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| SQUID Magnetometers for Imaging Hidden Corrosion (Tuesday March 13, 2001) |
| Detecting hidden corrosion by its magnetic emanations (March 14, 2001) |
| Analysis of Martian meteorite using unique magnetic microscope supports claim that meteorites could have carried life from Mars to Earth (Oct.16, 2000) |
FOR RELEASE: Tuesday, March 13, 2001, 15:30
SQUID Magnetometers for Imaging Hidden Corrosion
Grant Skennerton ((215) 753-4582, grant.skennerton@vanderbilt.edu
John Wikswo (615) 322-2977, john.wikswo@vanderbilt.edu
Vanderbilt University
Nashville, Tennessee, USA
Dr Wikswo's contact details during the meeting:
Westin Seattle, (202) 728-1000
Dr Skennerton's contact details during the meeting:
Dayton Marriott, (937) 223-1000
Popular Version of Paper L27.006
Tuesday, March 13, 2001, 15:30
APS March 2001 Meeting, Seattle, WA
Superconducting QUantum Interference Device magnetometers (SQUIDs) are extremely sensitive detectors of changes in the strength of magnetic field. Relative to their sensitivity, a tremendous amount of magnetic flux is generated when a small amount of metal corrodes. In addition to their sensitivity, SQUIDs do not need to make physical contact with the specimen to measure the corrosion, and they can measure the corrosion deep within the metal specimen, without the corrosion needing to be visually accessible for measurement to occur. Because SQUIDs provide a non-invasive, extremely sensitive technique for measuring the magnetic fields associated with corrosion, SQUIDs are unequaled in their ability to image the magnetic fields associated with ongoing, hidden corrosion in metallic specimens removed from retired military aircraft.
Of scientific interest is the effect that certain environmental parameters, such as moisture and salt, have on the corrosion of military aircraft. Specifically of interest is the hidden corrosion that can occur in critical components, such as the lap joints in the fuselage, where two overlapping sheets of metal are fastened together using rivets or spot-welds. While undergoing corrosion, the magnetic fields produced by a spot-welded lap joint are characteristically different to those produced by a riveted lap joint, as shown below.
| a) |  |
b) |  |
Magnetic map of corrosion activity in (a) a riveted lap joint and (b) a spot-welded lap joint. Red represents higher magnetic activity and blue represents no magnetic activity. Note: From one image to another, the color scales are not normalised. Red in image (a) does not necessarily represent the same absolute level of magnetic activity as red in image (b).
We designed a methodology that takes advantage of the sensitivity and non-invasiveness of SQUIDs to determine the effects of moisture and salt on lap joint specimens. One common assumption, valid in some instances, is that an increasing amount of salt will increase the amount of corrosion. To test this assumption in the instance of hidden corrosion within lap joints, we exposed a set of lap joint specimens to a range of increasingly corrosive environments while the specimens were scanned by the SQUID. Thus, we were able to measure the magnetic fields associated with the corrosion caused by each environment.
The environments we used were humid air, distilled water, and then three increasingly concentrated salt solutions. The difference in concentration between each salt solution was a factor of ten. We did not expect that humid air would generate much magnetic activity, and this was borne out in our experiments. There was a significant increase in the magnetic activity going from humid air to distilled water:
Comparison of magnetic activity versus time for a spot-welded specimen in humid air and distilled water.
However, what we then found ran counter to the assumption generally held going into this study: There was no statistically significant increase in the magnetic activity following an increase in the bulk salt concentration of the immersion solution. While this observation was unexpected, it has been confirmed by other investigators using more conventional corrosion measurements. This would indicate 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.
In addition to these measurements, we have conducted other studies which demonstrate that SQUIDs can measure the effect other parameters have on the magnetic fields associated with hidden corrosion in aerospace structures. Such parameters include the presence of corrosion prevention compounds, the level of repair performed on a specimen, and the degree to which the paint has degraded. In addition, we successfully demonstrated the SQUIDs' ability to detect exfoliation corrosion.
To date, all of our studies have been conducted in a laboratory on small aluminum samples inside of a magnetic shield. Because of the complex and time-varying contributions of ferromagnetic contamination, ferromagnetic fasteners, and the Earth's magnetic field, we are not yet convinced that SQUIDs can be used for measurements of corrosion activity on intact aircraft. Even if restricted to the laboratory, we are convinced that SQUIDs can provide unique and useful information about hidden corrosion activity in aluminum.
For more information about our work, please go to our site: Living State Physics
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