Bright deposits

BTighteners are used to obtain bright deposits direcdy from the bath (117). The additives currentiy used fall into two classes, which have vatiously been labeled primary and secondary, first class and second class, and carrier and brightener. The last is more commonly used in plating plants. 

While brass deposits have a somewhat higher protective value on steel than the equivalent thickness of copper, the deposits tend to tarnish, and when used for decorative purposes bright deposits are normally protected by a clear lacquer. 

A proprietary tetrachromate bath has been used in Germany under the name of the D process. By the use of additions of magnesium oxide and sodium tungstate it is claimed that the current efficiency of the bath can be raised to as high as 35-40%. Other additives such as indium sulphate, sodium selenate or sodium hexavanadate enable bright deposits to be obtained. 

A crack count of 30-80 cracks/mm is desirable to maintain good corrosion resistance. Crack counts of less than 30 cracks/mm should be avoided, since they can penetrate into the nickel layer as a result of mechanical stress, whilst large cracks may also have a notch effect. Measurements made on chromium deposits from baths which produce microcracked coatings indicate that the stress decreases with time from the appearance of the first cracks . It is more difficult to produce the required microcracked pattern on matt or semi-bright nickel than on fully bright deposits. The crack network does not form very well in low-current-density areas, so that the auxiliary anodes may be necessary. 

For ruthenium, electrolytes based on ruthenium sulphamate or nitrosyl-sulphamate have been described, but the most useful solutions currently available are based on the anionic complex (H2 0 Cl4 Ru N Ru-Cl4-OH2) . The latter solutions operate with relatively high cathode efficiency to furnish bright deposits up to a thickness of about 0-005 0 mm, which are similar in physical characteristics to electrodeposited rhodium and have shown promise in applications for which the latter more costly metal is commonly employed. Particularly interesting is the potential application of ruthenium as an alternative to gold or rhodium plating on the contact members of sealed-reed relay switches. 

Those types that contain grains of extremely small dimensions (less than 10 nm). Typically, bright deposits (as a result of additives, for instance) such as Ni-P... 

Sulfur compounds, e.g. arenesulfonic acids, used in conjunction with the unsaturated compounds mentioned above can provide level, bright films. After several hours of operation of the bath, the odour of the aromatic hydrocarbon can be detected and it is well known that the bright deposits contain sulfur. Thus reduction of organic species at the developing nickel surface is a general phenomenon. 

The deposit of nickel on top of the copper provides the essential resistance of the system to corrosion. Generally it is deposited from a solution based on the familiar Watts formulation—300 g 1 1 nickel sulphate, 30 g 1 1 nickel chloride, and 40 g 1 1 boric acid—and as in the copper processes the solutions contain additives to help give a bright deposit with good levelling characteristics. 

There is great commercial incentive to produce smooth and bright deposits. Consequently, there is a vast choice of additives for use to improve micro throwing power, making deposits smoother and more uniform. 

Kardos and Foulke [40] distinguish three possible mechanisms for bright deposition (1) diffusion-controlled leveling,... 

Electrolysis has been applied primarily to extract metallic Re from solutions, and to produce Re coatings. In electrolytic extraction, the metal can be obtained in the form of a bright deposit or a black powder, depending on the conditions of electrolysis. The... 

R. Bunsen suggested that it would be worth trying to find if allotropic forms of chromium could be produced by electrolyzing green and blue chromic salt soln. Subsequent work, however—by W. R. Whitney, etc.—has shown the hypothesis to be untenable. 8. 0. Cowper-Coles obtained a bright deposit of chromium from a soln. of 25 parts of chromic chloride in 75 parts of water at 88°, with a current of 0-04-0-05 amp. per sq. cm. With a cold soln., gas is evolved at both electrodes, but no metallic deposit is obtained until an excess of hydrochloric acid is added. 

The main mechanisms of bright deposition are diffusion-controlled leveling. 

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