Thursday, February 28, 2008

Effect of Other Variables - Nickel Plating

Other factors also influence the nature of the cathode film and thereby the quality of the deposit. They are current density, temperature, pH, and degree of agitation or relative motion of cathode and solution. These are all interrelated; if a variable such as current density is altered considerably without the other factors being altered of the same time; adverse effects on the deposit may be noted. For example, typical modern Watt bath can be operated at 55 oC and pH 2.0 to give sound, ductile, grey deposits at 6 A/dm2 with a moderate agitation. If the current density is lowered to 0.2 A/dm2 without reduction of the temperature or increase of the pH, bright, brittle deposits will be obtained.

 

The range of permissible current densities is greatly extended at low pH, where smoother deposits are obtained mainly because of less tendency for the precipitation of ferric hydroxide from traces of dissolved iron. With good filtration, the difference in smoother is negligible for baths of pH 2 to 4.5. This is especially true for decorative plate obtained at pH 3.5 to 4.5. Disadvantages of lowering the pH too far appear in the later discussion of current efficiency and throwing power. A bath operated in the pH range 1.5 to 2.0 is sometimes used for an original flash nickel coating on steel for producing a smooth, adherent, rather lustrous base plate for further plating of nickel from baths of pH 2.5 to 4.5 or of copper from acidic baths.

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