Coating degradation monitoring through combined acoustic emission and electrochemical impedance spectroscopy measurements

Abstract

Corrosion is a leading damage mechanism in the degradation of marine assets. Organic barrier coatings are widely used as a corrosion mitigation measure, because their application suppresses the interaction between the metallic structure and the corrosive environment. Electrochemical impedance spectroscopy (EIS) is a well-established technique for the evaluation of coating performance. Acoustic emission (AE) monitoring has gained increasing interest as a technique for continuous monitoring of corrosion damage. In this experimental study, EIS measurements are combined with AE monitoring to investigate the degradation of different organic barrier coatings. Laboratory experiments were performed on 7 aluminum specimens covered with different types of coatings. Water uptake was achieved by immersing the samples in a salt solution for 24 hours prior to each test. Each sample was equipped with an electrochemical cell (Ag/AgCl reference electrode, platinum mesh counter electrode, acidic NaCl solution; pH=2) and an AE sensor (85-180kHz). The acidic salt solution was used to accelerate the degradation of the coatings for a period of 24 hours during which periodic EIS measurements were performed as well as continuous AE monitoring. AE signals detected during the test could be associated with a measured reduction in coating resistance and, in some cases, visible signs of surface degradation. The preliminary results from the study indicate that coating degradation generates AE signals in the same frequency range as that of typical AE from corrosion degradation. This is a promising perspective towards expanding the capabilities of AE based structural health monitoring systems.