Zinc flake coating

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

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

1. Cr (VI) -containing (hexavalent chromium) zinc flake coatings: Cr (VI) -containing surfaces offer higher corrosion protection with a lower layer thickness. However, Cr (VI) is carcinogenic and environmentally hazardous, which is why the automotive industry nowadays does without this ingredient in zinc flake coatings. European regulations prohibit the use of surfaces containing Cr (VI), such as the end-of-life vehicle regulation EG 2000/53 and EG 2002/95 on electrical and electronic equipment (RoHS directive). These coatings are still permissible for applications outside the automotive or electrical industry.
2. Cr (VI) -free zinc flake coatings: Cr (VI) -free layers are more environmentally friendly than Cr (VI) -containing surfaces, but have the disadvantage that the so-called "self-healing effect" does not exist.

Various designers, automotive companies and their suppliers have created their own specifications and delivery instructions to determine the requirements for these coating systems.

Zinc flake coating is an umbrella term for coating technology. This is offered by various providers under the respective brand names. The companies usually act as licensors for the individual coating companies.

history

Since electrolytically galvanized surfaces offer comparatively little protection against corrosion and there is a risk of hydrogen embrittlement with galvanized zinc layers on high-strength steel (e.g. high-strength screws with strength classes 10.9 and 12.9) , the industry needed a different corrosion protection system. High-strength steel parts (such as screws with strength class> 10.9, high-strength nuts with strength class> 9), structural parts with tensile strength > 1000 N / mm² or> 320 HV are sensitive to hydrogen embrittlement. Galvanic coating processes and acid pickling have a major influence on the formation of hydrogen-induced brittle fractures.

In the 1970s, a new coating system was developed in the USA: the zinc flake coatings (patent number 1376067). With a low layer thickness of typically 8–12 µm, this system provided a high level of corrosion protection and made it possible to avoid hydrogen embrittlement.

In the 1980s and 1990s, the application of these coating systems, e.g. B. in the automotive industry. The automotive industry requires coating systems with high corrosion resistance. Since zinc flake coatings do not generate hydrogen in the process, they were used as an alternative to galvanic surfaces in critical applications.

properties

Today these coatings are preferred for fasteners and other components in the automotive industry because they offer several advantages:

• good optics (coloring)
• very good corrosion protection (240 h to 1500 h in the salt spray test , depending on the requirements)
• Temperature load
• good chemical resistance
• environmental friendliness
• Frictional properties (for screws and nuts )
• no heat release behavior
• no risk of hydrogen embrittlement with high-strength fasteners
• electric conductivity
• further screw connection properties

In addition to applications in the automotive industry, these coating systems can also be found in wind turbines , the construction industry, electrical engineering ( plant construction ), trucks and also in other markets.

Zinc flake coatings create what is known as 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 µm and 15 µm, although thicker layers are also possible for special requirements. With metric thread parts it is necessary to adhere to the tolerances according to ISO 965 so that the thread of the screw does not stick and the coefficients of friction can be adjusted accordingly. The average minimum layer thickness of hot-dip galvanized fasteners is 50 μm, regardless of the thread size.

In contrast to paints, where there is a risk of infiltration, this phenomenon is prevented by the sacrificial effect of the zinc. Zinc flake coatings achieve better results in the salt spray test than a typical galvanic zinc coating, which in the salt spray test (mostly in accordance with DIN EN ISO 9227) often only achieve 96 h to 200 h. In making such a comparison, however, it should be borne in mind that applying salt spray tests to zinc-coated steel will not produce a realistic result because these spray tests will incorrectly accelerate the failure mechanism.

Coating technology

Zinc flake coating: coated screws

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

• Spray process. The coating material is applied to the surface of the components with a spray gun. This can be done manually or in a fully automated spray system (for larger or bulky parts, also called rack goods, since the parts are brought into the coating process on a rack).
• Dip-spinning (English for dip-spin process). The parts are loaded into a basket. The coating is carried out by immersing the basket in a container with the prepared coating material. After immersion, the basket is centrifuged to remove the residues of the coating material (for smaller parts by mass / bulk goods, also called drum goods).
• Rack dip centrifuges. Parts that are placed in baskets or otherwise fixed are immersed, centrifuged and driven through the oven with the rack.
• Dip pulling. By dipping into the paint and pulling it out in a defined manner, it is possible to coat the outside and inside of, for example, pipes in one process step. However, the parts should have enough openings so that the material can run off again, otherwise a perfect coating is not possible, as bubbles form due to accumulations.

Before coating, the surface of the parts must be pretreated. Pickling with acids (e.g. sulfuric acid, hydrochloric acid) generates atomic hydrogen, which can penetrate the steel structure and make it brittle. In order to avoid pickling processes, other pre-treatment processes are necessary. The typical cleaning methods 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 through the mechanical action of steel balls, which are accelerated onto the parts with the help of a turbine in a chamber. Both processes do not generate hydrogen, so there is no risk of hydrogen embrittlement of high-strength steel components.

After the pretreatment, the actual coating follows. The parts are sprayed with the zinc flake material on a rack (spray method) or immersed in a container and centrifuged (dip spinning). The coating material ideally forms a uniform layer on the surface of the parts. A baking process is required to produce the excellent properties of the zinc flake coatings.

The coated parts must be baked in an oven under a controlled temperature and time. This temperature-time constellation depends on the coating material and the product manufacturer, as every manufacturer of zinc flake products offers its own patented formula. Typical baking temperatures are 200 ° C, 240 ° C and 320 ° C. When baking, the coating is cross-linked and a uniform, dry, adherent layer is created. An air-drying version has been available since 2014. This zinc flake coating dries within 48 hours at room temperature, but it can also be dried in the oven at a faster rate at 80 - 100 ° C. The coating is suitable for dip-spin and spray processes. Depending on the system, air bubbles can become coated in the applied layer. These air bubbles are then points of condensation for penetrating water vapor. This results in a kind of delamination or low-oxygen corrosion with decomposition of the water (hydrolysis) with the simultaneous effect of low temperatures (<0 ° C).

application

Zinc flake coatings are used worldwide in the automotive and construction industries as cathodic corrosion protection layers. In combination with post-treated thin organic or inorganic coatings, these can also offer color (black, silver, green, blue, etc.), chemical resistance, low electrical conductivity (due to the influence of the organic layer) and screw connection properties. If required, relubrication or screw locking (patch) is also possible.

Steel parts that can be coated with zinc flake coatings are e.g. B. Bolts, nuts, springs and metal sheets as well as structural parts.

In wind turbines (see renewable energy ), these coatings are often applied to threaded fasteners. Good systems are certified by Germanischer Lloyd, among others.

Zinc flake coatings are particularly suitable for high-strength screws (FK 10.9 and higher), high-strength nuts (FK 10 and higher), structural parts with tensile strength> 1000 N / mm² or> 320 HV, because hydrogen embrittlement is avoided. However, due to the system, there is a higher risk of operational hydrogen embrittlement due to the layer structure. The non-compact layer can lead to hydrolysis of the corrosion electrolyte. The pH value of the corrosion medium is lowered by the decomposition of water. This causes an increased supply of atomic hydrogen (Tafel; Heyrowsky reaction). With this system, there is therefore a fundamental risk of operational hydrogen embrittlement in the case of extremely high-strength materials that are sensitive to hydrogen.

swell

• ISO International Organization for Standardization. ISO 10683 Fasteners - Non-electrolytically applied zinc flake coatings, 2000
• CEN Comité Européen de Normalization. EN 13858 Corrosion protection of metals. Non-electrolytically applied zinc flake coatings on iron or steel components
• Qualicor - European Quality Label Association. Vademecum - non-electrolytically applied zinc flake layers
• https://www.doerken-mks.de/de/beschichtungloesungen/zinklamelle/
• ISO International Organization for Standardization. DIN EN ISO 9227 - Salt spray tests

Individual evidence

1. ↑ Fasteners - hot-dip galvanizing (ISO 10684: 2004 + Cor. 1: 2008); German version EN ISO 10684: 2004 + AC: 2009, page 12
2. ↑ DIN EN ISO 14713-1 - Zinc coatings - Guidelines and recommendations for protecting iron and steel structures from corrosion - Part 1: General design principles and corrosion resistance (ISO 14713-1: 2009); German version EN ISO 14713-1: 2009, page 25