Tuesday, February 26, 2008

Anticipating Agents

The cathode efficiencies for nickel deposition is high in the Watts Bath; nevertheless sufficient hydrogen ions are discharged that pitted plate result from slowly forming hydrogen gas bubbles clinging to the growing cathode deposit. To prevent pitted deposits, certain anionic wetting agents which do not appreciably affect the ductility or stress of the plate are used. Before the advent of bright plating using organic additives to produce bright deposits, hydrogen peroxide was used in the plain baths to prevent pitting, and is still used in some cases. When it is carefully used dull nickel plate of very high ductility is obtained. It function by depolarization of the cathode with respect to hydrogen evolution, and by oxidation of traces of organic contaminant in the bath. The usual recommendation for a Watt bath is an addition. Once a day of 0.5 ml of 30% hydrogen peroxide per liter of solution. A grade free of organic and metallic stabilizers should be used, used and localized additions next to the cathode should be avoided. An excess of hydrogen peroxide seriously increased the tensile stress and caused burning, a decrease in the limiting current density for the given plating conditions, and embrittlement of the plate. For this reason, the use of a proper wetting agent should always be given first consideration.

 

It is essential that oil, grease, and other undesirable organic contaminants be prevented from entering the bath as these contaminants may make the peroxide as well as the wetting agent ineffective.

 

Fractional percentage of cobalt derived from anodes or salts have no significant effect on the properties of nickel deposits. The effect of appreciable additions of cobalt have received much study, and in general, the alloy plate is harder and more highly tensile stressed than nickel. The effects of cobalt's in semibright  and bright nickel are discussed under next heading.

 

Composition Range

The typical formula is excellent for an average cathode current density of 5 A/dm2 at a temperature of 50oC. Under such conditions local current densities of one-half or twice this value will still result in excellent deposits. For plating at lower current density for example 2 A/dm2, the nickel sulfate and nickel chloride contains may be halved. For higher density than normal, their concentration can be increased, but the practical difficulty of  excessive dragon losses and crystallization set a limit to such changes. Current density higher than 10 A/dm2 are better achieved by increase in agitation, temperature and ratio of chloride to sulfate.

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