Accelerated corrosion tests play an important role in the development and quality control of coatings. As a result of increasingly stringent environmental legislation, coating developers are under increasing pressure to find greener alternatives that maintain performance. Atmospheric corrosion testing is often relied upon to compare quality between different coating systems.
The question “can we predict the behaviour of coatings in practice on the basis of a simple, rapid test?” was the starting point for the development of the first salt spray tests in 1914. The tests were further developed and eventually the neutral salt spray test was incorporated into ASTM under the name B117. The ASTM B117 test and the equivalent ISO 9227 test are continuous salt spray tests. In a continuous salt spray test, the conditions are kept constant for the duration of the test. For B117, this means exposure to a mist of 5% NaCl salt solution at a temperature of 35°C. Although the test has been used for a long time, a good correlation between the test and practical results has never been found. Nevertheless, this test is the most common test today.
One may wonder why this test is so popular. Due to the long existence of the B117 test a lot of test results are known for different coatings and coating systems. Therefore, when developing new coatings/coatings systems, performance is compared with that of other coatings/coatings systems using the same B117 test. In that respect, this test does indeed have its use. This test has also proven to be useful for general quality screening. For example, in the context of quality control companies often specify a minimum number of hours that the coating must withstand the test conditions before blistering of the coating or corrosion of the substrate can be observed.
In the search for accelerated tests that are closer to the real conditions, cyclic corrosion tests were developed (e.g. SAE J2334, JASO M609-91, VW PV1210, …). Many car brands have developed their own test standards. These tests are conducted in climate chambers in which temperature and humidity can be controlled in different regimes, typically combined with salt spraying. Since coatings in outdoor applications are exposed in most applications to alternating dry – wet cycles and also hot – cold cycles, this type of test can be expected to give a better match to reality. Even for these cyclic tests one must be careful when interpreting the results. Detailed correlations between test results and practice should be investigated for the coating system before any prediction of expected service life in real conditions can be made. These tests are however very suitable for making qualitative comparisons or for quality control purposes. Examples of different tests that METALogic can perform for the automotive industry:
On this page you can find more information with regard to the ISO 17025 scope of METALogic.