Inhibiting Metal Galvanic and Carbon Corrosion in Aluminum Alloy-Carbon Fiber Reinforced Composite Joints by Spontaneous Deposition of Diazonium Adlayers on Exposed Carbon Fibers

Abstract

The spontaneous formation of a diazonium molecular adlayer on the exposed fibers of a carbon fiber reinforced polymer (CFRP) composite was investigated as a surface treatment. The purpose of the surface treatment is to reduce the extent of galvanic corrosion on a trivalent chromium process (TCP) conversion-coated aluminum alloy when the two materials are mechanically joined as is common in aircraft structures. The adlayer functions to reduce the rate of dissolved oxygen reduction on the more noble carbon thereby reducing the rate of galvanic corrosion on the more active metal alloy. The formation of the adlayer (4-nitrophenyl (NP), 4-nitroazobenzene (NAB), or 2-fluorene diazonium tetrafluoroborate (FL)) was achieved by immersing the CFRP composite specimen in a solution of 5 mM diazonium salt dissolved in acetonitrile for a specified time (1–24 h) under open circuit conditions (so-called spontaneous deposition). Cyclic voltammetry (CFRP composite) and rotating disk voltammetry (glassy carbon) were used to investigate the oxygen reduction reaction kinetics in naturally aerated 0.5 M Na2SO4 (pH 5–6). The oxygen reduction reaction current was suppressed by greater than 90% by the NP and NAB adlayers and 80% by the FL adlayer. The surface treatment (24 h immersion) with NAB significantly reduced the galvanic corrosion damage on TCP conversion-coated aluminum alloy during a 14-day continuous neutral salt spray (NSS) exposure. The mass loss (mg/cm2) and corrosion intensity (g/m2y) were reduced by over 10×. The spontaneously formed NAB adlayers are stable during the salt-spray exposure and provide better inhibition of galvanic corrosion on the aluminum alloy and resistance to carbon corrosion than do NAB adlayers formed by an electrochemically assisted process reported on previously.