Mar. 30, 1998 COLUMBUS, Ohio -- Researchers at Ohio State University are developing an early warning system for aircraft degradation -- paint that changes color when the metal beneath it begins to corrode.
While maintenance crews can search for corrosion with several high-tech tools including electromagnetic currents and ultrasound, this paint is potentially both less expensive and more sensitive, because it makes tiny pockets of corrosion visible to the naked eye.
The paint reveals newly-formed corrosion cavities as shallow as 15 micrometers -- about one-fifth the width of a human hair. Gerald Frankel, associate professor of materials science and engineering, and Jian Zhang, a graduate student, presented their results at a recent meeting of the Materials Research Society.
“We would like to know early where corrosion is occurring on airplanes, but it starts in hidden crevices, so it’s hard to find,” said Frankel. “One solution is to implant corrosion sensors in an airplane, but sensors only detect corrosion in their immediate area, and there are lots of potential spots for corrosion on an airplane. That led us to this notion of modifying paint, because on an airplane paint is everywhere.”
Frankel cautioned that the paint would not be ready for actual use on airplanes for a number of years. He did say, however, that with further development, the paint could save military and commercial maintenance crews from having to tear aircraft apart to check for corrosion.
Maintenance crews can’t readily see the corrosion that’s hidden inside the crevices of aircraft until it spreads to a considerable extent. For this reason, Frankel and Zhang decided to colorize a normally invisible part of the corrosion process that takes place on the surface of the metal near the hidden corrosion site.
The chemical reaction that takes place during corrosion releases electrons that are consumed by a second reaction. This reaction that consumes electrons will often increase the pH of the region where it takes place. The researchers mixed clear paint with laboratory chemicals called pH indicators that change color when pH changes. They coated a piece of aluminum alloy with paint containing Phenolphthalein, a common pH indicator, and soaked it in salt solution to promote corrosion. After only 4 hours the phenolphthalein paint had turned red around pockets of corrosion.
To measure how well the paint worked, Frankel and Zhang passed electrical current through other pieces of painted metal to simulate the electron-consuming reaction that normally happens during corrosion. After some calculations, they determined that the change in color became visible to the naked eye after a small number of electrons had passed through the metal -- only enough to correspond to a pit of corrosion about 15 micrometers deep.
Right now, the researchers are only working with clear paint, and Frankel admits that this method wouldn’t work for regular colored airplane paints, because the pigments would cover up the color change effect of pH-sensitive chemicals.
But, Frankel said, much corrosion happens inside airplanes, where colored paint isn’t necessary.
“Lots of corrosion on airplanes happens near galleys and latrines; there are many corrosive fluids in those areas. There are gangways and other areas where moisture can build up. Water can slosh around in the belly of a plane, and corrode the metal from the inside out,” Frankel said.
He added that maintenance crews could easily climb inside the plane and shine a flashlight to see whether any areas of the paint have changed color because of corrosion.
Frankel also said that a paint doesn’t have to visibly change color to indicate corrosion. An alternate plan would be to mix the paint with chemicals that fluoresce with pH changes, so a quick scan with a black light would make corroded areas glow.
Frankel and Zhang will continue this work, which was funded by a grant from the Wright-Patterson Air Force Base Materials Directorate through the Air Force Office of Scientific Research.
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