Zinc flake coating

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

The requirements of the zinc flake coatings are defined in the international standard ISO 10683 and also in the European standard DIN EN 13858. DIN EN ISO 10683 specifies the requirements of the zinc flake coatings for fasteners with thread, and DIN EN 13858 describes the requirements of the zinc flake coatings for Non-threaded fasteners and also for other components. There are two groups of zinc flake coatings:

Various designers such as automobile companies and their suppliers have created their own specifications and delivery conditions to determine the requirements of these coating systems.

Zinc flake coating is an umbrella term for the coating technology, this is offered by different suppliers under the respective brand names. The companies most often occur as a licensor for the individual coating operations.

History

Since electrolytically galvanized surfaces offer a comparatively low corrosion protection, and galvanic zinc coatings on high-strength steels (eg high -strength bolts of strength class 10.9 and 12.9) there is a risk of hydrogen embrittlement, the industry needed a corrosion protection system. High-strength steel parts (like screws of strength class > 10.9 high-strength bolts with strength class > 9), construction parts with tensile strength > 1000 N / mm ² or > 320 HV are susceptible to hydrogen embrittlement. Electroplating method and pickling with acids have a major impact in the development of hydrogen-induced brittle fractures.

In the 70s, a new coating system was developed in the USA: the zinc flake coatings (patent number 1,376,067 ). Through a small layer thickness of typically 8-12 microns, this system showed high corrosion protection and enabled the avoidance of hydrogen embrittlement.

In the 80s and 90s spread the application of these coating systems, eg in the automotive industry. The automotive industry requires coating systems with high corrosion resistance. Since zinc flake coatings do not generate hydrogen during the process, they have been used as alternatives to galvanized surfaces in critical applications.

Properties

Today, these coatings are preferred for connecting elements and other components in the automotive industry because they have several advantages:

  • Good appearance ( color )
  • Very good corrosion protection (240-1500 hours in the salt spray test, depending on requirements)
  • Temperature stress
  • Good chemical resistance
  • Environmental
  • No hot solving behavior
  • No risk of hydrogen embrittlement in high-strength fasteners
  • Electrical conductivity
  • More Verschraubungseigenschaften

In addition to the applications in the automotive industry is also found these coating systems for wind power plants, construction, electrical engineering ( plant ), trucks and also in other markets.

Zinc flake coatings produce the so-called cathodic protection; the less noble zinc " sacrifices " itself to protect the base metal. Steel can be protected in this way. The layer thickness is often between 5 microns and 15 microns, which for special requirements, thicker layers are possible. For metric threaded parts, it is necessary to hold the tolerances according to ISO 965, so that the threads of the screw does not stick and the friction coefficients are also adjustable. Hot dip galvanized fasteners with a typical thickness of 80-200 microns must be trimmed to expose the thread.

Unlike paints, where there is a risk of infiltration, this phenomenon is prevented by the sacrificial corrosion of zinc. Zinc flake coatings show the salt spray better corrosion protection than a typical galvanic zinc coating, often reach the salt spray test ( usually according to DIN EN ISO 9227 ) only 96 to 200 hours.

Coating technology

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

  • Spraying. The coating material is applied with a spray gun onto the surface of the components. This can be done manually or in a fully automated spray system (for large or bulky items, also called rack parts, since the parts are placed on a rack in the coating process ).
  • Dip - spinning (English for dip-spin process). The parts are loaded into a basket. The coating is achieved by immersion of the basket in a container with the prepared coating material. After dipping the basket is centrifuged to the residues of the coating material to remove (for smaller parts by mass / bulk, also called barrel plating ).
  • Frame - dip spin. Parts that are placed in baskets or otherwise fixed to be dipped, centrifuged, and drove to the frame through the furnace.
  • Dip-Drain. By dipping into the paint and defined pulling the coating of exterior and interior, for example, pipes in a single process step is possible. However, the parts should have sufficient openings to allow the paint to run again, otherwise no proper coating is possible because of the formation of accumulations of paint bubbles.

Before coating the surface of the parts to be treated. Pickled with acids (e.g., sulfuric acid, hydrochloric acid) produces atomic hydrogen, and can penetrate into the steel structure and they become brittle. To avoid pickling, other pretreatment processes are necessary. The typical purification procedure are degreasing with an alkaline aqueous solution and then beams with very small steel balls ( blasting agent ). Detergent remove grease, oil and dirt from the metallic surface. Rays eliminates scale and rust by the mechanical action of the steel balls, which are accelerated by means of a turbine to the parts in a chamber. Both methods produce no hydrogen, for this reason there is no risk of hydrogen embrittlement in high-strength steels.

After the pretreatment, then comes the coating process. The parts are sprayed onto a base with the zinc flake material ( spray test) or immersed in a vessel and centrifuged ( dip- spinning). On the surface of parts of the coating material is a liquid and uniform layer. To form the excellent properties of zinc flake coatings, a burn-in is required.

The coated parts must be baked in an oven under controlled temperature and time. This temperature-time constellation depends on the coating material and product manufacturers, as each manufacturer of zinc flake products has its patented formula. Typical baking temperatures are 200 ° C, 240 ° C and 320 ° C. After baking, the coating is cured and a uniform, thin, adherent and dry layer is generated.

Application

Zinc flake coatings are used worldwide in the automotive and construction industries as a cathodic corrosion protection layers. In combination with the post-treated thin organic or inorganic coatings, this can also add color (black, silver, green, blue, etc. ), chemical resistance, have low electrical conductivity ( due to the influence of the organic layer ) and Verschraubungseigenschaften. If necessary, re-lubrication or screws securing (patch ) is possible.

Steel parts, which can be coated with zinc flake coatings are, for example, screws, nuts, springs and sheets, and structural parts.

Wind turbines (see renewable energy ) are applied, these coatings for fasteners with thread often. Good systems include certified by Germanischer Lloyd.

Zinc flake coatings are particularly suitable for high-strength bolts ( FK 10.9 and higher), high-strength nuts ( FK 10 or higher), construction parts with tensile strength > 1000 N / mm ² or > 320 HV suitable because hydrogen embrittlement is avoided.

Swell

  • ISO International Organization for Standardization. ISO 10683 Fasteners - Non- electrolytically zinc flake coatings Applied, 2000
  • CEN Comité Européen de Normalisation. EN 13858 Corrosion protection of metals. Non- electrolytically zinc flake coatings on Applied iron or steel components
  • Qualicor - European Quality Label Association. Vademecum - Non - electrolytically applied zinc flake coatings
  • Http://www.doerken-mks.de/de/produkte-systeme/applikationstechnik.html
  • ISO International Organization for Standardization. DIN EN ISO 9227 - Salt spray tests
  • Coating
  • Corrosion
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