Activation of new zinc-nickel baths and zinc baths with black passivation, green passivation and sealant. In the zinc-nickel baths the metal is deposited, and then the black passivation is carried out with surface conversion, in order to increase the corrosion resistance. Finally, the material is immersed in the bath with a sealant to further increase corrosion resistance.
The zinc bath, on the other hand, is followed only by green passivation, still to increase corrosion resistance.
Electroplating of zinc-nickel alloys was the first process for zinc alloys plating to be studied and applied on an industrial scale, in order to meet the special protection needs of automobile components subjected to extreme environmental conditions (engine compartment).
The following table shows the most commonly used formulations:
|Alloy composition and baths components||Acidic bath Alloy with medium-high Ni content||Alkaline bath
Alloy with low Ni content
|Alkaline bath Alloy with high Ni content|
|Nickel content in the alloy||10-15%||6-8%||12-15%|
|Zinc as the metal||20 g/l||8 g/l||8-10 g/l|
|Nickel as the metal||3 g/l||1.8-2.2 g/l||1.8-2.2 g/1|
|Ammonium chloride||10 g/l||–||–|
|Sodium hydroxide||–||110 g/l||120-140 g/1|
|Sodium carbonate||–||–||<60 g/l|
|According to instructions|
The first formulation is derived from the one used for zinc electroplating from ammonium chloride galvanizing acidic baths; the zinc alloy so obtained has a nickel content of 10-15%. The nickel required for the alloy is added according to two different operating modes: the first uses nickel and zinc anodes connected with two separate power supplies; the second, much easier and more practical, feeds the bath with concentrated solutions of nickel.
Two later formulations, especially the third, able to provide deposits with high nickel content in the alloy – 12-15% – have had greater success. Zinc is restored in the electrolyte using an external dissolution tank. The operating temperature is maintained between 20 and 28 °C; the anode-cathode ratio is 2:1, using insoluble nickel anodes in the work tank. The applicable cathodic current densities are, of course, based on the type of installation, the maximum reference values are 1 A/dm2 and 3.5 A/dm2, respectively, for barrel plating and static installations. Nickel and its complexing products are added after carrying out an analysis and based on the A/h absorbed. The zinc-nickel ratio should be maintained between 4 and 4.5. In any case, it is advisable to use a bath under continuous filtration.
If the reasons in favour of alkaline processes against acidic ones are evident (better distribution of deposits and lower operating costs), there are not too many differences between the two alkaline solutions Zinc-nickel deposits with a higher nickel content (12-15%) are more resistant to corrosion and temperature, while operating with a lower nickel content (6-8%) is simpler and the deposits obtained are easier to passivate. The presence of foreign metal pollutants is deleterious, especially as regards the passivation characteristics. Iron is tolerated up to a maximum of 70-75 ppm, chromium adversely affects the deposits in high current density areas and the distribution of nickel in the alloy. The defects caused by these two metal pollutants can be eliminated by a treatment with zinc powder. The characteristics that have made this process particularly attractive for the electroplating industry may be summarized as follows:
– Exceptional resistance to corrosion of the deposits;
– Possibility of black chromium plating without the use of silver salts;
– Possibility of silver finishes without passivation containing hexavalent chromium;
– Consistency of the alloy at all current densities;
– High hardness of the deposits (no scratches in the barrel plating operations); possibility of assembly of pieces coated with aluminium parts;
– Very limited corrosion;
– Facilitation of subsequent painting and rubber-coating operations.