Friday, March 7, 2008

Effect of Bath Variables - Nickel Plating

Regardless of the mechanism, the practical platter wants to know how the major plating variables influence internal stress in nickel deposits. The preponderance of evidence supports the following conclusions.

1.       Tensile stress increases with increase in chloride content of the bath.

2.       The effect of bath temperature on stress is not consistent; it varies with the composition of the bath (principally the chloride content) and the current density.

3.       The effect of pH varies with composition of the electrolyte, however for a Watts bath it is definitely advisable to keep the pH below 5, not only because deposits with lower stress are obtained but also because the harmful effects of some impurities on stress may be much more pronounced at pH 5 or above.

4.       The effect of current density is not marked over the range 1 to 5 A/dm2 but is usually in the direction of an increasing tensile stress with increasing current density.

5.       Superimposing alternating current on the direct plating current tends to reduce stress, other conditions remaining the same.

6.       Agitation has little effect in purified plain Watts's baths. In bright plating baths containing excessive concentration of class II brighteners or harmful impurities, decreased agitation (decreased diffusion rate) will result in decreased tensile stress.

7.       Hydrogen peroxide, dissolved inorganic impurities such as lead, zinc, iron, chromium, aluminum, and phosphate, organic impurities such as sizing from unwashed anode bags, amines from improperly cured rubber linings, and excessive concentrations of class II brighteners can all act to increase the tensile seriously.

8.       Fluorides and fluoborates tend to reduce tensile stress slightly. Class I brighteners can decrease the tensile (contractile) stress to zero and then with increasing concentrations, reverse the direction so as to make the nickel expand or be under compression if restrained from expanding.

9.       Stress may be induced into the first thin layers of a plate that is deposited on a structurally different substrate with different lattice spacing or on a substrate that has a high internal stress in the surface.

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