The shield keeps out the ambient magnetic field that would interfere with the delicate measurements. These joints consist of two overlapping sheets of aluminum that are fastened together using rivets or spot-welds. 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.