Detecting Hidden Corrosion By Its Magnetic Emanations

Physicists at Vanderbilt University have developed a new remote sensing technique that can detect corrosion hidden deep within metal joints where conventional electrochemical-detection methods fail.

This capability could prove to be extremely valuable. According to a 1996 study by the Battelle Institute, corrosion in roads, bridges, passenger and freight railway systems, pipelines, harbors, airports, water treatment plants, solid waste disposal facilities, chemical processing plants and virtually every part of our complex infrastructure may be costing the United States as much as $300 billion per year—more than the cost of losses from fires and floods combined.

About a third of this insidious deterioration can be prevented using available means, the study estimates. But corrosion is not limited to places where it can be detected with conventional methods. It also occurs on hidden surfaces where it is extremely difficult to detect and study.

In a paper presented at the March meeting of the American Physical Society, John W. Wikswo, the A. B. Learned Professor of Living State Physics, and research associate Grant Skennerton reported that they have successfully used a super-sensitive piece of microelectronics, called a Superconducting QUantum Interference Device or SQUID, to detect the subtle changes in magnetic field strength that are generated when small amounts of metal corrode.

"SQUIDs are extremely sensitive magnetometers," said Wikswo. "Relative to their sensitivity, a tremendous amount of magnetic flux is generated when a small amount of metal corrodes." Moreover, the high-tech devices do not need to make physical contact with a metal specimen to detect the presence of corrosion and they can measure corrosion even when it is totally hidden from view.

The study, which was funded by the United States Air Force, specifically tested the SQUID’s ability to image the magnetic fields associated with ongoing, hidden corrosion in metal samples removed from aging military airplanes.

"Our study shows that SQUIDs are unrivaled in their ability to detect many types of hidden corrosion activity," Skennerton said. The technique does have an important limitation. To date, all of the studies have been conducted in a laboratory on small aluminum samples inside a magnetic shield. The shield keeps out the ambient magnetic field that would interfere with the delicate measurements.

"Unless we can find a way to compensate for the complex and time-varying contributions of ferromagnetic contamination, ferromagnetic fasteners, and the Earth's magnetic field, we will not be able to use SQUIDs to detect corrosion in the field," Wikswo said. "But we are convinced that SQUIDs can provide unique and useful information about hidden corrosion activity even if they are restricted to the laboratory."

An example of the role that SQUIDs can play in the fight against corrosion is an experiment the physicists did to test the relationship of the salt content of water to the corrosion of critical lap joints used in aircraft fuselages. These joints consist of two overlapping sheets of aluminum that are fastened together using rivets or spot-welds.

Wikswo and Skennerton designed an experiment that took advantage of the SQUID’s sensitivity and non-invasiveness to determine the effects of moisture and salt on lap joint samples. They exposed a set of lap joints to increasingly corrosive environments—humid air, distilled water, and then three increasingly concentrated salt solutions—and measured the magnetic fields associated with the corrosion caused by each environment.

"We did not expect that humid air would generate much corrosion, and this was borne out in our experiments," Wikswo reported. "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. Instead of finding that the corrosion increased when the metal was immersed in the saltier solutions as they expected, "there was no statistically significant increase in corrosion following the increases in the concentration of the salt solutions," Wikswo said.

Since then other investigators working on the Air Force project have confirmed this result using more conventional corrosion measurements. "This indicates that the chemistry within a lap joint might be different from that for exposed aluminum surfaces, where corrosion has a documented dependence on salt concentration," Wikswo said.

In follow-on studies, the researchers have begun to test the effectiveness of different corrosion-prevention compounds and the impact that different kinds of repairs have on corrosion activity. In addition, they have determined that the SQUID can detect a particularly nasty kind of hidden corrosion, called exfoliation corrosion. 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.