Corrosion protection for trucks and trailers

Functional components on trucks and trailers are subjected to high stress and therefore need to be offered enduring protection against environmental influences and corrosion. Depending on the prevailing framework conditions and other requirements to be fulfilled, various coatings can be used in this.

Functional components on trailers need to be able to withstand high levels of stress and may be protected by different forms of coating, depending on their requirements.

Hot-dip galvanisation

In addition to chassis components, fasteners that are in permanent external use are also protected by hot-dip galvanisation. External temperatures represent a challenge for coatings, especially in countries with extreme winters, such as in Scandinavia and Russia. There are two fundamental coating processes in hot-dip galvanisation: batch galvanising (discontinuous) and strip galvanising (continuous).

Chassis components are typically hot-dip galvanised, as they are exposed to high external influences.

In both cases the component is dipped in a liquid zinc bath at a temperature of around 450°C. This forms an insoluble zinc-iron alloy between steel and zinc. The coating thicknesses for batch galvanising are typically between 50 - 150 µm. Hot-dip galvanisation is a highly capable procedure for protecting components. The zinc-iron alloy is almost impervious to damage from mechanical stress and offers active cathodic corrosion protection: the surface reacts in combination with air and water, with white rust subsequently forming. Layers are removed continuously in this process. However, the galvanisation process is associated with a high consumption of energy, which needs to be included in the cost calculations. The high layer thicknesses also have an effect on the total weight of the chassis and often result in the need for recutting of precise boreholes. In addition, in-house coating is usually not possible for the user, as the plant involved is very large and the procedure complex.

KTL coating

Cathodic dip coating (KTL) is an electro-chemical coating procedure in which the component is coated under direct current in a tank filled with a watery, electrically-conductive dip spin coating. The coating layer deposited is typically between 10 - 40 µm thick. In the annealing process the coating layer is cross-linked to form a homogeneous, cohesive film. The KTL coating protects largely via the barrier effect and good coating cross linking. KTL coatings are often used for visible parts, as they are standard and easily available. In addition, they also offer an optically appealing surface in a wide range of colours. The spectrum of uses ranges from small stamped parts (rack or drum coating) to complete bodies. The encompassing properties of KTL coating enable it to penetrate even the smallest of cavities, which means that even components with complicated structures can receive an even coating. A second top coat – typically powder coating – enables further characteristics such as colouring or chemical resistance to be achieved.

However, a KTL coating has no cathodic corrosion protection effect. If the coating is damaged down to the substrate, rust and the risk of disbonding soon arise. This problem can be resolved via an innovative system from Dörken MKS, which combines cathodic dip coating with a zinc or zinc flake base coat from the Delta MKS series, thereby achieving a highly-capable degree of cathodic corrosion protection.

Zinc flake

Zinc flake systems are used for bulk products such as fasteners, springs and smaller stamped and bent components, as well as being sprayed onto larger components such as chassis parts. The zinc flake system from Dörken is also suitable for high-tensile steel, as no hydrogen is involved in the coating process.

A zinc flake coating comprises a base coat and a top coat. The top coat enables defined coefficients of friction to be achieved - a key issue in the coating of wheel bolts and nuts.

The base coat consists of flake-like zinc alloy particles embedded in a binder matrix, with corrosion protection achieved via cross-linking on the component. An additional top coat lends the coating multi-functional characteristics such as defined coefficients of friction or chemical resistance, for example against cleaning substances. Despite this, the system comprising base and top coat has a thickness of just 12 - 20 µm. The individual coats are dried at up to 220°C. Zinc flake systems offer active cathodic protection. If the coating is damaged, on contact with water and oxygen the base metal zinc sacrifices itself in favour of the more noble steel. This means high-performance corrosion protection in the micro-layer range. The low coat thicknesses result in considerable weight savings of up to 80 kg per vehicle. In addition, the risk of setting problems is also minimised. The modest space requirements of the coating facilities and spraying booths mean that zinc flake coating is often conducted in-house or integrated directly into the manufacturing line. This results in significant savings in pre-treatment and logistics, with significantly lower complaint costs incurred.