Electrogalvanization
Encyclopedia
Electrogalvanization is a galvanization
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.
ZincElectroplating
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 60’s, with the introduction of the first acid chloride based electrolyte . The 80’s saw a return to alkaline electrolytes, only this time, without the use of cyanide. Compared to hot dip galvanizing, electroplated zinc offers these significant advantages:
- Lower thickness deposits to achieve comparable performance
- Broader conversion coating
availability for increased performance and color options
- Brighter, more aesthetically appealing, deposits
(CrVI). 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 passivate conversion coating
s.
(SCE), 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 (hexavalent chromium (CrVI) or trivalent chrmoim (CrIII) depending upon OEM requirements) 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:
For ideal metal distribution, Zn metal evolutes between 6-14 g/L (0.8-1.9 oz/gal) and NaOH at 120 g/L (16 oz/gal). But for the highest productivity, Zn metal is between 14-25 g/L (1.9-3.4 oz/gal)and NaOH remains at 120 g/L (16 oz/gal).
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.
> 6 years without a change in appearance and > 12 years without functional corrosion. Alkaline Zn/Ni (12-15% Ni) has a microhardness of 450 HV15 and can replace hard steel components for various equipment manufacturers.
Besides Automotive, the Electrical, Building, Aerospace and Fastener Industries utilize zinc and zinc alloy electrodeposited coatings.
Galvanization
Galvanization is the process of applying a protective zinc coating to steel or iron, in order to prevent rusting. The term is derived from the name of Italian scientist Luigi Galvani....
process in which a layer of zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
is bonded to steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
in order to protect against corrosion
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
. The process involves electroplating
Electroplating
Electroplating is a plating process in which metal ions in a solution are moved by an electric field to coat an electrode. The process uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal...
, running a current of electricity through a saline/zinc solution with a zinc anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
and steel conductor.
ZincElectroplating
Electroplating
Electroplating is a plating process in which metal ions in a solution are moved by an electric field to coat an electrode. The process uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal...
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
Plating
Plating is a surface covering in which a metal is deposited on a conductive surface. Plating has been done for hundreds of years, but it is also critical for modern technology...
of Zinc was developed at the beginning of the 20th century. At that time, the electrolyte was cyanide
Cyanide
A cyanide is a chemical compound that contains the cyano group, -C≡N, which consists of a carbon atom triple-bonded to a nitrogen atom. Cyanides most commonly refer to salts of the anion CN−. Most cyanides are highly toxic....
based.. A significant innovation occurred in the 60’s, with the introduction of the first acid chloride based electrolyte . The 80’s saw a return to alkaline electrolytes, only this time, without the use of cyanide. Compared to hot dip galvanizing, electroplated zinc offers these significant advantages:
- Lower thickness deposits to achieve comparable performance
- Broader conversion coating
Conversion coating
Conversion coatings are coatings for metals where the part surface is converted into the coating with a chemical or electro-chemical process. Examples include chromate conversion coatings, phosphate conversion coatings, bluing, black oxide coatings on steel, and anodizing. They are used for...
availability for increased performance and color options
- Brighter, more aesthetically appealing, deposits
History
Zinc plating 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 1930’s with the alkaline cyanide electrolyte. Much later, in 1966, the use of acid chloride baths improved the brightness even greater. The latest modern development occurred in the 80’s, with the new generation of alkaline, cyanide-free zinc. Recent European Union Directives (ELV/RoHS/WEEE)http://ec.europa.eu/environment/waste/elv_index.htm prohibit automotive, other Original Equipment Manufacturers (OEM) and Electrical and Electronic Equipment manufacturers from using hexavalent chromiumHexavalent chromium
Hexavalent chromium refers to chemical compounds that contain the element chromium in the +6 oxidation state. Virtually all chromium ore is processed via hexavalent chromium, specifically the salt sodium dichromate. Approximately of hexavalent chromium were produced in 1985...
(CrVI). 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 passivate conversion coating
Conversion coating
Conversion coatings are coatings for metals where the part surface is converted into the coating with a chemical or electro-chemical process. Examples include chromate conversion coatings, phosphate conversion coatings, bluing, black oxide coatings on steel, and anodizing. They are used for...
s.
Processes
The corrosion protection afforded by the electrodeposited zinc layer is primarily due to the anodic potential dissolution of zinc versus iron (the substrate in most cases). Zinc acts as a sacrificial anode for protecting the iron (steel). While steel is close to ESCE= -400 mV (the potential refers to the standard Saturated calomel electrodeSaturated calomel electrode
The Saturated calomel electrode is a reference electrode based on the reaction between elemental mercury and mercury chloride. The aqueous phase in contact with the mercury and the mercury chloride is a saturated solution of potassium chloride in water...
(SCE), 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 (hexavalent chromium (CrVI) or trivalent chrmoim (CrIII) depending upon OEM requirements) 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:
Cyanide electrolytes
Contain sodium cyanide (NaCN) and sodium hydroxide (NaOH). All of them utilize proprietary brightening agents. Zinc is soluble as a cyanide complex Na2Zn(CN)4 and as a zincate Na2Zn(OH)4. Quality control of such electrolytes requires the regular analysis of Zn, NaOH and NaCN. The ratio of NaCN : Zn can vary between 2 to 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:| Zinc | Sodium hydroxide | Sodium cyanide | |
|---|---|---|---|
| Low cyanide | 6-10 g/L (0.8-1.3 oz/gal) | 75-90 g/L (10-12 oz/gal) | 10-20 g/L 1.3-2.7 oz/gal) |
| Mid cyanide | 15-20 g/L (2.0-2.7 oz/gal) | 75-90 g/L (10-12 oz/gal) | 25-45 g/L (3.4-6.0 oz/gal) |
| High cyanide | 25-35 g/L (3.4-4.7 oz/gal) | 75-90 g/L (10-12 oz/gal) | 80-100 g/L (10.70- 13.4 oz/gal) |
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 (into low current density areas).For ideal metal distribution, Zn metal evolutes between 6-14 g/L (0.8-1.9 oz/gal) and NaOH at 120 g/L (16 oz/gal). But for the highest productivity, Zn metal is between 14-25 g/L (1.9-3.4 oz/gal)and NaOH remains at 120 g/L (16 oz/gal).
High speed electrolytes
Dedicated to plating at high speed in plants where the shortest plating time is critical (ie steel coil or pipe that runs at up to 200 m/min (ft/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 saccharineTraditional 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 (plating speed), 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 (2.7-6.7 oz/gal). The pH varies between 4.8 and 5.8 units. The following chart illustrates a typical all potassium chloride bath composition:| Parameters | Value in g/L (oz/gal) |
|---|---|
| Zinc | 40 g/l (5.4 oz/gal) |
| Total chloride | 125 g/l (16.8 oz/gal) |
| Anhydrous zinc chloride | 80 g/l (10.7 oz/gal) |
| Potassium chloride | 180 g/l (24.1 oz/gal) |
| Boric acid | 25 g/l (3.4 oz/gal) |
Typical grain refiners include low soluble ketones and aldehydes. These brightening agents must be dissolved in alcohol or in hydrotrope
Hydrotrope
A hydrotrope is a compound that solubilises hydrophobic compounds in aqueous solutions. Typically, hydrotropes consist of a hydrophilic part and a hydrophobic part but the hydrophobic part is generally too small to cause spontaneous self-aggregation.Hydrotropes do not have a critical concentration...
. 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.
Business Fields
Initiated by the Automotive Industry, Zinc Alloy deposits (ie Zn/Co, Zn/Fe, Zn/Ni, Sn/Zn) are applied for all applications where the performance expectations exceed> 6 years without a change in appearance and > 12 years without functional corrosion. Alkaline Zn/Ni (12-15% Ni) has a microhardness of 450 HV15 and can replace hard steel components for various equipment manufacturers.
Besides Automotive, the Electrical, Building, Aerospace and Fastener Industries utilize zinc and zinc alloy electrodeposited coatings.