Sunday, March 9, 2008

The Bright Nickel Baths

The nickel baths to which brighteners, mainly bath-soluble organic compounds, are added, are mostly of the Watts type with widely varying proportions of chloride and sulfate. Boric acid is always present, usually not less than 35 g/l. Condition in the cathode film must be favorable not only for the deposition of nickel but also for the functioning of the brighteners function best at pH value of about 3 to 5 and bath temperatures of about 50 to 65oC.

The unique decorative and corrosion resistant coating that can be obtained with nickel plate underneath a thin, hard, wear-resistant, and non tarnishing chromium plate was responsible for the intensive research that led to the development of modern bright nickel plating. These brighteners have received more attention than those for any other plating bath, owing to the economic importance of eliminating the expensive polishing of dull nickel deposits, and the fact that the organic brighteners for nickel were definite compounds and therefore their mode of action could be more accurately studied.

The nickel brighteners fall into two distinct classes. Aromatic sulfonic acids, sulfonamides have been designed class I nickel brighteners. The aromatic rings to which the sulfon group is attached are generally benzene and naphthalene rings. These brighteners produce almost bright or cloudy bright plate on buffed surfaces but are unable to build higher luster with continued plating. Class I aromatic sulfon brighteners have no critical upper concentration and are used in relatively high concentration and are used in relatively high concentration (1-10 g/l) without significantly affecting the adhesion and the limiting current density. The decrease the tensile stress of the deposit and can with increasing concentration endow the plate with compressive stress. Sulfur is introduced into the plate, about 0.03% at pH value 3 to 5 when the class I compounds are used in warm baths without class II brighteners. The sulfur is present as sulfide, and the plate is harder and of brighter tensile strength, but less ductile than the normal dull Watts nickel plate.

The brighteners designed as class II nickel brighteners are use together with class I brighteners to obtained a brilliant leveling deposit of increasing luster with continued plating to the point of highest luster. Class II brighteners are principally unsaturated compounds which produce the leveling and increased luster of the deposit when used in conjunction with class I brighteners. They introduce carbonaceous material into the plate. Most class II brighteners cannot be successfully used alone to obtain bright plate because they produce excessive brittleness and tensile stress, and inferior adhesion to the substrate. A certain few class II compounds, especially coumarin, which are used to obtain ductile, semibright, sulfur-free nickel plate do make possible ductile, bright or almost bright plate but with decreased leveling when extremely high rates of solution agitation are used, unlike the case of normal use with the usual agitation which aids leveling.