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Coating

Overview coatings

Coating Description
Nickel plating Serves both decorative purposes and corrosion protection. Because of the hard layer, application in the construction of electrical apparatus, as well as in the telephone industry. Especially for screws no abrasion of the coating. Nickel-plated iron parts are not recommended for outdoor use. Improvement of corrosion protection by impregnation.
Veralize Special hard nickel plating
Chrome plating Mostly after nickel plating, layer thickness approx. 4 µm Chrome has a decorative effect, increases the tarnish resistance of nickel-plated workpieces and improves corrosion protection.
Bright chrome plating: high gloss.
Matte chrome plating: matt gloss (silk gloss).
Barrel chrome plating is not possible.
Brass plating Brass plating is mainly used for decorative purposes. Steel parts are often brass plated to improve the adhesion of rubber to steel.
Copper plating Often used as an intermediate layer before nickel, chrome and silver plating. As a top coat for decorative purposes.
Silver plating Silver orders are used for decorative and technical purposes.
Tin plating Tin plating is mainly used to achieve or improve the solderability (soft solder). Also serves as corrosion protection. Thermal post-treatment is not possible.
Anodizing Anodic oxidation creates a protective layer on aluminum that acts as a corrosion inhibitor and prevents staining. For decorative purposes, virtually any color shade can be achieved.
Hot dip galvanizing Dipping in zinc bath, the temperature of which is approx. 440° - 470°C. Layer thickness min. 40 µm. Surface matt and rough, staining possible after a short time. Very good corrosion protection.
Applicable for threaded parts from M8. Ensure thread runnability by suitable measures.
Abbreviation: tZn
Anorganic
Zinc coating
Excellent high zinc coating (silver-gray color) for parts with tensile strength Rm ≥ 1,000 N/mm² (strength classes ≥10.9, hardness ≥ 300 HV).
In this coating process, hydrogen-induced embrittlement is excluded by process engineering. Temperature resistant up to approx. 300 °C. Applicable for threads ≥ M4
Mechanical
Galvanizing
Chemo-mechanical plating process. Degreased parts are placed in a plating drum together with a special mixture of glass beads and zinc powder. The glass beads act as a carrier for the zinc powder grains and bring them to the workpiece surface, where they adhere by cold welding.
Chemical
Blackening
Chemical process for sharpening stainless steels for decorative purposes
Burnishing Chemical process, bath temperature approx. 140 °C with subsequent oiling. For decorative purposes only light corrosion protection.
Phosphating Only light corrosion protection. Good adhesion for paints. Appearance gray to gray-black. Subsequent oiling improves corrosion protection.
Impregnate Especially for nickel-plated parts, post-treatment in dewatering fluid with wax additive can seal the micropores with wax. Significant improvement in corrosion resistance. The wax film is dry and invisible.
Tempering In the case of electroplated fasteners made of steels with tensile strengths Rm ≥ 1,000 N/mm² or hardness ≥ 320 HV which are under tensile stress, there is a risk of failure due to hydrogen embrittlement. Hydrogen can be partially eliminated by tempering at approx. 180°C to 230°C (below the tempering temperature). However, according to the current state of the art, this process does not offer a 100% guarantee. Tempering must be carried out immediately after the galvanic treatment.
Tribotechnical
Coating
Form friction-reducing and wear-inhibiting layers. Protection against high friction (galling).
Waxing Sliding layer to reduce the insertion torque of thread-forming screws.

Galvanic coatings

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.

Abbreviations galvanic coatings

Abkürzung Galvanische Überzüge
Table 1: Coating metals
Coating metal
Identification letter
Abbreviation Element
Zn Zinc A
Cd* Cadmium B
Cu Copper C
CuZn Copper-Zinc D
Ni b* Nickel E
Ni b Cr r* Nickel-Chrome F
CuNi b* Copper-Nickel G
CuNi b Cr r* Copper-Nickel-Chrome H
Sn Tin J
CuSn Copper-Tin K
Ag Silver L
CuAg Copper-Silver N
ZnNi Zinc-Nickel P
ZnCo Zinc-Cobalt Q
ZnFe Zinc Iron R

The use of cadmium is restricted in certain countries
*The ISO classification code is in ISO 1456
     
Table 2: Layer thickness
Layer thickness (µm)
Code
One coating metal Two coating metals*
no layer thickness specified - 0
3 - 1
5 2 + 3 2
8 3 + 5 3
10 4 + 6 9
12 4 + 8 4
15 5 + 10 5
20 8 + 12 6
25 10 + 15 7
30 12 + 18 8
*The thicknesses specified for the first and second coating metals apply to all combinations of coatings with the exception that chromium is the top layer, which always has a thickness of 0.3 µm.
Table 3: Post-treatment and passivation by chromating
Gloss level Passivation by chromating*
Self color
Identification letter


matt
No color A
bluish to bluish iridescent² B
yellowish shimmering to yellow-brown, iridescent C
olive green to olive brown D


plain
No color E
bluish to bluish iridescent² F
yellowish shimmering to yellow-brown, iridescent G
olive green to olive brown H


shiny
No color J
bluish to bluish iridescent² K
yellowish shimmering to yellow-brown, iridescent L
olive green to olive brown M
high gloss No color N
arbitrary Like B, C or D P
matt brown black to black R
plain brown black to black S
shiny brown black to black T
all gloss levels without chromating³ U

*Passivation is only possible for zinc or cadmium coatings.
²Applies only to zinc coatings
Example of such a coating: A5U

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 general, galvanic coating should not be applied at all in order to exclude the risk of hydrogen embrittlement fracture.
If an electroplated coating is nevertheless to be applied, the parts must be annealed at approx. 200°C for approx. 6 hours at the latest 4 hours after the electroplating treatment. Subsequent heat treatment reduces the risk of hydrogen embrittlement, but complete elimination cannot be guaranteed.

Sources:
DIN EN ISO 4042 - Fasteners - Electroplated coatings
DIN EN ISO 15330 - Fasteners - Tension test for detection of hydrogen embrittlement - Method with parallel bearing surfaces
DIN EN ISO 1456 - Metallic and other inorganic coatings - Electroplated coatings of nickel, nickel plus chromium, copper plus nickel and copper plus nickel plus chromium
http://de.wikipedia.org/wiki/Galvanotechnik (09.09.2011)

Ban on coatings containing chromium(VI)

Cr6-Verbot

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!):

Cr6-Verbot

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

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)