How Zinc Flake Systems Ensure High Corrosion Protection

Q: How can a zinc flake system help protect components from corrosion and preserve the component functionality in the long term?

A: Zinc flake coatings offer a high degree of cathodic protection with thin-film coating thickness. As a result, zinc flake is in demand wherever especially high corrosion and challenging requirements exist. From small nuts and bolts to large steel components, suitable corrosion protection must be considered for every metallic component right from the planning stage. Electrochemical reactions alter metal properties and can considerably impair its functionality. A reliable solution to this is cathodic corrosion protection via a zinc flake coating.

How is a zinc flake coating created? A zinc flake coating is essentially a “paint” with numerous, microscopic flakes (corrosion protection pigments). Per ISO 10683 and ISO 13858, these are a combination of zinc and aluminum flakes held together by an inorganic matrix. A zinc flake coating is a modular system, comprising a base coat and topcoat. Since its creation approximately 40 years ago, corrosion experts have focused on chromium (VI)-free and resource-saving ingredients.

The base coat is the zinc flake coating and it is applied directly to the substrate that is to be treated. This base coat determines the corrosion protection characteristics, with the zinc that it contains ensuring a high level of cathodic corrosion protection for the parts. As the less noble metal, zinc corrodes and protects the steel from deteriorating. This means that, in the event of damage to the coating, the ignoble zinc in the base coat sacrifices itself on contact with water and oxygen in favor of the more noble steel base material. A topcoat is typically applied to the zinc flake base coat. This increases corrosion protection as the dissolution of the zinc and iron is inhibited. Corrosive media (water, salts, cleaning agents) are kept away from the surface. The corrosion protection remains intact even in the event of mechanical damage (such as via a scratch or stone impact) to the surface, extending down to the base metal. Only after the zinc in the vicinity of the damage is dissipated and its distant effect is no longer sufficient does the base metal begin to corrode (red rust). However, the topcoat not only increases the corrosion protection it also gives the coating additional multifunctional properties, such as enhanced chemical or mechanical resistance, coloring or a defined coefficient of friction setting for threaded parts.

The overall coat thickness of these coatings is on average just 8-20 μm — making it thinner than a human hair. Despite this, a high protective effect is achieved, with up to 1,000 hours against base metal corrosion (in salt spray testing, per ISO 9227).

A further advantage is that the zinc flake systems are also suitable for high-tensile steel, as no hydrogen is generated in the coating process and there is no risk of hydrogen-induced stress corrosion cracking. For this reason, the zinc flake is suitable for high-tensile steels of the categories 10.9 and higher (> 1,000 MPa).

As a rule, pretreatment is undertaken prior to the coating process. The actual coating process is completed using a range of different application techniques, depending on the size and geometry of the part. Parts can either be sprayed with the prepared coating material or immersed in a filled container in the dip-spin process. In the latter, bulk products or racked goods are immersed before being spun with a centrifuge, ensuring that excess coating material is removed. This means that the desired coating thickness can be achieved — even for difficult geometries. Parts with suitable geometry can also be coated efficiently using the dip-drain process. Here the coating thickness is achieved via a defined withdrawal speed and defined viscosity of the coating material.

The application procedure is followed by pre-drying and crosslinking in an oven. The low crosslinking temperature of 220°C also avoids damaging the steel through excessive heat.

Thanks to its thin character and reliable corrosion protection, zinc flake technology is ideally suited for components with standards that require a high level of precision, such as screws with a metric thread. In addition, large parts such as chassis components for cars (for example, suspension components, axle parts or control arms) can be optimally protected against corrosion. Finally, the zinc flake technology also supports flexible elastic and/or plastic deformation without inhibiting the functional properties of the coating. This makes the coating suitable for springs of all kinds.

Zinc flake technology is not only used in the automobile industry, but is also found in the wind energy and construction sector, rail infrastructure, agriculture and other markets. Its advantages mean that zinc flake is primarily used in fastenings for the automobile industry. Every second bolt from the leading manufacturers is coated with zinc flake systems. In the construction sector, stainless steel remains the preferred material, and zinc flake offers an alternative here with regard to cost and performance. ASTM specifications F3125 and F3019 approve use of these surfaces for construction applications.

Albert Gelles is technical director at Doerken Corporation USA, a member of Dörken MKS. Email him at agelles@doerkenusa.com, or visit doerkenusa.com.