Electrogalvanization


Electrogalvanizing is a process in which a layer of zinc is bonded to steel in order to protect against corrosion. The process involves electroplating, running a current of electricity through a saline/zinc solution with a zinc anode and steel conductor.
Zinc electroplating maintains a dominant position among other electroplating process options, based upon electroplated tonnage per annum. According to the International Zinc Association, more than 5 million tons are used yearly for both hot dip galvanizing and electroplating. The plating of zinc was developed at the beginning of the 20th century. At that time, the electrolyte was cyanide based. A significant innovation occurred in the 1960s, with the introduction of the first acid chloride based electrolyte. The 1980s saw a return to alkaline electrolytes, only this time, without the use of cyanide. The most commonly used electrogalvanized cold rolled steel is SECC steel. Compared to hot dip galvanizing, electroplated zinc offers these significant advantages:
was developed, and continues to evolve, to meet the most challenging corrosion protection, temperature, and wear resistance requirements. Electroplating of zinc was invented in 1800 but the first bright deposits were not obtained until the early 1930s with the alkaline cyanide electrolyte. Much later, in 1966, the use of acid chloride baths improved the brightness even further. The latest modern development occurred in the 1980s, with the new generation of alkaline, cyanide-free zinc. Recent European Union directives prohibit automotive, other original equipment manufacturers and electrical and electronic equipment manufacturers from using hexavalent chromium. These directives, combined with increased performance requirements by the OEM, has led to an increase in the use of alkaline zinc, zinc alloys and high performance trivalent passionate conversion coatings.

Processes

The corrosion protection afforded by the electrodeposited zinc layer is primarily due to the anodic potential dissolution of zinc versus iron. Zinc acts as a sacrificial anode for protecting the iron. While steel is close to ESCE= -400 mV, depending on the alloy composition, electroplated zinc is much more anodic with ESCE= -980 mV. Steel is preserved from corrosion by cathodic protection. Conversion coatings or trivalent chromium are applied to drastically enhance the corrosion protection by building an additional inhibiting layer of Chromium and Zinc hydroxides. These oxide films range in thickness from 10 nm for the thinnest blue/clear passivates to 4 µm for the thickest black chromates.
Additionally, electroplated zinc articles may receive a topcoat to further enhance corrosion protection and friction performance.
The modern electrolytes are both alkaline and acidic:

Alkaline electrolytes

Cyanide electrolytes

Contain sodium sulphate and sodium hydroxide. All of them utilize proprietary brightening agents. Zinc is soluble as a cyanide complex Na2Zn4 and as a zincate Na2Zn4. Quality control of such electrolytes requires the regular analysis of Zn, NaOH and NaCN. The ratio of NaCN : Zn can vary between 2 and 3 depending upon the bath temperature and desired deposit brightness level. The following chart illustrates the typical cyanide electrolyte options used to plate at room temperature:
ZincSodium hydroxideSodium cyanide
Low cyanide6-10 g/L 75-90 g/L
Mid cyanide15-20 g/L 75-90 g/L 25-45 g/L
High cyanide25-35 g/L 75-90 g/L 80-100 g/L

Alkaline non-cyanide electrolytes

Contain zinc and sodium hydroxide. Most of them are brightened by proprietary addition agents similar to those used in cyanide baths. The addition of quaternary amine additives contribute to the improved metal distribution between high and low current density areas. Depending upon the desired performance, the electroplater can select the highest zinc content for increased productivity or lower zinc content for a better throwing power.
For ideal metal distribution, Zn metal evolutes between 6-14 g/L and NaOH at 120 g/L. But for the highest productivity, Zn metal is between 14-25 g/L and NaOH remains at 120 g/L.
Alkaline Non Cyanide Zinc Process contains lower concentration zinc metal concentration 6-14 g/L or higher zinc metal concentration 14-25 g/L provides superior plate distribution from high current density to low current density or throwing power when compared to any acidic baths such as chloride based - or or sulfate baths.

Acidic electrolytes

High speed electrolytes

Dedicated to plating at high speed in plants where the shortest plating time is critical (i.e. steel coil or pipe that runs at up to 200 m/min. The baths contain zinc sulfate and chloride to the maximum solubility level. Boric acid may be used as a pH buffer and to reduce the burning effect at high current densities. These baths contain very few grain refiners. If one is utilized, it may be sodium saccharine.

Traditional electrolytes

Initially based on ammonium chloride, options today include ammonium, potassium or mixed ammonium/potassium electrolytes. The chosen content of zinc depends on the required productivity and part configuration. High zinc improves the bath's efficiency, while lower levels improve the bath's ability to throw into low current densities. Typically, the Zn metal level varies between 20 and 50 g/L. The pH varies between 4.8 and 5.8 units. The following chart illustrates a typical all potassium chloride bath composition:
ParametersValue in g/L
Zinc40 g/l
Total chloride125 g/l
Anhydrous zinc chloride80 g/l
Potassium chloride180 g/l
Boric acid25 g/l
 
Typical grain refiners include low soluble ketones and aldehydes. These brightening agents must be dissolved in alcohol or in hydrotrope. The resultant molecules are co-deposited with the zinc to produce a slightly leveled, very bright deposit. The bright deposit has also been shown to decrease chromate/passivate receptivity, however. The result is a reduction in the corrosion protection afforded.

Footnotes