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.
The initial introduction of this process on the market has been rather slow, due to the problematic gloss and the passivability of the deposits (blue passivation, etc.). After overcoming these limitations, this process has been meeting a tremendous success, especially in frame applications. Solutions have low metal content and are completely free from complexing products, with interesting economic and operational benefits for the treatment and discharge of waste and washing waters.
The zinc concentration in the bath is of extreme importance and must be strictly maintained within the indicated limits. This is achieved, in an optimal way, by dissolving zinc in a tank external to the work tank. The solution is circulated with a pump filter, that picks it from the dissolution tank and sends it to the work tank; from here, it returns to the dissolution tank by overflow. The zinc concentration is kept by operating with mild steel anode baskets, filled with zinc spheres and immersed in the dissolution tank. Corrections of the concentration of zinc can be readily carried out by increasing or decreasing the number of baskets immersed in the dissolution tank or by varying the liquid level in the dissolution tank, with a variation of the surface concerned by the chemical dissolution.
The current processes are capable of providing very shiny and levelled deposits, with good distribution also at different current densities. This allows optimum use of cathodic frames. The deposits are glossy but little strained, even at high thicknesses. The chromic conversion processes available are able to satisfy all the requirements for aesthetic appearance and resistance to corrosion.
The determination of zinc and caustic soda concentrations is extremely easy. The analysis methods are the same as those used for the cyano-alkaline processes. This process also allows to control the additives, with Hull cell tests, supplemented by accurate measurement of the zinc coating thickness on test plates.