Accelerated aging of anticorrosive coatings: Two-stage approach to the AC/ DC/AC electrochemical method

Abstract

Organic coatings are nowadays the most common and cost-effective approach to extend the service life of metallic structures subjected to weathering conditions. The development of new and more effective protective coatings is closely related to the rapid assessment of the anticorrosive performance before industrialization. Currently, the coating/metallic substrate system evaluation is made using accelerated standardized test methods. However, the time needed to perform such tests can be long, and in some cases can hold up for thousands of hours. The AC/DC/AC method is an accelerated cyclic test, consisting of a combination of electrochemical impedance spectroscopy (EIS/AC) characterization and cathodic polarisations (DC). The latter technique has been widely used to assess the adhesion of coatings to the metallic substrate but, to the best knowledge of the authors, no clear evidences have been provided that the aging promoted by AC/DC/AC is comparable to the one promoted by standardized accelerated aging tests, such as the neutral salt spray test. Said comparison is necessary to be able to support the use of the AC/DC/AC method to assess the anticorrosive performance of coatings. This work addresses this gap and goes further, proposing an innovative two-stage AC/DC/AC procedure that allows a better control of the physical delamination inherent to this technique, while promoting a chemical and electrochemical attack of coating and metallic substrate. Two water-based organic primers applied on cold rolled steel were aged by neutral salt spray (aging characterized by EIS) and by the proposed AC/DC/AC procedure. The obtained results were critically compared based on the EIS analysis. It was observed that the two-stage AC/DC/AC procedure mimicked the degradation mechanisms occurring during the neutral salt spray test, but producing results in less than 25% of the aging time.