Electrolytic zinc coating of steels
N.B.: The information contained in this sheet comes from reliable sources. Nevertheless, it is provided without any guarantee, express or implied, of its accuracy.
Principle:
Electrolytic zinc plating has a dominant position in the field of electroplating by the tonnage of metal electroplated. Since the beginning of the 20th century, the processes have used grain refiners capable of brightening the deposits. Bright zinc deposits are widely used and electrolytes have been developed to give the deposits desirable properties in terms of decorative appearance, corrosion resistance and chromium passivation.
History
Electrolytic zinc plating is applied to resist corrosion before any aesthetic or functional consideration. At the beginning of the 20th century, the first electrolytes used were cyanide compounds, which were very effective but harmful to the environment. The first high-gloss deposits did not appear until 1966 with the invention of acid zinc chloride baths and the incorporation of ketones as a glossing agent. In the 1980s, alkaline zincate baths gradually replaced the old cyanide baths and dominated the market with their excellent anti-corrosion properties. The acceleration of the abandonment of cyanide processes also corresponds to the imposition of European Directives, which banned hexavalent chromium in zinc passivation (Directive 2000/53 on end-of-life vehicles or ELV). The cyanide-free alkaline zinc deposit contains little organic brightener and its columnar structure allows a thicker, more protective passivation.Processes
The corrosion protection of steel is primarily the result of the anodic potential difference between zinc (ESCE = approx. -980 mV) and steel (ESCE = approx. -400 mV). The steel is thus protected by cathodic protection as long as the zinc is not completely oxidized. The ability of the deposit to reduce the corrosion rate can be simply summarized in 4 variables:
The thickness of the deposit commonly located at 10 μm today
Its ability to receive protective conversion layers with thicknesses of the order of 200 to 400 nm in the case of trivalent chromium conversions
The reduction of the potential difference with steel by the use of more noble alloys such as Zn-Ni with 12-15% nickel
The deposition of a reinforced organo-mineral finish using silica, lubricants and inhibitors to a thickness of 1 μm
Zinc electrolytes are divided into two types: alkaline or acidic
1. Alkaline baths:
Cyanides: they contain zinc, sodium cyanide and soda. Zinc is soluble in the form of Na2Zn(CN)4 but also in the form of zincate Na2Zn(OH)4. The maintenance of the baths consists in regular control of the Zn, NaOH and NaCN content. The NaCN/Zn ratio can vary from 2 to 3 depending on the bath temperature.
The following table shows the usual values to be respected at room temperature:
Composition of cyanide electrolytes
Low cyanide:
ZINC G/L
WELDING G/L
75-90
6-10
SODIUM CYANIDE G/L
10-20
Medium cyanide:
WELDING G/L
ZINC G/L
75-90
15-20
SODIUM CYANIDE G/L
10-20
High cyanide :
WELDING G/L
ZINC G/L
75-90
25-35
SODIUM CYANIDE G/L
80-100
Commercially available brighteners use amino grain refiners and sodium benzyl nicotinate. Because of the significant health and safety risks associated with cyanide baths, these electrolytes are being replaced by cyanide-free alkaline baths.
Non-cyanide alkalis: These consist of zinc and soda. The grain refiners are the same as those used in cyanide processes, but they also contain quaternary amines capable of reducing the differences in thickness between zones of different current densities. Control is achieved by careful monitoring of zinc and soda according to the attached table. A high zinc content favors the faradic yield but decreases the penetration power of the deposits at low current densities.
Composition of alkaline electrolytes without cyanide
Best Metal Distribution:
WELDING G/L
ZINC G/L
120
6-14
Better productivity:
WELDING G/L
ZINC G/L
120
14-25
2. Acid baths:
Very high speed baths:
They are reserved for the continuous treatment of wires, strips or tubes. The substrate runs at very high speeds of up to 200 m/min and requires particularly short galvanizing times. The baths consist of zinc sulfate or chloride up to the solubility limits. An addition of boric acid for low concentrations limits the burning at high current densities and has a buffering effect on the pH. They contain few grain refiners including sodium saccharinate.
Traditional chloride baths:
These are the most widespread. Initially launched in ammonium base, these processes have evolved in potassium base in the West mainly because of the contamination of wastewater. They contain zinc chloride, ammonium or potassium chloride and boric acid for potassium baths. The zinc content depends on the degree of productivity required in bulk or batch treatment. It generally varies from 20 g/L to 50 g/L. The pH is 4.8.
Usual composition of a chloride bath
CONTENT IN G/L
PARAMETER
40
Zinc
125
Total Chloride
80
Anhydrous zinc chloride
180
Potassium chloride
25
Boric acid
Grain refiners are ketones or aldehydes with low solubility. They have to be dissolved in alcoholic solvents or better hydrotropic surfactants. These molecules are co-deposited with zinc and generate a very high gloss but interfere with passivation and reduce the protective power of zinc.
Implementation
Main equipment (furnace, reactor, line, machine...)
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