Detection of Hidden Corrosion of Aircraft Aluminum Alloys by Magnetometry Using a Superconducting Quantum Interference Device

The distribution and magnitude of the magnetic field resulting from hidden corrosion of Al 2024-T3 (UNS A92024) alloy plates in solutions containing different concentrations of chloride (Cl−) ion were measured using a superconducting quantum interference device (SQUID) magnetometer, with no electrical or mechanical connections intruding on the sample. The magnetic field strength resulting from hidden corrosion occurring on the bottom surface of a 3-mm (0.118-in.) Al 2024-T3 plate in 3.5% (35,000 ppm) sodium chloride (NaCl) solution was measured as a function of distance between the SQUID pickup coils and the top surface of the sample. Results showed magnetic field strength decreased with increasing distance from the top of the sample, approaching the background value at a distance of 13 mm (0.512 in.). When the SQUID pickup coils were located 2 mm (0.079 in.) above the top surface of a sample consisting of a stack of aluminum plates, it was possible to detect hidden corrosion on the bottom surface of a stack as thick as 10.2 mm (0.402 in.) in 3.5% (35,000 ppm) NaCl solution. The magnetic field strength from aluminum alloy corrosion was shown to decrease with decreasing NaCl concentration. The SQUID was able to detect very weak corrosion signals in a solution containing only 1 ppm NaCl, 1/35,000 of that found in seawater.