Zinc alloy
Encyclopedia
Electroplated Zinc alloys have been developed for meeting the most challenging specifications in terms of corrosion
protection, temperature and wear resistance.
Only Zn/Ni (86%/14%) is an alloy while lower content of Iron, Cobalt and Nickel leads to co-deposits. Zn/Ni (12%-15%) in Nickel in acidic and alkaline electrolytes is plated as the gamma crystalline phase of the binary diagram Zn-Ni.
zinc is much more anodic with -980 mV. Steel
is preserved from corrosion by cathodic protection. When alloying zinc with cobalt or nickel at levels less than 1% has minimal affect on the potential but both alloys improve the capacity of the zinc layer to develop a chromate film by conversion coating. This enhances corrosion protection further.
On the opposite Zn/Ni between 12% and 15% of Ni (Zn/Ni 86/14) has a potential around -680 mV closer to Cadmium -640 mV. During corrosion, the attack of zinc is preferred and the dezincification leads to a consistent increase of the potential towards steel. Thanks to this mechanism of corrosion, this alloy offers much greater protection than all other alloys.
For cost reasons the existing market is dividing between alkaline Zn/Fe (99,5%/0,5%) and alkaline Zn/Ni (86%/14%). The use of former alkaline and acidic Zn/Co (99,5%/0,5%) is disappearing from the specifications because Fe gives similar results with less environmental concern. The former Zn/Ni (94%/6%) that was a blend between pure zinc and the crystallographic gamma phase of Zn/Ni (86%/14%), was withdrawn from the European specs. A specific advantage of alkaline Zn/Ni (86%/14%) involves the lack of hydrogen embrittlement by plating. It was proved that the first nucleation on steel starts with pure nickel and that this layer is plated 2 nm thick prior to the Zn-Ni. This initial layer prevents hydrogen
from penetrating deep into the steel
substrate thus avoiding the serious problems associated with hydrogen embrittlement. The value of this process and the initiation mechanism is quite useful for hard strength steel, tool steels and other substrates susceptible to hydrogen embrittlement.
A new acidic Zn/Ni (86%/14%) has been developed that produces brighter deposit but offers less metal distribution than the alkaline system and without the aforementioned Nickel underlayer, does not offer the same performance in terms of hydrogen embrittlement.
Additionally, all the zinc alloys receive the new CrVI free conversion coating films that are frequently followed by a top-coat to enhance corrosion protection, Wear resistance and to control the coefficient of friction.
, Zinc Alloys are applied for all applications where the cosmetic requirements require more than 6 years without a change in appearance and 12 years for avoiding functional corrosion. Alkaline Zn/Ni 86/14 has a microhardness of 450 HV15 and can replace hard steel components for various equipment manufacturers. Besides Automotive, Electrical, House Building, Aerospace, Fastener Industries all find benefits from Zinc Alloys.
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...
protection, temperature and wear resistance.
History
The modern development started during the 80’s with the first alkaline Zn/Fe (99,5%/0,5%) deposits and Zn/Ni (94%/6%) deposits. Recently, the reinforcement of the corrosion specifications of the major European Car Makers and the Directive ELV that banished the use of hexavalent Chromium (CrVI) Conversion Coating required greater use of alkaline Zn/Ni between 12 and 15% of Ni (Zn/Ni 86/14).Only Zn/Ni (86%/14%) is an alloy while lower content of Iron, Cobalt and Nickel leads to co-deposits. Zn/Ni (12%-15%) in Nickel in acidic and alkaline electrolytes is plated as the gamma crystalline phase of the binary diagram Zn-Ni.
Processes
The corrosion protection is primarily due to the anodic potential dissolution of zinc versus iron. Zinc is acting as sacrificial anode for protecting iron (steel). While steel is close to -400 mV, depending on alloy composition, electroplatedElectroplating
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...
zinc is much more anodic with -980 mV. 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...
is preserved from corrosion by cathodic protection. When alloying zinc with cobalt or nickel at levels less than 1% has minimal affect on the potential but both alloys improve the capacity of the zinc layer to develop a chromate film by conversion coating. This enhances corrosion protection further.
On the opposite Zn/Ni between 12% and 15% of Ni (Zn/Ni 86/14) has a potential around -680 mV closer to Cadmium -640 mV. During corrosion, the attack of zinc is preferred and the dezincification leads to a consistent increase of the potential towards steel. Thanks to this mechanism of corrosion, this alloy offers much greater protection than all other alloys.
For cost reasons the existing market is dividing between alkaline Zn/Fe (99,5%/0,5%) and alkaline Zn/Ni (86%/14%). The use of former alkaline and acidic Zn/Co (99,5%/0,5%) is disappearing from the specifications because Fe gives similar results with less environmental concern. The former Zn/Ni (94%/6%) that was a blend between pure zinc and the crystallographic gamma phase of Zn/Ni (86%/14%), was withdrawn from the European specs. A specific advantage of alkaline Zn/Ni (86%/14%) involves the lack of hydrogen embrittlement by plating. It was proved that the first nucleation on steel starts with pure nickel and that this layer is plated 2 nm thick prior to the Zn-Ni. This initial layer prevents hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
from penetrating deep into the 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...
substrate thus avoiding the serious problems associated with hydrogen embrittlement. The value of this process and the initiation mechanism is quite useful for hard strength steel, tool steels and other substrates susceptible to hydrogen embrittlement.
A new acidic Zn/Ni (86%/14%) has been developed that produces brighter deposit but offers less metal distribution than the alkaline system and without the aforementioned Nickel underlayer, does not offer the same performance in terms of hydrogen embrittlement.
Additionally, all the zinc alloys receive the new CrVI free conversion coating films that are frequently followed by a top-coat to enhance corrosion protection, Wear resistance and to control the coefficient of friction.
Bath compositions
- Composition of electrolyte for plating alkaline Zinc-Iron at 0,5% in Fe:
| Parameters | Composition in g/L |
|---|---|
| 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... |
6-20 |
| Iron Iron Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust... |
0.05-0.4 |
| Caustic soda | 120 |
- Composition of electrolyte for plating Acidic Zinc-cobalt at 0,5% in Co:
| Parameters | Composition in g/L |
|---|---|
| Zinc | 25-40 |
| Cobalt Cobalt Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal.... |
2-5 |
| Total Chloride Chloride The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water... |
130-180 |
| Potassium Potassium Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are... Chloride |
200-250 |
| Boric Acid Boric acid Boric acid, also called hydrogen borate or boracic acid or orthoboric acid or acidum boricum, is a weak acid of boron often used as an antiseptic, insecticide, flame retardant, as a neutron absorber, and as a precursor of other chemical compounds. It exists in the form of colorless crystals or a... |
25 |
- Composition of electrolyte for plating Alkaline Zinc-nickel 4-8% in Ni:
| Parameters | Composition in g/L |
|---|---|
| Zinc | 7.5-10 |
| Nickel Nickel Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile... |
1.8-2 |
| Caustic Soda | 100-120 |
- Composition of electrolyte for plating Alkaline Zinc-nickel at 12-15% in Ni:
| Parameters | Composition in g/L |
|---|---|
| Zinc | 7-12 |
| Nickel | 1-2.5 |
| Caustic Soda | 120 |
- Composition of electrolyte for plating Acidic Zinc-nickel at 12-15% in Ni:
| Parameters | Composition in g/L |
|---|---|
| Zinc | 30-40 |
| Nickel | 25-35 |
| Total Chloride | 150-230 |
| Boric Acid | 25 |
Business Fields
Initiated by the Automotive IndustryAutomotive industry
The automotive industry designs, develops, manufactures, markets, and sells motor vehicles, and is one of the world's most important economic sectors by revenue....
, Zinc Alloys are applied for all applications where the cosmetic requirements require more than 6 years without a change in appearance and 12 years for avoiding functional corrosion. Alkaline Zn/Ni 86/14 has a microhardness of 450 HV15 and can replace hard steel components for various equipment manufacturers. Besides Automotive, Electrical, House Building, Aerospace, Fastener Industries all find benefits from Zinc Alloys.