A typical method used to mitigate galvanic corrosion when bolting Al to CFR PMCs is to insert an insulating layer (G-10 fiberglass or E-glass–reinforced PMC) between the members. Although the direct contact between the aluminum and CFR PMC is eliminated, the aluminum and the CFR PMC are still galvanically coupled through the Ti fastener, and galvanic corrosion on Al will occur if the electrolyte bridges over the insulating layer. Galvanic corrosion can be mitigated by increasing the IR loss across the skirt, which can be achieved by lengthening the skirt or breaking up the continuity of the salt bridge that forms on the skirt. The continuity of the salt bridge can be disrupted by applying hydrophobic coatings on the insulative skirt.
In this study, galvanic corrosion rates were monitored for 6061-T6 Al—CFR PMC couples (with insulating skirts) subjected to the following conditions:
1) Insulting skirts having coatings (silane-based coating, epoxy, polyurethane, or latex) of various levels of hydrophobicity
2) Salt solutions of various concentrations (0, 10, 100, 1000, 10000, and 20000 ppm of chlorides)
3) Various insulating skirt lengths (0.25 inch or 0.5 inch),
4) Various exposure conditions (controlled humidity laboratory exposure, outdoor exposure with sheltered and unsheltered conditions).
The effect on galvanic corrosion by the different coatings was studied as a function of hydrophobicity, as determined by the contact angle for wetting. The contact angle was measured on the coating surface before and after exposure to the outdoor test sites to study the effects of environmental degradation on hydrophobicity. The degradation of the coatings was also studied using other techniques such as Raman spectroscopy. The ingress of environmental constituents (i.e., chlorides, sodium ions, etc.,) towards the anode and cathode surfaces was analyzed using scanning electron microscopy (SEM) and energy dispersive-X-ray analysis (EDXA).