Corrosion protection with zinc flake systems   – yesterday, today, tomorrow

Durability, flexibility and further development have been writing the success story of zinc flake technology since the 1970s. Numerous sectors employ this effective coating of steel components, with continuous research ensuring that they satisfy the constantly rising requirements of corrosion protection. 

One especially significant step: with the availability of zinc flake systems that harden at room temperature it is not just small parts that can be coated, but also increasingly large and heavy parts that cannot be heated in the furnace – a benefit for uses such as electromobility or bridge construction. 

 

What are zinc flake coatings?

Zinc flake coatings are so-called dispersion coatings, they can be applied with a variety of procedures. For small parts, dip-spin coating in centrifuge baskets is common practice. After immersion in the liquid coating the excess material is spun off and reused. The remaining coat is then cross-linked to the part in a furnace - retaining a coat of approx. 60-70 % zinc and up to 10 % aluminium. When it comes to bulk parts, this process is typically repeated. The reason: agglomeration points from the first coating are reliably covered. The zinc flakes lie largely parallel to one another and the surface, arranged in a polymer film. This results in a primary barrier effect against moisture and oxygen. 

Compared to conventional industrial coatings, the zinc flake coatings are characterised in particular by the fact that extremely thin coats of approx. 8-20 µm achieve excellent active corrosion protection. This means, for example, that they are ideally suited for coating the thin flanks of nut and bolt threads, without affecting the fit. A further advantage is the inorganic binder system, which minimises setting behaviour below the head for screw and bolt coatings in particular. As zinc flake systems do not generate any hydrogen in their application[1]and consequently eliminate the problem of hydrogen-induced stress corrosion cracking that affects other coatings, these systems are used in particular for the corrosion protection of high-tensile bolts and spring steel. In addition to small parts (“bulk parts”), zinc flake technology now increasingly also protects larger components against corrosion.  

 

Zinc flake - a success story

The invention originates with a small start-up from the US, which sold its development to the Japanese firm Dacral around 1970. In 1974 Dacral, a subsidiary of Nippon Oil Fat Comp (NOF), began searching for a contract coater in America, finding Magni. Dörken took over the Magni technology to adapt it to the requirements of the European market. At that time an inorganic titanate-based binder system was already in use, although cathodic protection was provided via zinc dust.   

Since the invention of the system, zinc flakes have been used as pigment in coatings. Due to their “self-healing” or passivating effect, chromium(VI)-based systems were very successful in the early phase. They enabled significantly thinner coats to be achieved than the chromium(VI)-free systems, with the consequence that the chromium-based Dacromet system long enjoyed a monopoly in the coating of screws and bolts. However, the water-based system requires a very high annealing temperature, at 300°C. In contrast, the solvent-based, chromate-free Delta Tone system developed by Dörken only requires 180-200°C – an advantage that opened up the field of spring coating to the company. In 2000 the Magni and Dörken firms went their separate ways, with the consequence that three globally active providers have since existed on the market, with the third being Dacral. Four to five years prior to the entry into effect of the End-of-life Vehicles Directive (Directive 2000/53/EC[3]), around 2002, chromium(VI)-based coatings were gradually replaced by chromate-free systems. 

 

 

Developments and areas of application

The zinc flake system is frequently adapted to new applications and developed further to satisfy changing market requirements. These include: 

Bolts and screws: Thin coats are a prerequisite here, in order to coat diverse sizes and dimensions. Zinc flake coatings have established themselves here over the course of many years and also offer good protection for smaller screws (<M6) and therefore very low diameters of threaded goods. In addition, the coating also satisfies the significantly increased requirements of corrosion protection. Where in 1980 240 h in salt spray testing were standard, today at least 720 h are expected.

Engine components: As the majority of motor vehicles have increasingly compact and encapsulated engines, an increasing level of heat resistance is required. The car industry has agreed on preconditioning of 4 days at 180°C with subsequent corrosion testing. In contrast to passivated galvanic systems, zinc flake technology satisfies this requirement and is therefore the preferred choice for the coating of components in the engine compartment.

 

Large components: After zinc flake had successfully established itself for small parts, the aim was to use it increasingly for larger and heavier parts as well. In the case of bolts, M14 is already too large for coating in the drum process. The own weight of the parts (typically 180 g for M16 x 80) leads to massive coating damage and hinders the formation of the coat. Against this background, the application process in particular was adapted. Whilst wind energy bolts (M24-M38) can still be coated with the spray procedure, more complex formed parts require a sophisticated approach to coating. As a result, for example, the square-metre-sized engine mount of a German luxury limousine with its complicated structures, perforations and undercuts was serially coated using the adapted dip-spin process in 2017. This enabled 700 g of weight to be saved compared to standard coating, for just one part.

Appearance of components: One of the most important requirements of OEMs in the motor vehicle sector was and remains an attractive appearance of black surfaces and components under service stress, or after corrosion testing. The contrast of the black topcoat to the white zinc oxide layer that forms with corrosion is noticeably disturbing. As damage to the black topcoat cannot be avoided with the coating of bulk products, the chemical make-up of the zinc flake base coat was altered to inhibit oxidation. This results in two benefits: less unsightly white corrosion and more effective use of the zinc quantity solely for the cathodic protection of the component. This is enabled by adapting the electrical resistance of the coat. The result is a dark black surface that after a long period of corrosive stress displays no more change than the “white haze” of a zinc-nickel coat.

Room temperature hardening systems: With specific parts the coating must occur with a very low annealing temperature (room temperature). This is the case, for example, with assembled parts in combination with plastic. In this case, lowering the temperature by just 20-30°C is enough, depending on the character of the plastic. This can typically be achieved via application technology adjustments. The case is different with large components that do not fit in available furnaces or that cannot be heated using standard processes - for example the metre-sized cogs used in construction cranes or other large cast steel parts. Coating systems that harden at room temperature have recently come into use, with these applied using the spray process and offering superb protection.

 

Bibliography

1. Belz, Hans W., DELTA-TONE, eine anorganische Beschichtung mit hohen Korrosionsschutzeigenschaften. Galvanotechnik 83(1992)1927.

2. DIN EN ISO 10683 Fasteners – Non-electrolytically applied zinc flake coatings, 2014.

3. DIRECTIVE 2000/53/EC, End-of-life Vehicles Directive, 18/09/2000