancing rather than a receding contact angle, a fine mist of water is apphed to the metal surface and the spreading of water is observed. On a clean surface, water spreads to a uniform film. With oleic acid as the test soil, the atomizer test can detect the presence of 10 mg of soil per cm, less than a monomolecular layer (115). For steel that is to be electroplated, the copper dip test is often employed. Steel is dipped into a cupric salt solution and the eveimess of the resulting metallic copper deposit is noted. 

It will be seen that the design of articles to be electroplated can have a considerable effect on the corrosion resistance of the electrodeposited coating. The chief effects are the result of variations in deposit thickness, but also important are features which can influence the adhesion, porosity and physical properties of the deposit. Good design will also avoid features of the plated article capable of trapping liquids or solid contaminants which might cause more rapid corrosion. 

Chemical reduction is used extensively nowadays for the deposition of nickel or copper as the first stage in the electroplating of plastics. The most widely used plastic as a basis for electroplating is acrylonitrile-butadiene-styrene co-polymer (ABS). Immersion of the plastic in a chromic acid-sulphuric acid mixture causes the butadiene particles to be attacked and oxidised, whilst making the material hydrophilic at the same time. The activation process which follows is necessary to enable the subsequent electroless nickel or copper to be deposited, since this will only take place in the presence of certain catalytic metals (especially silver and palladium), which are adsorbed on to the surface of the plastic. The adsorbed metallic film is produced by a prior immersion in a stannous chloride solution, which reduces the palladium or silver ions to the metallic state. The solutions mostly employed are acid palladium chloride or ammoniacal silver nitrate. The etched plastic can also be immersed first in acidified palladium chloride and then in an alkylamine borane, which likewise form metallic palladium catalytic nuclei. Colloidal copper catalysts are of some interest, as they are cheaper and are also claimed to promote better coverage of electroless copper. 

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