In the development phase of a new light-alloy wheel, particularly high-sheen two-tone wheels, information relating to corrosion resistance is very important.
This is where the staff of the corrosion test centre and those involved in paint finish development take on an important role and examine material samples or complete wheels under standard conditions. In addition they also check the materials and processes used in ongoing production. A whole series of effective procedures are available for this purpose.
Special test series for high-sheen wheels
High-sheen light-alloy wheels require a very special paint structure which provides reliable protection against corrosion for the bare-look metal surfaces. As part of the production process, first of all these wheels are completely painted. Then the paint structure is removed again in specific areas of the wheel disc, mostly the spokes, using a turning machine fitted with extremely fine machine tools. This creates a high-gloss finish on the metal surface which is then sealed again with a transparent coating in another painting process. The special appeal of such wheels lies in the contrast which is created between the coloured painted areas and the high-gloss metal surface.
Mercedes-Benz has played a decisive role in developing reliable anti-corrosion systems for high-sheen wheels and is currently considered to be a pioneer in this area. During the development phase, the staff involved in paint finish development set some challenging objectives. One main requirement was to find a way to reliably prevent the “filiform” corrosion which occurs in the case of high-sheen wheels – this is where the paint in the high-gloss metallic areas is initially undercut in a filiform manner. The filiform threads mainly follow small, microscopic score marks which are caused by the turning tool when applying the high-sheen finish. At an advanced stage, the paint layers can then become loose in parts, which results in so-called delamination. However, thanks to newly developed anti-corrosion paints which are used in the production process by all suppliers exclusively for high-sheen Mercedes-Benz wheels, filiform corrosion is now a thing of the past.
Paintwork “torture chambers” put the wheels under stress
In developing new anti-corrosion paints for high-sheen wheels, Mercedes-Benz introduced the toughest of test methods – a series of ultimate tests for wheel coatings. To begin with, staff at the corrosion test centre prepare individual wheel segments by scribing deep marks on high-sheen areas which penetrate down to the bare light alloy. These prepared wheel segments then undergo the so-called CASS test for 24 hours. In this chamber-like system, the paint finishes are permanently subjected to different, highly-corrosive salt sprays. The next stage for the test samples is the filiform chamber. Here the test samples – now partly coated in a crust of salt from the CASS test – must withstand a 28-day variable climate programme. As part of the final evaluation, the employees assess the filiform corrosion around the scribed marks. Approval is only granted for those anti-corrosion paints which reduce undercutting corrosion to a minimum and at the same time do not negatively impact on other properties.
The variable climate test-D (D = Daimler), or “KWT-D” to give it its short name
(after the German “Klimawechseltest-D”), is geared towards the world’s most extreme climate requirements. Individual wheels are exposed to constantly changing environmental conditions in special climate chambers. The different climate stresses are based on global weather data, meltwater analyses and data on air pollutants. The KWT-D models different climate zones. For example, a hot/humid phase has therefore been incorporated to simulate a tropical climate, and a cold phase to simulate cold countries. The different humidity and climate variations simulate everyday motoring, such as the condensation of the morning, or the dryness of the afternoon. In addition, the test wheel is sprayed four times a week with a salt solution, so as to initiate and increase the corrosive stresses.
The KWT-D subjects the anti-corrosion materials used in the light-alloy wheels to significantly higher stresses than the variable climate test specified by the VDA (Association of German Automobile Manufacturers) and also does not take as long to complete. More test sequences can therefore be carried out in the same timeframe. Ultimately, customers in countries which do not push the anti-corrosion system to its limits also benefit from this stress marathon and the enhanced protection measures which are designed as a result. This is because there is no difference between the wheels supplied in regions with extreme climates or those supplied to the domestic market.
Laboratory tests examine paint finish resilience
To supplement the variable climate test-D, and also the CASS and filiform tests, the laboratories also carry out the widest variety of stone chip, scribe and scratch tests, or a steam-jet test, examine resistance to low temperatures and at the same time assess the mechanical resistance of the paint finish under standard laboratory conditions. In addition, samples which have been initially damaged in this way can also be subjected to aggressive corrosion stresses. These series of tests help the laboratory technicians to assess the undercutting caused to the paint layers by corrosion on conventional painted wheels too.
The practical experience of the staff also plays an extremely important role in all of the assessment techniques and laboratory analyses. Long-serving members of the Mercedes corrosion test centre are able to “feel” the quality of a paint finish quickly and accurately. To do this, they use a knife to scratch a small paint sample off and rub the material between their fingers. The consistency and specific resistance of the paint sample give them an initial impression of the quality of the paint finish.
Colour specialists, meanwhile, assess attainment of the colour and the shine of the paint finish in their own special light cabins which maintain constant colour temperature and light intensity. To help them with their task, they also have access to a comprehensive archive of paint sample cards.
Practical test on the roofs of the production facilities
As part of the “outdoor exposure” test series, wheels are extracted from the current production process as well as from operational trials on new paints and are then prepared for a special long-term test. After staff have conducted a detailed quality check of the paint finish, the painted surface is subjected to initial damage by scribing deep marks in specifically defined areas which penetrate down to the light alloy. The wheel sections which have been prepared in this way spend the next year on the roof of one of the production facilities, where they have to withstand the natural environmental conditions of the Swabian town of Sindelfingen as well as additional regular salt spray attacks by one of the staff members. This series of tests is carried out on all light-alloy wheels and supplements the comprehensive test programme conducted in the laboratories.
250 wheel models pass through the corrosion test centre each year
Around 250 wheels per year undergo this test marathon. But it is not only new wheel models which are approved on the test bench. The specialists also carry out the comprehensive test programme when a wheel supplier wants to introduce new materials, such as a base coats, clear coats, functional coatings such as anti-corrosion paints, and also primers or pre-treatments. The changes only receive a “basic material approval” once all tests have been passed without any issues. At the same time, wheels from current production are also tested to ensure that they meet strict Mercedes-Benz quality criteria. The severity of the test series conducted in the climate chamber is demonstrated by the enormous amount of salt which is consumed at the corrosion test centre: the plant requires around 5 tons of salt each year. Used salt is recycled in the basement of the test facilities.
Source: Daimler AG