Long-term protection for chassis components

Car chassis components, such as rear subframes, track rod ends and anti-roll bars, are exposed to extreme stresses. Zinc flake coatings can provide long-term corrosion protection for these parts and also meet the stringent standards set by global carmakers.

Zinc flake systems have been used suc­cessf ully in the automotive industry to protect vehicle surfaces for many years. One of their main advantages is that they can be applied in very thin layers between 8 and 12 µm thick at relatively low cross-linking temperatures. In addi­tion, no hydrogen is produced during the coating process which prevents the risk of stress corrosion cracking occurring. This makes zinc flake coatings the ideal solu­tion for protecting high-strength steels of 1000 MPa and above. Combined with a top coat that meets the requirements of the ap­plication in question, zinc flake systems guarantee long-term, effective cathodic corrosion protection, even in the event of repeated stone chips. 

High level of protection for complex components

In the automotive industry, zinc flake systems are primarily used to coat small parts such as bolts, spring band clamps and springs. But these high-performance micro-coatings are also applied to !arge chassis components with complex, chal­lenging shapes. These parts are exposed to extreme stat­ic and dynamic stresses and constant me­chanical impacts in the form of stone chips. As a result, they need high-quali­ty corrosion protection systems. 

For example, wishbones with a zinc flake coating meet all the requirements of the salt spray test, even after being exposed to a large number of stone chips, and can al­so withstand an hour at a very high tem­perature of 400 ° C without evidence of any changes. Brake discs can also undergo the required 500-hour salt spray test in ac­cordance with ISO 9227 without rusting. The coating system from Dörken MKS used in these cases has been applied to hydraulic brake discs for vehicles over a number of years and provides the high standard of corrosion protection needed for these safety components, while also ensuring that they have a high-quality ap­pearance. More recently the coating has been used on leaf springs for pick-ups and meets the requirement for a salt spray test lasting over 720 hours without red rust forming. 

Wishbones with a zinc flake coating meet all the requirements of the salt spray test, even after being exposed to a large number of stone chips. 

For comparison purposes a wishbone without a zinc flake coating is also shown. 

Corrosion tests too complex and time-consuming

A range of different tests are used to iden­tify whether a coating provides long-term, effective protection and meets the require­ments of the manufacturers' own road tri­als and the increasing quality standards of the carmakers. 

Testing the corrosion resistance of com­ponents for the automotive industry is a difficult undertaking because of the com­plex requirements and the varying climat­ic conditions. In the case of outdoor tests, for example, corrosion damage often only becomes visible after several years. Out­door weather conditions in particular can vary significantly over time. 

In order to simulate the variety of weath­er conditions, different climate settings are used, from a dry desert climate to salty sea air. However, this approach general­ly takes too long for intensive testing to be feasible and makes it difficult to achieve reliable quality and development results. Against this background, the automotive industry has developed its own test proce­dures for corrosion resistance. These tests, which can be very demanding and high­ly extreme, simulate the life cycle of a car and provide comprehensive information for the evaluation and improvement of the required corrosion protection systems. 

Brake discs coated with a zinc flake system can undergo a 500-hour salt spray test without rusting,... 

...in contrast with brake discs without a zinc flake coating. 

Constant salt spraytest in accordance with ISO 9227 

Alongside the carmakers' special tests, a less complex and time-consuming process for assessing the corrosion resistance of automotive components has proved to be very useful. This is the constant salt spray lest in accordance with ISO 9227, which is regularly used in the laboratories at Dörken MKS. lt involves spraying the coated test samples with a five percent salt solution at a tem­perature of 35°C with 100% ambient hu­midity. In order to achieve reliable results concerning the corrosion resistance 0f the coating, the temperatures, the purity of the salt and the quality of the water are care­fully defined. In addition, the condensate is captured and measured on the basis of clearly specified criteria. 

As part of the precise calibration process, the test samples are weighed before and af­ter the lest to allow the amount of weight lost through rust to be determined. Be­cause of the standardised test set-up and the clearly defined conditions, there is plenty of empirical data available for this test procedure and different types of test chambers are supplied by a variety of man­ufacturers.

Alternating climate tests and other test methods

Another procedure that is frequently used in Germany is the alternating climate test. This generally includes a salt spray test, sometimes with salt concentrations that differ from ISO 9227, a specified dry phase and a stress phase with pure water mist. The components are also exposed to ex­treme temperatures ranging from -40° C to +80°C.

In Sweden a different test process has been developed by Volvo and Scania. The ACT I (accelerated corrosion test) involves sprin­kling the salt solution onto the components several times a day rather than spraying it. Because of the temperature in the test chamber, steam repeatedly rises around the components. In the modified version of the test, known as ACT II, salt solution is only sprinkled on once a day, but the salt concentration is changed. ACT II has proved to be a tougher test for a variety of lest surfaces. 

A special procedure developed in Japan is the CCT-A (cyclic corrosion test) used by Toyota. In this process the parts are first exposed to the normal salt spray test and then immersed in a salt solution. Dörken MKS also uses other methods such as the PV 1210 lest from VW and the L 467 test from Ford to ensure that its coatings pro­duce the required results.