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Taking corrosion's magnetic pulse.

Taking corrosion's magnetic pulse

By detecting and analyzing the tiny magnetic fields associated with electrical currents that occur during corrosion-causing chemical reactions, researchers are beginning to develop methods for monitoring corrosion effects even before they are visible to the eye. The technique is also proving to be a useful tool for investigating what happens at the atomic level during corrosion. This research, by physicist Margaret L.A. MacVicar and graduate student James G. Bellingham of the Massachusetts Institute of Technology, is an extension of the discovery two years ago that such magnetic fields are detectable (SN: 8/30/86, p.132).

Using a method for detecting magnetic fields that is now about 100 times more sensitive to corrosion currents than the original equipment, the investigators can study how the electrochemical current changes over time at a given location on a metal surface. "We see the fluctuations of electrochemical, or corrosion, current as a noise in the magnetic field," says MacVicar. "By looking at the frequency profile of the magnetic field, we are getting a snapshot of the mix of the kinds of atomic activities going on." The noise profiles provide new information about corrosion mechanisms. That information may make it easier to intervene during, say, manufacturing processes, to avert potential corrosion problems. "The power of the method is now becoming clear," she says.

Bellingham and MAcVicar typically work with zinc immersed in hydrochloric acid. At a given location, zinc atoms may be losing electrons to become positive ions or hydrogen ions from the hydrochloric acid may be picking up electrons to form hydrogen gas. When two zinc wires touch the metal surface, a current flows if there is an electrochemical difference between the two spots. A magnetic-field detector picks up the magnetic field caused by the current.

"You're looking at two sites interacting electrically," says Bellingham. Sometimes that interaction leads to strong oscillations, in which the current changes direction at regular intervals. "What's happening is that there are some random processes in the reaction that occasionally set up the circumstances to drive an oscillation. But those conditions don't always exist." The occasional occurrence of oscillations, however, provides powerful clues about possible corrosion mechanisms.
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Title Annotation:research on monitoring magnetic fields associated with corrosion
Author:Weiss, Rick
Publication:Science News
Date:Feb 27, 1988
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