SQUID can catch concealed corrosion

Even a careful eye and conventional technology can’t spot one of the most insidious effects of age and weather on aircraft–the corrosion of aluminum joints deep below the surface.

But now a new magnetism-based technique has proven itself able to locate that hidden damage, report physicist John Wikswo and engineer Grant Skennerton of Vanderbilt University in Nashville. The technique has already produced a surprise: Salt solutions of increasing concentration don’t cause increasing amounts of corrosion at subsurface joints, as they do on airplane surfaces. The researchers say this suggests that different corrosion chemistries are at work.

The technique works because magnetic fields associated with a piece of metal change in strength as corrosion worsens. Taking advantage of this trait, Wikswo and Skennerton analyzed retired aluminum aircraft parts with a sensitive instrument known as a Superconducting Quantum Interference Device, or SQUID, which can sense subtle magnetic changes. Each aircraft part the researchers measured contained a hidden joint, where overlapping sheets of metal are spot welded or fastened together with rivets.

Wikswo and Skennerton analyzed the parts with a SQUID while each piece sat in a bath of distilled water or one of three solutions of different concentrations of sodium chloride.

Contrary to expectations, the salt solutions–no matter the concentration–caused the same amount of corrosion-induced magnetic field changes, says Wikswo, who presented the results March 13 in Seattle at the 2001 meeting of the American Physical Society. What’s more, no salt solution caused more damage than exposure to distilled water did.

Other researchers confirmed the results with a widely used corrosion-measuring technique, says Skennerton. They weighed metal pieces containing subsurface joints before and after they were soaked in water or a salt bath. The SQUID technique has advantages, however, because it can detect hidden damage in parts as it occurs, he notes.

SQUIDs currently can’t locate corrosion in intact aircraft due to interferences from Earth’s magnetic field and other sources, note Wikswo and Skennerton. Still, they say, the technique will be useful in the lab for teasing apart the detailed chemistry behind corrosion of different aircraft parts.