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 hi dden corrosion of Al 2024-T3 (UNS A92024) alloy plates in solutions co ntaining different concentrations of chloride (Cl-) ion were measured using a superconducting quantum interference device (SQUID) magnetomet er, with no electrical or mechanical connections intruding on the samp le. The magnetic field strength resulting from hidden corrosion occurr ing 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 func tion of distance between the SQUID pickup cells and the top surface of the sample. Results showed magnetic field strength decreased with inc reasing distance from the top of the sample, approaching the backgroun d value at a distance of 13 mm (0.512 in.). When the SQUID pickup coil s were located 2 mm (0.079 in.) above the top surface of a sample cons isting 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 Na Cl concentration. The SQUID was able to detect very weak corrosion sig nals in a solution containing only 1 ppm NaCl, 1/35,000 of that found in seawater.