Accelerated corrosion tests are commonly used across the coatings industry to predict performance of coating systems and aid development of new coating systems for a variety of exposure environments. Variability in the results exist in almost all accelerated testing. To improve efficiency in coatings development and to gain better confidence in test results, it is important to understand the root cause of variability. This work investigates the effect that different scribe requirements (such as dimension or orientation) have on the outcome and comparability of accelerated corrosion test results. Discussion will focus on coating application and sample preparation prior to accelerated corrosion test exposure, as well as evaluation of scribe corrosion after various preparation protocols.
The coatings industry has made widespread use of a variety of accelerated test methods to quickly and effectively evaluate coating performance. Such accelerated methods are advantageous for predicting coating system performance where real-time testing is impractical. For example, it is not practical to evaluate coatings in harsh environments where coatings are expected to last for decades when the pace of innovation and new coating development is faster than the test time would need to be. Therefore a variety of test methods exist to evaluate coatings on metal substrates, such as steel or aluminum. Coatings that will be subjected to corrosive environments require testing in environments to simulate the effects of corrosion, typically involving exposure to moderate salt concentration and elevated temperatures for a specified amount of time.1 Such tests, testing environments, and evaluation methods include ASTM B117,2 ISO 9227,3 and ISO 12944, to name a few.
Despite each method outlining many of the protocols for testing and evaluation, some variables or aspects of the test protocol remain unspecified. This lack of specificity in any given test allows for variability in the performance testing across the industry. One such example is the method for introducing a scribe to the test panels. Each of the corrosion evaluation methods in the documents mentioned above requires that a scratch or scribe be made through the coating down to bare substrate on test panels, but they do not necessarily specify the tool or method to be used. Previous studies by Maples et al. in 2018 and Rakers et al. in 2020 evaluated the effect of different scribe tools for narrow (<0.8 mm) and wide (ca. 2 mm) scribes to understand whether the scribe tool had any effect on the extent of corrosion after exposure.5,6 In both cases, the authors found that the scribe tool had no notable effect on the scribe creep results.