Sunday, March 2, 2008

Metal Distribution - Nickel Plating

Electrochemical properties which determine metal distribution over a cathode of a given shape are cathode polarization, solution conductivity, and cathode efficiency. A steep slope of the cathode potential current density curve, a small or negative slope of the cathode current efficiency – current density curve, and a low specific resistivity of the electrolyte favor more uniform metal distribution or higher throwing power. How these characteristics of Watt bath are affected by changes in the plating variables indicate their probably effect on throwing power.

 

The specific resistivity of the Watt bath is about 11 ohm-cm at 60oC and it decreases in the usual way with increase in termperature. It also decreases with increasing total concentration and increasing chloride on concentration. It is but little affected by pH within the range 2.0 to 5.5.

 

Like the cathode efficiency of all plating bath, that of the Watt bath rises with increase in current density until the limiting current density is reached. It also rises with increase in temperature and with total concentration. Increase in chloride ion concentration has a more markedly beneficial effect, but the effect of pH is the most pronounced.

 

The throwing power of the Watts bath is slightly improve by an increase in pH, temperature, and nickel and chloride contents. As high current densities, the low pH watts bath is only slightly inferior to other nickel baths in throwing power, and can be made superior be sufficient increase in its chloride content. High throwing power in acidic nickel baths can be obtained by using low nickel salt concentration, for example, 100 to 120 g/l NiSO4.H2O, and high sodium chloride concentration of 160 g/l, with 30 g/l boric acid, pH of 4, 50 to 60 oC and no agitation. Agitation greatly diminished the throwing power of such baths. This bath can be made to produce bright, stress-free, high throwing power, leveling nickel plate by the use of class I together with class II addition agents.