Coating

In electroplating, an electrical current is sent through an electrolytic bath. The metal to be applied (e.g. zinc or nickel) is located at the positive pole (anode), while the item to be coated is located at the negative pole (cathode). The electric current thereby detaches metal ions from the consumable electrode and deposits them on the article by reduction. In this way, the object to be plated is uniformly coated on all sides with copper or another metal. The longer the object is in the bath and the higher the electric current, the stronger the metal coating (e.g. zinc coating) becomes.

Since September 21, 2017, chemicals containing chromium(VI) may no longer be used or placed on the market. In order to keep open the possibility of continued use under the REACH regulation, applications for authorization had to be submitted by March 21, 2016. Since very elaborate proofs or studies are required to obtain authorization, it can be assumed that the number of electroplating companies that will still be offering chromium(VI)-containing coatings (e.g. "yellow galvanized") from 2017 onwards will fall steadily.

The following coatings are affected:

• Galvanized with yellow chromate (e.g. A2C)
• Galvanized with olive chromate coating
• Galvanized with black chromate coating
• Zinc flake coating containing chromium-6 (e.g. Dacromet^®)

For this reason, we would like to present you with suitable alternatives (chrome-6 free!):

Notice:

If the component hardness exceeds 320 HV or the tensile strength Rm exceeds 1,000 MPa, the manufacturing process must be checked using a hydrogen embrittlement detection test. In this case, galvanic coating should generally be avoided altogether in order to exclude the risk of hydrogen embrittlement fracture. An alternative in these cases can be a zinc flake coating.
All data without warranty.

Zinc flake coatings are non-electrolytically applied coatings that provide good corrosion protection. These coatings consist of a mixture of zinc and aluminum flakes bonded by an inorganic matrix.

In addition to galvanizing, zinc flake coatings provide what is known as cathodic protection; the coating "sacrifices" itself to protect the base metal. Steel can be protected with these coatings. The coating thickness can range from 5 µm to 15 µm. Thick coatings provide more corrosion protection; thinner coatings provide less corrosion protection but do not affect the functionality of the coated surface, e.g. for nuts and bolts. For metric threaded parts, it is necessary to comply with the tolerances according to ISO 965. The coating thickness of zinc flake coatings must give good corrosion protection with a thin layer so that the connection between bolt and nut is not negatively affected. Compared to the thickness of hot-dip galvanized threaded fasteners, zinc flake coatings have an advantage because the thickness of the coating is thinner.

Unlike paints, corrosion does not spread under the coating. In the salt spray test, zinc flake coatings give 480 hours without red rust (RR). It is also possible to achieve 720 or 840 hours without red rust in the salt spray test, with or without post-treatment. These results in the salt spray test show better corrosion protection than a typical electroplated zinc coating, which can give from 96 to 200 hours in the salt spray test. In addition to corrosion protection, these coatings provide medium temperature resistance, good electrical conductivity, and also good chemical resistance (e.g., to cleaners, fuel, coolants, oils).

The coating material of the zinc flake coatings is supplied in liquid form; it must be prepared to the desired application conditions before use. The viscosity, temperature, stirring time before application play an important role here. The material can be applied using the following application techniques:

Spraying method
The coating material is applied to the surface of the components using a spray gun. This can be realized manually or in a fully automated spraying system.

Dip-Spinning
The parts are loaded into a basket. Coating is realized by dipping the basket into a container with the prepared coating material. After dipping, the basket is centrifuged to remove the residues of the coating material.

Before coating, the surface of the parts must be pretreated. Pickling with acids (e.g. sulfuric acid, chloric acid) generates atomic hydrogen and can penetrate the steel structure and make it brittle. To avoid pickling processes, other pretreatment processes are necessary. The typical cleaning processes are degreasing with an alkaline aqueous solution and then blasting with very small steel balls (abrasives). Detergents remove grease, oil and dirt from the metallic surface. Blasting removes scale and rust by the mechanical action of steel balls accelerated by a turbine onto parts in a chamber. Both processes do not produce hydrogen, for this reason there is no risk of hydrogen embrittlement in high strength steels.

After the pretreatment comes the coating process. The parts are sprayed with the zinc flake material on a rack (spray process) or dipped in a container and centrifuged (dip spinning). On the surface of the parts, the coating material forms a liquid and uniform layer, which is not yet compact and does not show all its properties. In order to form the excellent properties of zinc flake coatings, a baking process is required.

The coating is still highly reactive and a compact layer must be formed by heat. The coated parts must be baked in an oven under controlled temperature. This temperature depends on the coating material and product manufacturer, as each zinc flake product manufacturer has its patented formula. Typical burn-in temperatures are 200 °C, 240 °C and 300 °C. After baking, the coating is cross-linked and a uniform, thin, adhesive and dry layer is produced.

Source:
http://de.wikipedia.org/wiki/Zinklamellen%C3%BCberzug (09.09.2011)