Corrosion of Graphite-Fiber-Reinforced Composites I — Galvanic Coupling Damage Available to Purchase

As part of an investigation into the long-term environmental degradation of composite materials in seawater, the effects of electrochemical polarization on graphite/epoxy have been investigated in a two-part study. In the present study, the effects of cathodic polarization (as caused by galvanic coupling to magnesium) are investigated. Graphite lies very high (noble) in the galvanic series, and graphite-fiber-reinforced composites will almost certainly form galvanic couples with structural metals when the two are used together in marine applications. Oxygen will then be reduced at the graphite/polymer cathode with an associated rise in the local pH. This high pH at the fiber/matrix interface may damage the polymer matrix resin and reduce the structural integrity of the composite. T-300/934 graphite/epoxy coupons were galvanically coupled to magnesium in natural seawater for 140 days. After the exposure period, the apparent shear strength of the coupons was reduced by approximately 30% because of localized attack of the matrix or fiber/matrix interface. The most probable damage mechanism is nucleophilic attack by hydroxyl and perhydroxyl ions, which form as a result of the oxygen reduction reaction on graphite. The damaged resin system contained phthalate linkages, i.e., ester bonds, which are susceptible to nucleophilic attack. Localized delamination/ penetration rates were estimated to be on the order of 5 rnm/y. Techniques for avoiding galvanic coupling damage in composites are suggested.