Development of Guidelines to Attenuate Galvanic Corrosion between Mechanically-Coupled Aluminum and Carbon-Fiber Reinforced Epoxy Composites Using Insulation Layers

Abstract

The use of an insulating skirt to attenuate galvanic action between mechanically-coupled 6061-T6 aluminum (Al) and electrically conductive carbon fiber reinforced (CFR) polymer-matrix composites (PMCs) was assessed, analyzed and modeled. The galvaniccorrosion current as a function of time between the 6061-T6 Al and CFR PMCs for a range of insulative skirt lengths, l s , and sodium chloride (NaCl) salt loadings, m , in a humidity chamber at 30 • C and 90% relative humidity (RH) were monitored. Polarization experiments were conducted on 6061-T6 Al and conductive CFR PMC electrodes to study the mechanisms governing galvanic corrosion in solutions ranging from 0 ppm chloride (Cl − ) to 20,000 ppm Cl − concentrations. Based on the electrochemical characteristics of Al and the CFR PMC and the ohmic loss within the galvanic couple, a model was developed that related the galvanic corrosion rate (i Galv ) to the value of l s /m , which is the ratio of the insulating skirt length to salt loading. Experimental data from the polarization experiments as well as data from monitoring the galvanic couples in the humidity-chamber galvanic experiments were used to generate plots of log l s /m vs log i Galv , which can be used in design to determine proper skirt length for various environments (based on salt-loading levels) to limit the galvanic corrosion rate to a specified value. Today, many structures, machines and components are comprised of a material system rather than a single type of material.1 Polymers and composites are also being used with greater frequency. For example, only 3 kilograms of polymers were used in automobiles in the 1980s: 2 whereas, today, more than 100 kilograms of polymers are used in a typical automobile.3 Polymer-matrix composites (PMCs) are also being used with greater frequency in commercial, defense, aerospace, automotive and civil-infrastructure industries. 4,5 The advantages of PMCs, especially lightweight carbon-fiber reinforced (CFR) PMCs, are their higher specific strength and stiffness over those of conventional metal alloys. 1,6,7 Some specific examples of CFR PMC products are aircraft fittings and bicycle frames. [8][9][10] As PMCs are being incorporated into material systems to reduce weight and improve performance, they are often put in intimate contact with metal alloys such as aluminum (Al), 1,11 which can induce crevice and galvanic corrosion at the aluminum interface. 1,12,13 If the PMC is insulative such as glass-fiber reinforced (GFR), only crevice corrosion can be induced at the aluminum interface. If the PMC is conductive such as CFR, then galvanic action will also be induced at the aluminum interface.14-20 Figure 1 shows corrosion of a CFR PMC/6061-T6 Al couple exposed outdoors at Marine Corps Base Hawaii (MCBH) (a severe marine test site) for 12 months. Notice that galvanic corrosion also extends beyond the CFR perimeter under the salt and corrosionproduct deposits.Carbon fibers are electrically conductive, relatively inert, and have more noble corrosion potentials compared to ...

Effect of the Microstructure of Carbon Fiber Reinforced Polymer on Electrochemical Behavior

“…[17][18][19][20][21] CFRPs are often used together with other engineering metals, such as steels, Al alloys, Mg alloys and Ti alloys in industry. 14,15,22,23 As carbon has an open-circuit-potential (OCP) more positive than most engineering metals, strong galvanic cells can be formed between CFRPs and these metals in aggressive service environments, 24-27 thus galvanic corrosion of the coupled metals has been always a big concern.In the past decades, many studies have been focused on the galvanic effect of CFRPs on the coupled metals, including the influence of metallurgical and environmental factors on the galvanic corrosion. 10,25,[28][29][30][31] These have led to some common knowledge.…”

Utilization of hydrophobic coatings on insulative skirts to attenuate galvanic corrosion between mechanically-fastened aluminum alloy and carbon-fiber reinforced polymer-matrix composites

“…The model chosen for this study was developed by Srinivasan et al [11]. Experimental analyses of the galvanic corrosion mechanism of the CFR PMC and 6061-T6 Al were used to develop a model linking the galvanic corrosion rate (i Galv ) to salt loading (m') and length of the insulating skirt (l s ).…”

“…For a given salt loading (m'), the ohmic loss increases as the skirt length (l s ) increases. The ohmic loss is expressed as a function of m' and l s [11] and it is given by…”

“…where i Galv is the galvanic corrosion rate, L is exposed anode length, W NaCl is the atomic weight of NaCl, F is Faraday's constant, Z is the magnitude of charge on the ionic species, and µ j [cm 2 sec -1 V -1 ] is the mobility of species j [11].…”

“…The direct contact of CFR PMCs with Al alloys can result in accelerated galvanic corrosion of the Al alloys [6,7], where the CFR PMC serve as the cathode and the Al serves as the anode [8][9][10][11]. In chloride-containing solutions, oxygen reduction and/or hydrogen evolution cathodic reactions can take place on the electrically conductive carbon fibers that have corrosion potentials more positive than that of Al alloys [9,10].…”