Zinc flake coatings are non-electrolytically applied coatings, which provide good protection against corrosion. These coatings consist of a mixture of zinc and aluminium flakes, which are bonded together by an inorganic matrix.
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The specifications for zinc flake coatings are defined in international standard ISO 10683 and also in European standard EN 13858. ISO 10683 sets out the requirements for zinc flake coatings for threaded fasteners and EN 13858 describes the requirements for zinc flake coatings for fasteners with no thread and for other parts as well. There are three groups of zinc flake coatings:
Cr(VI)-free coatings are more environmentally friendly than surfaces with a Cr(VI) content. No zinc flake coatings used in the automotive industry nowadays contain this substance. Various manufactures, such as car companies and their suppliers, have produced their own specifications and supply rules in order to define the requirements for these coating systems.
Zinc flake coating is a generic term for the coating technology and this is marketed by the different suppliers under their respective brand names. The companies generally act as licensors for the individual coating businesses.
History
Because electrolytically zinc-plated surfaces provide comparatively little corrosion protection, and in the case of galvanic zinc coatings on high-strength steel (e.g. category 10.9 and 12.9 high-strength bolts) there is a risk of hydrogen embrittlement, the industry needed a better corrosion protection system. High-strength steel parts (such as bolts of strength category > 10.9 and nuts of > 9) and components with tensile strength of > 1000 N/mm2 or > 320 HV are susceptible to hydrogen embrittlement. Galvanic coating processes and pickling with acids have a major influence on the development of hydrogen-induced brittle fractures.
In the 1970s, a new coating system was developed in the United States: zinc flake coating (patent number 1376067). By virtue of a thin coat thickness of typically 8-12 µm this system produced a high level of protection against corrosion and made it possible to avoid hydrogen embrittlement.
In the 1980s and 1990s, the use of these coating systems spread, e.g. within the automotive industry. This industry needs coating systems offering a high level of corrosion resistance. As zinc flake coatings do not create any hydrogen in the process, they were used for critical applications as an alternative to electroplating.
Characteristics
Today, these are the preferred coatings for fasteners and other parts in the automotive industry, as they offer various advantages:
The requirements of the automotive industry have set new objectives. Corrosion protection and appearance are no longer the only and most important characteristics.
In addition to the applications in the automotive industry, these coating systems are also found in wind power systems, the construction industry, electrical equipment (plant construction), trucks and other markets as well. Zinc flake coatings create what is known as cathodic protection: the less noble zinc ‘sacrifices’ itself in order to protect the underlying metal. Steel can be protected in this way. The coating thickness is often between 5 µm and 15 µm, with thicker layers also possible where there are special requirements. When coating metric threaded parts it is necessary to keep to the tolerances defined in ISO 965 so that the bolt’s thread does not get gummed up and the coefficient of friction can be set accordingly. Hot-dip galvanised fasteners with a typical coating thickness of 80-200 µm have to be grooved again retrospectively in order to expose the thread.
In contrast to paints where the risk of sub-surface corrosion creep exists, this phenomenon is avoided through the sacrificial effect of the zinc. In salt spray tests zinc flake coatings demonstrate better protection against corrosion than a typical galvanic zinc coating, which in the tests (generally run in accordance with ISO 9227) often achieve only 96 to 200 hours.
Coating technique
The material for the zinc flake coatings gets supplied in liquid form and needs to be prepared to the desired conditions before application. The viscosity, temperature and stirring time prior to application all play an important role here. The material can be applied using the following application techniques:
Prior to coating, the parts’ surface needs to be pre-treated. Pickling with acids (e.g. 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 processes are removal with an alkaline aqueous solution and then blasting with very small steel balls (shot). Cleaning solutions remove grease, oil and dirt from the metal surface. Blasting removes scaling and rust through the mechanical action of the steel balls, which are fired at the parts inside a chamber using a turbine. Neither process produces any hydrogen, so there is no danger of any hydrogen embrittlement when pre-treating high-strength steels in this way.
After the pre-treatment then comes the coating process. Mounted on a rack the parts get sprayed with the zinc flake material (spraying process) or, placed inside a container, they get dipped and spun (dip-spinning). The coating material forms a liquid, uniform layer on the parts’ surface. In order to develop the excellent properties of zinc flake coatings, an annealing process is required.
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.
Applications
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 colour (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.
In the case of wind turbines these coatings are frequently used for threaded fastening elements. Good systems are certified by, among others, the GL Group with the certificate “Germanischer Lloyd”. Zinc flake coatings are particularly well suited to high-strength bolts (strength category 10.9 and above), high-strength nuts (strength category 10 and above) and structural parts with tensile strength of > 1000 N/mm2 or > 320 HV because hydrogen embrittlement is avoided.