Coating chassis parts correctly

The high quality of the protective zinc flake coatings on chassis parts is the result of a carefully monitored process that is specially designed for the parts in question. Components with complex shapes can also be reliably coated using a range of different application methods. 

Chassis parts are exposed to extreme stat­ic and dynamic stresses and to constant mechanical impacts in the form of stone chips. In order to provide the very high level of corrosion protection needed and to ensure that the vehicle has a lang use­ful life, highly resistant coating systems must be used. Special requirements, such as coating the inside of tubular structures, can only be met by specially designed cor­rosion protection solutions. In addition, the choice of a high-performance coating system can allow for a thinner coating and therefore a reduction in weight. 

Zinc flake systems have been used for many years in the automotive industry to protect rear subframes, track rod ends and springs from corrosion. As the coatings are very thin and can be applied using a va­riety of processes, they are also ideal for chassis components with complex shapes, cavities and recesses. No hydrogen is pro­duced during the coating process, which means that there is no risk of hydrogen­induced stress corrosion cracking. As a re­sult, zinc flake coatings can also be ap­plied to high-strength steels. Base and topcoat provide cathodic protection The system, which consists of a basecoat and a topcoat, typically has a thickness of 8 to 20 µm. 

SEM image of a zinc flake coating with a thickness of 20 µm. 

The basecoat consists of flake-shaped zinc particles which are embedded in a bind­er matrix. The binder in the liquid coat­ing material cross-links when it is heated and forms a robust protective coating with good adhesion. The topcoat functions in a similar way and gives the system mul­ti-functional properties that include re­sistance to chemicals such as cleaning agents. 

The temperatures used to eure the coating are so low that there is no risk of the steel component being damaged by the heat. The combination of the basecoat and top­coat provides cathodic protection. If the coating is damaged, the zinc in the base­coat sacrifices itself when it comes into contact with water and oxygen to protect the steel substrate.

Specially designed coating process

Before the coating is applied, the surface of the components undergoes a pre-treat­ment process. Pickling should ideally be avoided because it can produce harmful hydrogen that can penetrate the structure of the steel and make it brittle. A typical cleaning process involves degreasing with an alkaline aqueous solution to remove grease, oil and dirt, followed by an option­al blasting phase using very fine steel shot. The parts are placed in a chamber where they are blasted with shot that is propelled by a blast wheel or compressed air in or­der to remove scale and rust.

Because of the range of different application processes available, zinc flake coatings can be used not only for small bulk parts, but also for chassis components. 

Depending on the size and shape of the parts, the coating can be applied using a number of different methods. The parts can be sprayed or immersed in a tank. Dip­ping is often used for small bulk parts or rack parts, which are subsequently centri­fuged to remove superfluous coating ma­terial. 

Parts with a suitable shape can be coat­ed efficiently using a dip-drawing pro­cess. By dipping the parts in the coating material and extracting them in a pre-de­fined way, it is possible to coat the outside and inside of tubes, for example, in one step, provided that the parts have enough openings to allow the coating material to flow out again and to prevent air bubbles from forming. Using all these application methods, the coating material should ide­ally form a uniform layer on the surface of the parts. 

The components are then pre-dried and cured in an oven. The temperature and the dwell time depend on the coating material and the products. The coating cross-links during the curing process and forms an even, dry layer with good adhesion to the substrate. The cross-linking process takes place at relatively low temperatures. This is an advantage, because the properties of some iron substrates start to change at around 220°C, which can cause problems, in particular in the case of flexible compo­nents such as clamps and springs. Cooling is the final phase of the coating process.

Monitoring and controlling influencing factors

A variety of parameters can influence all types of coating processes. The most important include the viscosity and sol­id content of the paint and the temper­ature of the paint, the component and the environment. Other factors that can affect the thickness of the coating in the dip-spin process, for example, are the du­ration of the dipping phase, the spinning speed and the diameter of the basket hold­ing the parts. The spinning time, the pivot­ing movement and/or the change of direc­tion can also have an impact on the qual­ity of the coating. 

A suspension arm with zinc flake system shows after 10 cycles no red rust (after stone impact test iaw DIN EN ISO 20567-1 and VDA 233-102). 

The key consideration in spraying appli­cations is the quantity of coating material applied to the component over a specific period of time and a specific area. Typical parameters include the air pressure, spray nozzle setting and material output quanti­ty. In this case too, the design of the racks or fixings have a major influence on the results of the coating process. 

In order to guarantee that the quality of the coating and the processes is consist­ent, tests are carried out during the appli­cation phase. These include investigating the properties of the coating material, such as viscosity, solid content and dilution, the application parameters and the properties of the coated component, including corro­sion resistance and coating thickness. The majority of the tests involve measuring the component itself in order to demonstrate that all the end customer's requirements have been met.

Measuring corrosion resistance and adhesion

A variety of tests can be used to measure a component's corrosion resistance. Along­side field testing in realistic conditions, it is also possible to use standardised envi­ronmental simulations. Generally, a neu­tral salt spray test in accordance with DIN EN ISO 9227 or a combined cyclic corro­sion test is carried out to determine the time taken for white or red rust to form.

The climate change test simulates the most extreme requirements for a component and its coating.

The temperatures, the purity of the salt and the quality of the water are clearly defined, which ensures that the corrosion resistance results are reliable and accu­rate. An adhesive tape test is often used to measure the adhesion of the coating. Another method is the cross-hatch test. The implementation and assessment of this test, like many other test procedures, are defined in a standard. Finally, it is im­portant to demonstrate that the required coating structure has been created and to measure how evenly the coating is distrib­uted over the surface of the component. The quality of zinc flake systems is in­fluenced by the application methods and the process parameters. However, indus­try standards and test procedures help to guafantee the consistently high quali­ty and·corrosion resistance of coatings on chas.sis parts and allow customers' indi­vidual requirements to be met.