Monday, April 21, 2008

Effect of Contaminants

In the modern acidic nickel plating baths, small percentages of metallic contaminants such as copper, lead, zinc, or tin exert profound effects on the appearance and properties of the nickel deposit, especially in the low current density plating range. The harmful effects of these impurities are additive.

Copper in concentrations over about 5 mg/l is detrimental to adhesion to steel, or leads to poor adhesion when current is interrupted or decreased to very low current density during the nickel plating. This is also true far similar concentrations of lead. Copper in these and higher concentrations cause dark, low current density plate especially evident in bright nickel plating.

Lead in concentration higher than 5 mg/l cause dark, fine grained, brittle, poorly adherent deposits. Stannous tin in low concentrations cause dark, low current density plate, it can be removed by oxidation to the insoluble stannic form, and thus this impurity can be confused with an organic impurity in the rate case where a tin anode get mixed in with nickel. Zinc impurity in bright nickel baths, while not as eritical as copper, lead and tin can cause whitish and with higher concentrations, dark plate in the low current density areas and even cause shiny black streaks in the plate. In general, the bright nickel baths can tolerate about 20 to at least 100 mg/l of zinc, depending on the configuration of the work. Iron (III) impurity tends to precipitate readily at pH values of about 3.5 and higher, and can cause speckling on settling surfaces, unlike the case of iron (II) which codeposits compatibly with nickel. Good filtration minimized this iron (III) problem.

Hexavalent chromium in concentrations of 3 to 5 mg/l introduced from spray from chromium plating baths or dragin from faulty rack coatings causes skip plating in low current density areas, and in higher concentrations, streaked, peeled plate. Small concentrations of sodium bisulfite reduce the hexavalent chromium to the very much less harmful trivalent form. Tirvalent chromium and aluminum in appreciable quantities lower the limiting cathode current density and tend to precipitate out at pH values above about 3.5 and cause roughness especially on settling areas of a cathode. Nitrate ion is reduced to ammonium ion at the cathode and, like anodizing agents, decreases the cathode efficiency and in appreciable concentrations causes skips plating in the low current density areas. Nitrate is not nearly as critical as hexavalent chromium. Phosphate ion lowers the limiting current density through formation of ion nickel phosphate in the cathode film and the precipitated phosphate particles also cause roughness.

Many organic compounds in small concentrations as well as large excesses of many class II brighteners can cause brittle plate and dark low current density plate. Grease, oil, improper surfactants, rubber accelerators, and so on, can cause brittle, cloudy plate.

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