Corrosion under control
Trends and developments in corrosion testing
In the field of light-weight automotive design, the steels are becoming stronger and the sheets thinner. New materials and material component are being used. Scientific research and testing in the area of corrosion are making an important contribution. Current trends and developments in corrosion testing were the topics discussed at the Tri-country Corrosion Conference held in Linz in mid October. The video clip provides an overview.
A new corrosion test becomes the standard in the automotive industry. Quality, time and cost are decisive parameters in corrosion testing. The objective is to be able to make statements about real-life conditions in a rapid and economic efficient fashion. VDA 233-102, which is the new cyclic corrosive environment lab test developed by a working committee under the direction of voestalpine experts, has become the new industry standard used by European OEMs to test their specimens, components and vehicles.
New and improved. The old test, VDA 621-415, which had been in use for 40years, had substantial weaknesses for the testing of modern zinc-coated sheets in comparison with practice-based situations. For example, the creepage results did not reflect reality in that the test showed that galvanized steel experienced creepage more quickly than uncoated cold-rolled strip, a result that did not conform to the facts. Another problem was that corrosion mechanisms such as those found in filiform corrosion on aluminum were not represented in the corrosion test. Also, the test did not feature any sub-zero phase, nor did it monitor humidity.
Tests for steel and aluminum. The advantages of the new VDA 233-102 test are convincing:
- Test duration reduced from 10 to 6 weeks
- True-to-life corrosive situations for steel with bare surfaces, galvanized steel and aluminum
- Correct creepage ratio for mechanical damage between uncoated and galvanized steels
- Correct results in flange corrosion (corrosion in joined areas)
- True-to-life filiform corrosion (worm-shaped creepage) on aluminum and steel
- Consideration of world-based temperature and climate relationships (desert and tropical climates)
- Sub-zero phase (-15 °C) after storage under humid conditions in order to create thermal stress in adhesives and paints
A working committee collaborated with customers from the automotive industry to develop a new cyclic corrosive environment lab test. The specimens are submitted to stone chipping, scratching and other damage before they are introduced to the testing station. Each test phase consists of three cycles, each of which is conducted for a period of 24 hours.
- A: Salt spray cycle: Temperature (ranging between 35 and 50 °C) and humidity (ranging between 50 and 100%) are altered during the 24-hour period, whereas the specimens are continually sprayed with a 1% NaCl solution for the first three hours at 35 °C.
- B: Observation cycle: Temperature and humidity are included in the first cycle, but here the salt spray test is replaced by a three-hour dry phase in which the temperature and humidity are reduced. This way the specimens can be inspected in dry condition.
- C: Cold cycle: This is a five-hour sub-zero phase at –15 °C. It ensures that all climates are included.
Within a given testing week, the three tests are performed in a prescribed sequence: A-B-A-C-A-B-B. The testing ensures that true-to-life corrosion situations are created, particularly for the substrates of non-galvanized steel, galvanized steel and aluminum. The testing methods were derived from true-to-life corrosive environments and provide significant results for a large number of applications in the automotive industry.