Degradation of selective solar absorber surfaces in solar thermal collectors - An EIS study

•EIS was successfully applied in the assessment of degradation of five solar coatings.•Equivalent circuits were proposed for each system.•The effect of the electrolyte (0.5 M Na2SO4 and 0.5 M NaCl solutions) was discussed.•The coatings were ranked according to their performance.

The selective solar absorber surface is a fundamental part of a solar thermal collector, as it is responsible for the solar radiation absorption and for reduction of radiation heat losses. The most common absorbers are nowadays produced by vacuum deposition, presenting disadvantages, such as lower durability, lower resistance to corrosion, higher cost and complex production techniques. Spectrally selective paints are a potential alternative for absorbing surfaces in low temperature applications, with attractive features such as ease of processing, durability and commercial availability with low cost. Thus, two PVD coatings and three organic coatings obtained by projection on aluminium substrates were studied.Electrochemical impedance spectroscopy (EIS) allows for the assessment of mechanistic information on the degradation processes, especially if equivalent circuits are used, providing quantitative data that can easily relate to the kinetic parameters of the system. EIS measures were carried out in NaCl and Na2SO4 solutions at different immersion times up to 4 weeks.The performance of the coatings, based on coupons as-received and exposed to atmosphere, is discussed and a ranking is proposed.