Articles

28.02.2017

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Zinc flake coatings are non-electrolytically applied coatings, which provide good anti-corrosion protection. These coatings consist of a mixture of zinc and aluminum flakes, which are bonded together by an inorganic binder.

The requirements for zinc flake coatings are defined in the international standard ISO 10683: 2014 and in the European Standard EN 13858. ISO 10683 sets the requirements for zinc flake coatings for threaded fasteners and EN 13858 does the same for fasteners with no thread and other structural parts as well.

There are 2 groups of zinc flake coatings:

1) Zinc flake coatings containing Cr (VI) hexavalent chromium. Surfaces with this coating provide greater anti-corrosion protection with a thin layer. According to European decrees it is prohibited to apply coatings containing Cr (VI) (e.g EC 2000/53, EC 2002/95 for automotive and electronic industries).

2) Cr (VI) free coatings. Such coatings are ecologically safer for the environment, thus no zinc flake coatings used in the automotive industry contain this element. Car manufacturers and their suppliers have set their own specifications and supply rules to define the requirements for these coating systems.

History

Electrolytically zinc plated surfaces provide quite a little corrosion protection. In case of galvanic zinc plating of high-strength fasteners there is a risk of hydrogen embrittlement. Thus a better corrosion protection system was required in the industry. High-strength fasteners (bolts, nuts, washers) and components with tensile strength of >1000 N/mm2 or <320HV are susceptible to hydrogen embrittlement. The methods of galvanic coating application on fasteners and acid pickling contribute to the appearance  of hydrogen-induced brittle fractures.

In the 1970s, a new coating system was developed in the U.S.: zinc flake coating. Due to a thin coat thickness (typically 8-12 µm) this technology had a high level of anti-corrosion protection and made it possible to avoid hydrogen embrittlement.

In the end of the 20th century this technology became popular in the automotive industry. As zinc flake coatings don’t produce any hydrogen in the process, they were used for critical applications as an alternative to electroplating.

Properties:

  • Good appearance (coloring);

  • High protection against corrosion (240-1,500 hours, depending on specification and coating thickness);

  • Resistance to extreme temperatures;

  • High chemical resistance;

  • Ecologically safe;

  • Good friction coefficient;

  • No risk of hydrogen embrittlement;

  • Electrical conductivity;

  • Other assembly properties.

As far as other branches of industry are concerned, zinc flake coating is also used for wind power systems, construction, automobile, railway, oil and gas, agricultural and food manufacturing industries, etc.

The coating thickness is often from 5 to 15 µm with thicker layers if required.

In comparison with other types of coatings zinc flake coatings show better anti-corrosion protection than a typical galvanic zinc coating, which achieve only 96 to 200 hours in the tests (acc. to ISO 9227).

Coating technique

The material for the zinc flake coating is supplied in the liquid form and before application all the required preparations should be held. The viscosity, temperature and homogeneity play an important role here. The material can be applied using the following techniques:

  • Spraying. The coating material is applied to the parts’ surface using a spray gun. It can be done manually or in a fully automated spraying facility.

  • Dip-spinning. The parts are loaded into a basket. The coating is done by dipping into a container filled with the prepared coating material. After the dipping, the basket gets spun around in order to remove the fragment of the material (for smaller and high-volume parts).

  • Rack dip-spinning. Parts are placed or fixed in baskets and dipped spun and passed through the furnace with the rack.

  • Dip coating with extraction. The parts are dipped into the liquid material and drawn out.

Before the coating the parts’ surface is pre-treated. Acid pickling  (sulfuric or hydrochloric acid) produces atomic hydrogen and can penetrate into the steel structure and make it brittle. In order to avoid pickling procedures other pre-treatment processes are required. The typical cleaning process is surface degreasing by means of alkaline aqueous solution and blasting with small steel balls.

The cleaning solutions remove grease, oil and dirt from the metal surface. Blasting removes scales and rust through the mechanical action of the steel balls, which are fired at the parts inside a chamber using a turbine. Hydrogen is not produced during the process, so there is no risk of hydrogen embrittlement. Next to pre-treatment is the coating process. The parts on the lifting bars are sprayed with the zinc-flake coating material or placed inside a container, they are dipped and spun (dip-spinning). The coating material forms a liquid layer on the parts’ surface. In order to guarantee the excellent  properties of zinc flake coatings the annealing process is obligatory.

The coated parts have to be cured inside an oven at a controlled temperature for a set period. This temperature/time configuration is dependent on the coating material and the product manufacturer, as each manufacturer of zinc flake products has its patented formula. Typical curing temperatures are 200 °C, 240 °C and 320 °C. After the curing, a uniform, thin, firmly bonded and dry layer is produced.

Application

Zinc flake coatings are used as cathodic protective layers against corrosion all over the world in the automotive and construction industries. Combined with cured, thin, organic or inorganic coatings, these can also provide color (black, silver, green, blue, etc.), chemical resistance, low electrical conductivity (due to the influence of the organic layer) and assembly properties. If required, re-lubrication or a thread lock (patch) is also possible.

Steel parts that can be coated with zinc flake coatings include, for example, bolts, nuts, springs, panels and structural parts. Zinc flake coatings are particularly well applied to high-strength bolts (strength class 10.9 and above), high-strength nuts (strength class 10 and above) and structural parts with tensile strength of  > 1000 N/mm2 or  > 320 HV because hydrogen embrittlement is avoided.