Sunday, April 27, 2008

Anode for Nickel Plating

With the first introduced in 1916 and subsequent widespread use of the Watts bath, the old 90 to 94 % or the 95 to 97 % nickel anode (balance iron) used in the cold, low speed, pH 5.5 nickel bath, gave way to the chill-cast 99 % nickel anode. Next came the rolled 99 % nickel anode depolarized with small amounts of nickel oxide. This anode corrodes smoothly, forming a light brown film that washes off as it is formed. Since small amounts of loose nickel are formed on corrosion in all solutions based on the Watts formula, anode bags are generally necessary to prevent formation of nodular deposits as a resulat of the physical inclusion of these particles. This 99% nickel rolled anode is used in baths of PH above 4.

After the rolled depolarization anode came the cast carbon nickel anode, and still later, the rolled carbon nickel anode. Both of these contain more than 99% nickel and have the feature of forming their own bag on corrosion. This bag consists of a rather tenaciously adhering but porous carbon silica film that holds back loose anode particles provided they do not form in excessive amounts. The high purity carbon type anode are used only when the pH of the bath is 4.5 or lower. Such anode are sometimes used without any cloth bags in baths that are not air agitated.

The corrosion behavior of an anode depends on the pH and the chloride content of the bath. The uniformity of corrosion (sometimes called anode activity) should not be confused with anode efficiency. Only a small chloride content, 15 to 33 g/l nickel chloride is necessary to make the latter 100 % for all types of nickel anodes under the usual conditions where the anode area is at least equal to the cathode area. The selection of the best type of nickel anode for a given process is based not only on the purity of the anode current densities to be encountered.
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