Rust-free driving: Test procedures for corrosion protection cars

From bolts to chassis components and on to engine mounts: cars incorporate parts made from a range of different materials which, depending on use, need to satisfy numerous requirements with regard to corrosion protection. Each individual component requires a special coating precisely tailored to specifications, in order to guarantee long-lasting function and satisfy the strict automobile specifications of the national and international OEMs. Different test procedures are applied to test or examine the surface coatings of components used in cars.

Corrosion testing - complex and time consuming

Testing the corrosion resistance of components for the automotive market frequently presents problems for developers, due to the range of different stresses, complex requirements and different climatic conditions. Outdoor testing usually proves to be difficult, because: the first signs of corrosion damage often only become evident here after many years – in so-called outdoor weathering in particular, the conditions over time vary greatly and are seldom constant. To be able to simulate the various weathering conditions nonetheless, a distinction is made between different climates – from dry desert to tropical and/or salty conditions by the sea. The problem with this: for this approach, too, the time for intensive testing and therefore the achievement of dependable quality or development findings is generally too long.

Independent tests of the car manufacturers

Against this background, in some cases the car manufacturers have developed their own test procedures to test corrosion resistance. For example, the manufacturer Audi conducts one of the most exacting tests, the Ingolstädter Korrosions- und Alterungstest (Ingolstadt corrosion and ageing test - INKA). This involves the simulation of twelve car years within 19 weeks – in five stages: Firstly, the car is sprayed with salt in a climate chamber set to 35°C. After this it is exposed to tropical weathering conditions of up to 50°C and a maximum humidity of 100 %. In the next stage the bodywork is heated up to 90 degrees by 80 metal-halide lamps of 1,200 watts each. The colours in the interior are not allowed to fade – nor the materials become brittle. Stage four is the simulation of Arctic temperatures of minus 35°C. At the same time, a so-called hydropulser shakes the vehicle to recreate the twisting of the bodywork and stressing of the chassis parts on uneven roads. Finally, test drivers drive the car approximately 12,000 km on test tracks – including gravel roads and muddy trails. Mercedes has a similarly tough testing routine, the MEKO Test, whilst at BMW vehicles are required to prove their corrosion resistance in an extensive Dynamic Corrosion Test (DyCo).

One thing is clear, however: Even these extreme tests can only simulate the actual life of a car and not reflect it completely. Nevertheless, they provide comprehensive findings for the evaluation and optimisation of the necessary corrosion protection systems.

Constant climate testing in accordance with ISO 9227

In addition to the special tests of the car manufacturers, a less comprehensive test procedure for the corrosion resistance of individual parts and components in vehicle construction has also proved its value: the constant climate test in accordance with ISO 9227 NSS.

Test chamber for the salt spray test ISO9227 NSS

This involves the coated test pieces being sprayed continuously with a 5% salt solution at an ambient temperature of 35°C and 100° humidity. To ensure reliable and dependable test results for corrosion resistance are received the temperatures, degree of purity of the salt and the quality of the water are precisely specified. In addition, the condensation quantity is also collected according to defined criteria. Finally, a precise calibration is specified for the constant climate test. This means: the blank test pieces are weighed before and after testing – to identify loss of weight due to red rust. The uniform structure of the test and the specified framework conditions mean that numerous empirical values are available for this test procedure. Various test chamber manufacturers of different systems on the market.

Climatic extremes tests

So-called climatic extremes tests are also frequently used (in Germany). These typically combine the salt spray test partially with other salt concentrations than those in ISO 9227 with defined drying phases and a stressing stage with pure water spray.

Test chamber for the climatic extremes test (-40/+80°C)

In the process, the parts that are to be tested are exposed to in part extreme temperatures of -40°C to +80°C. A different test idea has established itself in Sweden – developed by vehicle manufacturers Volvo and Scania. In the so-called ACT I (Accelerated Corrosion Test) the salt solution is not applied as a spray, but instead sprinkled onto the components that are to be tested several times a day. The temperature in the test chamber means that the liquid evaporates again. In the modified test ACT II the solution is only sprinkled once a day, with the salt concentration also altered.

Test chamber for tests ACT I/ACT II/L467

On a range of different test surfaces the ACT II variant proves to be the tougher stress test. A separate procedure from Japan is the CCT-A (Cyclic Corrosion Test) applied by Toyota. This involves the parts initially undergoing the normal salt spray test, before being immersed in a salt solution.

All of the tests listed in the table below can also be conducted in the laboratories at Dörken MKS in Herdecke:

Comparison of test findings