Evo­lu­tion of cor­ro­sion test­ing

n practice – and in part demanded by the component manufacturers – various laboratory test procedures are applied, which can be divided into two categories: short-term tests and long-term tests. Short-term tests include constant climate tests and climatic extremes tests. Over a short, specific period of time these simulate the daily stresses, taking account of various parameters such as temperature and humidity or dryness and stress phases.

Alongside these standard test procedures, in some cases the car manufacturers have developed own test procedures to test corrosion resistance. These include, for example, the Ingolstädter Korrosions- und Alterungstest (Ingolstadt corrosion and ageing test - INKA) of Audi and the MEKO test of Mercedes. Here too, different stresses are simulated in various stages.

Corrosion Expert Florian Feldmann, Product Engineer at Dörken, tells us the significance of the test variants for material manufacturers and how they are implemented in day-to-day operations.

What corrosion tests are carried out at Dörken?Florian Feldmann: To reflect as great a variance of environmental stresses as possible, we conduct both short-term laboratory tests and long-term tests in the form of outdoor weathering studies. The short-term tests include, for example, salt spray testing according to DIN EN ISO 9227 and condensation testing as per DIN EN ISO 6270-2. Both of these are constant climate tests, but with different corrosive atmospheres. In addition, we also conduct a large number of climatic extremes tests for various car makers. In the area of long-term tests Dörken – often in collaboration with diverse universities – is currently conducting outdoor weathering studies on Heligoland, on the FINO II offshore platform in the Baltic, in Vienna, in Palavas (France) and also in Herdecke, near the company headquarters.

What components do you test at Dörken?Florian Feldmann: It varies: we test standard parts, but also special customer parts. These range from tiny screws to sheet metal and on to large components from diverse branches with diverse corrosion requirements. We coat the parts beforehand with various Dörken coating systems as well as reference systems. In addition, we also apply a mechanical stress to most, such as a crack or a stone impact, or subject them to thermal pre-stress to simulate corrosion behaviour in parts that are already damaged. Pre-stressing gives us a highly complex overall image of the corrosion behaviour of the surface. (Photo) documentation occurs at regular intervals, together with evaluation of the findings of the test series.

Are the same parts always subjected to outdoor weathering?
Florian Feldmann: Not as a rule. However, we have started a test series in which the same parts are exposed on the FINO II in the Baltic, in Vienna, Herdecke and France. Different climates result in different forms of corrosion. This test series therefore provides an excellent opportunity to compare corrosion behaviour under different climatic conditions: How does the surface of a part react to a more rural environment in Herdecke compared to a big city like Vienna, to the maritime climate on Heligoland, or the warm maritime climate in southern France?

Talking of comparison: Are the corrosion tests carried out in the lab compared with those in outdoor weathering?
Florian Feldmann: Comparing test findings is difficult and not even possible in some cases, because the conditions in the test processes are so different. Different corrosion mechanisms are involved, due to the different ambient conditions. For example, temperature, humidity and electrolyte vary depending on test. At the same time, laboratory testing can only simulate the day-to-day stresses on the components and never reflect the entire complexity, with the result that they cannot be compared to the outdoor weathering tests.

Is there a preferred test procedure?Florian Feldmann: Both test variants – laboratory testing and outdoor weathering tests – are thoroughly justified. The lab tests are standardised and have a relatively short duration. Typical specifications, such as from the car industry, are between 240-1500 hours in the salt spray testing according to DIN EN ISO 9227. They are also requested by numerous parts manufacturers. For us it is therefore necessary to carry out these tests and satisfy the requirements. At the same time, we also want to know what the corrosion behaviour of the parts is like in a real environment - including over a significantly longer time span. This is why we conduct the outdoor weathering tests. However, essentially, both tests provide numerous findings for the evaluation and optimisation of the corrosion resistance of surface coatings and therefore the parts themselves.  Even though we are not always able to put the tests in correlation, the results are a great help when it comes to assessing the performance of our coatings.

This technical article was also published in the international magazine IPCM^® Protective Coatings. You can read the original article here