Safety first: Corrosion protection for control and trailing arms

Chassis components play a key role in cars: joints, brakes, springs, steering, wheels and numerous other components form a complex unit to link the chassis to the road. Guaranteeing high safety and dynamism is therefore all the more important.

The chassis has a number of important tasks: on the one hand, it transfers energy between vehicle and road, on the other hand it is responsible for vertical vibration behaviour via springs and absorbers. In addition, it also enables the steering and braking of the vehicle. Technically sophisticated and safety-relevant systems are therefore essential. Malfunctions or corrosion damage would have fatal consequences for car passengers and bystanders. 

Today, the chassis as a whole and the steering elements in particular are exposed to extreme stresses. Potholes, cracks or unevenness on the roads are hard on the parts, with high quality a subsequent prerequisite. To avoid these problems, cars need different forms of arms and rods: control and trailing arms as well as twist beams. Depending on vehicle category, these components are made from steel or even aluminium. 

Coating options to combat corrosion

In addition to mechanical stresses, the arms and rods need to withstand the influence of water, heat and particle friction such as salt or grime. To avoid corrosion damage, a passive corrosion system such as a KTL coating is often used. KTL – cathodic dip coating – is an electro-chemical coating process in which the component is coated via direct current in a tank filled with an aqueous, electrically-conductive dip coating. The thickness of the deposited coat is typically between 10–40 μm. An annealing process cross-links the coating layer to form a homogeneous, cohesive film. The procedure is especially well-suited to complex structures and large batches, also penetrating hollow spaces. However, a consequence of the passive corrosion protection is that rust and the risk of disbonding arise where the coating is damaged down to the substrate.

Chassis components play a key role in cars: joints, brakes, springs, steering, wheels and numerous other components form a complex unit to link the chassis to the road. Guaranteeing high safety and dynamism is therefore all the more important. 

The alternative: zinc flake

This problem can be resolved with an innovative system that combines cathodic dip coating with a zinc or zinc flake basecoat from the Dörken MKS portfolio, achieving high-performance cathodic corrosion protection. 


Triangular control arm after stone impact test iaw DIN EN ISO 20567-1 and salt spray test iaw DIN EN ISO 9227: without Dörken MKS coating after 144 hours red rust (left), with Dörken MKS coating min. 720 hours without red rust (right).

The zinc flake coating systems are particularly well-suited for control and trailing arms, as they are applied in very thin layers (8–20 μm). Hollow spaces and weld nuts also present no problems. This thin coating also means that weight savings can be made – an aspect that is not only of significance in the field of lightweight construction, but also that of climate protection. The light coating results in excellent and long-lasting corrosion protection and thereby enduring functionality of the component.

A further advantage: the zinc flake systems from Dörken MKS are also suitable for high-tensile steel, as no hydrogen is generated in the coating process and there is therefore no risk of hydrogen-induced stress corrosion cracking. The low annealing temperature of 220 °C also avoids damaging the steel through excessive heat. In combination with a matching topcoat, a high degree of cathodic corrosion protection is provided – including after stone impact. This means that in the event of damage to the coating, the ignoble zinc in the basecoat sacrifices itself on contact with water and oxygen in favour of the more noble steel base material (see illustrations 2 and 3).

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After stone impact test iaw DIN EN ISO 20567-1 and salt spray test iaw DIN EN ISO 9227 for control arm no red rust after 1,000 hours.
 

In addition, zinc flake systems also meet the specifications of the manufacturers’ own driving tests, thus meeting the increasingly high quality requirements of the OEMs.


­A suspension arm with zinc flake system is after 10 cycles without red rust (after stone impact test iaw DIN EN ISO 20567-1 and VDA 233-102).