These joints consist of two overlapping sheets of aluminum that are fastened together using rivets or spot-welds. There was a significant increase in corrosion going from humid air to distilled water that we also expected. When they switched to the salt solutions, however, they were in for a surprise. Since then other investigators working on the Air Force project have confirmed this result using more conventional corrosion measurements. Take a sheet of metal, drill a hole in it and insert a rivet.
Exfoliation corrosion starts from the fastener hole and spreads laterally through the metal.
It is particularly difficult to detect because it doesn't come to the surface.