Zinc electrolytes
Zinc electrolytes are among the most important electrolyte types of electroplating . The element zinc , with an equilibrium potential Zn / Zn 2+ = −0.763 V, is one of the relatively base metals . Zinc is soluble in the acidic and alkaline range; therefore, both acidic and alkaline types of electrolytes could be developed.
Electrochemical deposition of zinc
Electrolytically deposited zinc layers have a metallic-bluish-white color. If the zinc layer has a high degree of gloss, there is a risk of confusion with chrome coatings. The color of electrolytically galvanized parts is often subsequently changed by passivation (bluish, iridescent, yellow, black). Small parts made of steel or zinc die-cast are galvanized as bulk goods in a drum, medium-sized or particularly decorative parts are placed on a frame for galvanizing. The workpiece to be galvanized is connected as a cathode ; in direct current at about 0.5-4.0 A / dm 2 , a finely crystalline zinc layer is then deposited on the surface.
Furthermore, steel strips (strip steel) for demanding applications (automotive, household appliance industry) can be galvanized in continuous electrolytic galvanizing plants (e.g. with the Gravitel process).
Zinc electrolyte types
A distinction is made according to their electrolyte composition
- weakly acidic zinc electrolyte (the supporting electrolyte is usually potassium chloride , the pH value is with boric acid ge buffers )
- Zinc electrolytes with a high cyanide content based on potassium or sodium cyanide and hydroxides (the cyanide content is between 75 and 150 g / l)
- Low-cyanide zinc electrolytes (the cyanide content is between 10 and 30 g / l)
- alkaline (cyanide-free) zinc electrolytes (the conductive salt is sodium or potassium hydroxide )
Weak acid and cyanide-free electrolytes place particularly high demands on the pretreatment of the surface, while cyanide-containing electrolytes (especially in the drum process) get by with a simple decoction degreasing.
The bath management must be particularly precise with the weakly acidic and cyanide-free electrolytes, while the cyanide-containing (especially the high-cyanide-containing electrolytes) work satisfactorily in a wide range of chemical concentration, current density and bath temperature.
A lot of hydrogen is produced when zinc is deposited in cyanide-containing electrolytes. Particularly in the case of steels with a higher strength, the hydrogen causes embrittlement, which must be eliminated again by subsequent tempering. For this reason, the acidic electrolyte type is often chosen for parts that are particularly prone to embrittlement.
Of the various electrolytes, the type with a high cyan content has the best throwing power and the acidic type the worst. Accordingly, after acid galvanizing, a component would have the greatest differences in layer thickness at various points on the surface.
The highest separation speed can be achieved with the weakly acidic electrolyte.
The cyanide bath types place special demands on wastewater treatment (detoxification) and occupational safety (handling of toxic substances).
For many galvanic galvanizing processes, organic additives are required so that instead of a powdery black layer, a firmly adhering and shiny layer is formed. These gloss additives are polar substances that accumulate on the surface in the electric field and act as inhibitors there, i.e. inhibit crystal growth and thus create very fine-crystalline structures. In the weakly acidic electrolyte, surfactants are also required to wet the surface and emulsify the gloss additives .
For the galvanizing of steel strip, acid sulphate electrolytes are used, which are operated with or without additives.