Paint Absorbs Corrosion-causing Chemicals, Kitty-litter Style

Engineers at Ohio State University have incorporated clay and other chemicals into a paint that keeps metal from corroding — and reveals when an airplane, boat, or bridge needs to be repainted.

Though the paint is still under development, early tests have shown that it prevents corrosion just as well as commercial paints that are less environmentally friendly.

The new paint is unique because its pigment contains tiny particles of clay that capture the chemicals that cause corrosion. It also releases just the right amount of a corrosion-fighting agent when needed, explained Rudolph Buchheit, professor of materials science and engineering. “It works kind of like high-tech kitty litter,” he said.

With further development, the pigment could enable maintenance crews to inspect surfaces using a common X-ray technique to determine when they need to be repainted. Buchheit and doctoral student Santi Chrisanti described the project Monday, August 23, at the meeting of the American Chemical Society in Philadelphia.

The pigment contains cerium, one of several natural anti-corrosion minerals known as rare earth elements. Coatings inside self-cleaning ovens often contain cerium, but those coatings are passive — they release cerium continually until the element is gone. Scientists have been working for years to create “smart pigments” that can do more.

“The challenge has been how to keep these rare earth elements stored in a paint and then release them on demand, just when conditions are right for corrosion,” Buchheit said.

Chloride is the chemical responsible for most metal corrosion. Water is another key ingredient, and water that contains salt, or sodium chloride, is particularly corrosive. When paint cracks or wears off, a chemical reaction with the chloride eats away the exposed metal — a serious problem for critical structures on vehicles or bridges.

To fight corrosion, the new pigment absorbs chloride, and releases cerium or other corrosion inhibitors to form a protective film over cracks in the paint. In tests, the engineers coated pieces of metal with the new paint formulation, and scratched the surface to simulate severe paint wear. Then they subjected the metal to a constant saltwater fog in a laboratory corrosion chamber.

After 1,000 hours, the metal remained corrosion-free — a performance comparable to commercial paints.

But those commercial paints prevent corrosion using chromate — a toxic chemical that rose to public awareness with the release of the film Erin Brockovich. Chromate must be carefully disposed of, to keep it from entering the water supply.

And if cerium or other another corrosion inhibitor were to enter the water supply? Buchheit admits that is a question better left to toxicologists than materials scientists, but to his knowledge the chemicals he is studying do not appear to pose the same health hazards.

In another result of their laboratory tests, the engineers confirmed that a technique called X-ray diffraction can be used to measure how much cerium was released to fill the cracks, and how much was left in the paint — an indicator of whether a piece of metal would need to be repainted.

With this technique, X-rays bounce off of the crystalline clay additives to form a pattern. Because the pattern is unique to every material, scientists can use X-ray diffraction to read a substance’s chemical fingerprint.

Buchheit pointed out that the use of a different X-ray technique, X-ray radiography, is now routine for studying airplanes, bridges and boats: “We want to make our replacement technology as much like the incumbent technology as we can, so people can use the same expertise and equipment to get the job done. X-ray diffraction is not as common outside of the research laboratory as X-ray radiography, but it’s not unprecedented, either.”

He envisions that maintenance crews would set up an X-ray diffraction machine on a rack that rolled over an object, such as an airplane wing. The process could be automated.

The engineers continue to work on the pigment, which should work with just about any corrosion inhibitor, not just cerium. Other possibilities that Buchheit’s team are currently studying include molybdenum and vanadium. Buchheit emphasized, however, that the new pigment is far from a commercial product. “Real corrosion-resistant paints are highly engineered,” he said. “They’ve been given all kinds of additives to make them flow better or to give them a fine gloss — things we haven’t yet worried about.”

The Air Force Office of Scientific Research funded this work.