CORROSION–Vol. 55, No. 10 957 CORROSION SCIENCE SECTION 0010-9312/99/000213/$5.00+$0.50/0 © 1999, NACE International Submitted for publication January 1999; in revised form, July 1999. * Fontana Corrosion Center, 477 Watts Hall, 2041 College Rd., The Ohio State University, Columbus, OH 43210. Corrosion-Sensing Behavior of an Acrylic-Based Coating System J. Zhang and G.S. Frankel* ABSTRACT Paint systems containing color-change or fluorescing com- pounds were found to be sensitive to underlying corrosion processes by reacting to the pH increase associated with the cathodic reaction that accompanies corrosion. The sensitivity of acrylic-based coating systems for detection of cathodic reactions associated with corrosion was determined by ap- plying constant cathodic current and measuring the charge at which color change or fluorescence was detected. Unaided visual observation of coated samples detected changes re- sulting from a charge corresponding to a hemispherical pit with a depth of 10 μm. Characteristics of modified acrylic coating systems were studied by titration tests. Electro- chemical impedance spectroscopy also was performed to test the influence of the indicating compound addition on the coating corrosion protectiveness. The time for color change was determined to be controlled by the sensitivity of the coating to pH increase, and not by the coating protectiveness. KEY WORDS: acrylic-based coatings, corrosion detection, corrosion sensing, electrochemical impedance spectroscopy, fluorescence, pH INTRODUCTION The early detection of corrosion in aging aircraft has economic and safety implications. In airplanes, cor- rosion may occur in relatively inaccessible locations, such as within the lap joints of the skin of an air- plane (Figure 1). 1 As a result, detection of localized corrosion in a large, complex structure such as an airplane is extremely difficult. Many different sensors and techniques are being developed to detect corro- sion. 2-4 However, these sensors are only effective if they are physically located at the place where the corrosion is happening or sensitive to corrosion occurring remotely. The idea of the present study was to modify paint to function as a sensor for corrosion since paint covers the entire surface of an airplane. The goal was to sense the cathodic reaction that accom- panies the oxidative corrosion reaction. The main cathodic reaction for any form of atmospheric corro- sion is oxygen reduction: O 2 + 2H 2 O + 4e – → 4 OH – (1) For localized corrosion such as pitting, crevice, and exfoliation corrosion, this cathodic reaction will tend to occur at more accessible locations than the anodic reaction (i.e., nearer to the source of oxygen in the air, Figure 1). This reaction will cause an increase in the local pH where it occurs, so a paint that is sensi- tive to pH increases generated by the cathodic reaction will sense corrosion occurring nearby. Others have pursued similar approaches. Color- change pH indicators have been incorporated into organic coatings as a tool for determining the pH gradients associated with filiform corrosion beads. 5 Fluorescent dyes were applied to microelectronic test vehicles to detect pH changes associated with corro- sion of Al or Au metallization under an applied bias