Corroded planes turn paint pink.

Airplanes get rained on, baked in the sun, and attacked by the wind--onslaughts that corrode metal and ultimately compromise safety. Detecting corrosion can be a tedious job for maintenance crews, since many imaging techniques work only on one small area of a large aircraft at a time.

Now, researchers are developing a way for airplanes to signal to their crews when it's time for repairs. At a meeting of the Materials Research Society in Boston this week, Gerald S. Frankel of Ohio State University in Columbus described paints that change color if the underlying metal becomes corroded.

The paints, created by Frankel and his colleague, Jian Zhang, detect changes in acidity and alkalinity, measured as pH. When water and air attack some metals, the resulting electrochemical reaction produces ions that increase pH.

One of the paints consists of a clear acrylic coating mixed with phenolphthalein, a chemical that turns from colorless to red above a given pH. Frankel and Zhang made test samples by covering pieces of an aluminum alloy with the special coating and a top layer of plain acrylic.

Passing a current through the samples stimulated the electrochemical reaction, and visible pink spots popped up. Later, Frankel says, "we will probe with electrochemical and other techniques to prove there is corrosion happening locally." From these measurements, the researchers calculate that the phenolphthalein coating could detect pits less than 15 micrometers deep.

The team has also looked at acrylic mixed with a compound that fluoresces under ultraviolet light when above a particular pH. By measuring the emitted light with a spectrophotometer, Frankel says, the researchers may be able to quantify the color change and thus the extent of corrosion.

The color change method, says William M. Mullins of Technical Management Concepts at Wright-Patterson Air Force Base in Ohio, is a "very clever idea. You could walk down the vehicle and see there's a pink spot. It would be very nice to be able to do that without actively going in with an instrument."

The new method would be especially useful for detecting the most troublesome corrosion, which is concealed around rivets and in the joints where sheets of metal overlap, says Mullins, who uses ultrasound to map out and model corrosion on the surface of materials.

Only clear coatings will work, but that limitation on airplane appearance may not matter if the coatings are used in hidden places, Frankel says.