Protective and ecofriendly at once: Zinc Flake Systems

A coating of zinc flakes is a ‚paint‘ made up of lots of little flakes, which primarily protects components of various types from corrosion. Through the sacrificial effect of the less noble zinc it provides active protection from environmental influences: this form of corrosion protection is called cathodic protection. Zinc flake coatings generally contain a combination of zinc flakes and aluminium flakes (as per DIN EN ISO 10683 or DIN EN 13858), which are fused together by an inorganic matrix. 
Extremely thin coats
Even with extremely thin coats – typically a system consists of a base coat and topcoat of 8-12 μm – it is possible to achieve protective effects against base metal corrosion (red rust) – as per DIN EN ISO 9227 – of up to 1,000 hours. By contrast, other conventional technologies require a greater coating thickness in order to offer similar corrosion protection or due to the way they are applied cannot work in the thinnest micro-layer range. In addition to the necessity for extremely thin layers due in part to the components‘ design, e.g. the need for threads to fit precisely, it is also worth mentioning from an ecological and economic perspective the low use of resources. 
During the coating process no hydrogen is produced and thus no danger exists of any hydrogen-induced embrittlement. For this reason zinc flake coating is also particularly well suited to high-strength steels of classes 10.9 and 12.9 / of 1,000 MPa and above. Due to its ability to perform well with coatings of minimal thickness the zinc flake concept has become most widely used for screws and fasteners in the automotive industry: every second screw used by the leading manufacturers is coated with zinc flake systems.
Complex requirements
In the field of automobile construction fasteners are safety-relevant components that are required to safely withstand corrosion. Black surfaces are in increasing demand for use in areas that are visible to the user. However, so far it has proved difficult to fulfil the complex requirements of the car manufacturers in a reliable manner, as these are not limited to capable corrosion protection and a premium appearance. The requirements regarding assembly characteristics, temperature stability of the coating and heat loosening torque in particular have become considerably more stringent.
KL 120 on the rise
Dörken MKS, the European market leader in micro layer corrosion protection systems with more than 30 years of experience, top technological performance and teamwork, considers itself and its employees as Corrosion Experts. This also includes the objective of constantly driving the industry forward with innovations. That is why Dörken MKS developed a system, which comprises the basecoat Delta-Protekt® KL 120 and a topcoat and which fulfils all requirements of the motor vehicle industry with regard to corrosion protection and the multifunctional characteristics of the coating, whilst at the same time giving the component a stable black surface. In this the choice of a matt basecoat was a conscious one, as it significantly improves the appearance and does not result in a noticeable silver-coloured shimmer even on points of impact and thread tips. With an overall layer thickness of 18 µm – depending on the structure of the layer, geometry of the parts and form of application – corrosion durability of 840 hours as per DIN EN ISO 9227 is achieved. White rust resistance of 240 hours is significantly exceeded. The basecoat guarantees active cathodic protection and optimised adhesion to steel. The topcoat ensures higher resistance of the coating to petrol, brake fluid, oil and cleaning agents; it improves the mechanical properties, ensures the consistency of the coefficients of friction and satisfies the temperature resistance requirement of 96 hours at 180° C.
Tailored processes
The system can be applied to racked goods or bulk parts using all standard methods, such as dipping, dip spin or spraying. The choice of procedure depends on the size and geometry of the parts. In conclusion, the coatings are cured at an object temperature of approximately 200° C to 250° C for an average of 30 minutes. The optimal combination of component geometry, coating structure, process and systems engineering is developed in close co-operation between all those involved in the process.

Date : 18.05.2018