Absorbing corrosion-causing chemicals
24 Aug 2004
Engineers at Ohio State University have incorporated clay and other chemicals into a paint that keeps metal from corroding and reveals when an aeroplane, 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 on Monday, August 23, at the meeting of the American Chemical Society in Philadelphia.
The pigment contains the rare earth element cerium, one of several natural anti-corrosion minerals. Coatings inside self-cleaning ovens often contain cerium, but those coatings are passive, meaning 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. 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.
In another result of their laboratory tests, the engineers confirmed that 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 aeroplanes, 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 aeroplane 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 emphasised, 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."
