DMA, EIS and thermal fatigue of organic coatings

Abstract

The importance of temperature variations on coatings aging is well known. The aim of this paper is to evidence how aging kinetic, monitored by EIS, can be complemented by dynamic mechanical analyses to access susceptibility of high performance corrosion protective composite coatings, designed for harsh conditions, when subjected to temperature variations, proposing methodology to aid in the selection criteria. Five coatings provided by recognized brands were submitted to corrosion tests with immersion and temperature cycling or shocking. The range of testing temperatures included the so-called humid Tg obtained by Arrhenius-type plot of low frequency impedance modulus. Different aging kinetics were evidenced by EIS for the coatings. Pull-off adhesion measurements complemented by SEM indicated loss of cohesion with cracks and voids nucleation at filler-matrix interfaces as one important feature of the coatings with more accelerated thermal aging kinetics. The irreversibility of impedance on Arrhenius-type plot, follows the thermal stress, but does not predict performance of the coatings. On the other hand, associating EIS with mechanical dynamic analyses allows understanding susceptibility to thermal aging, by considering the coatings viscoelastic properties measured within the range of testing temperatures. The input given by DMA testifies to the importance of having this technique included in coatings assessment protocols whenever temperature variations are in context. With this, one can not only better understand the behavior of the coatings, but also get clues to develop products with even better performance.