Evaluation of cure temperature effects in cataphoretic automotive primers by electrochemical techniques

Companies producing organic coatings for the automotive industry frequently need to test newly formulated products or materials, habitually according to different production parameters, in order to determine their actual behaviour before large-scale production of the coating is implemented. Proper choice of the test for this characterisation and the method of evaluation are critical. On the one hand, the choice is important because it is often necessary to distinguish coatings with very similar performance. On the other hand, results must be obtained in relatively short times, for it is impracticable for the manufacturers to wait for the completion of outdoor exposure tests. To reduce lead times in the development process, the use of accelerated aging tests to predict durability of the coating is essential (the time needed to perform them, however, is usually at least 500 h). This article proposes the use of EIS and an electrochemical test, AC/DC/AC, which consists in a combination of impedance results and cathodic polarisations, in order to find out the performance of coatings with precision and in a very short time. These techniques have been used to determine the optimum range of curing temperatures of a primer in order to obtain the maximum anticorrosion properties. Results show very good degree of concordance between the two electrochemical tests, although the AC/DC/AC method is much faster.