Cars And Bridges That Almost Never Rust -- Plastic Coating Creates "Organic" Metals

The coating, polyaniline, can be applied to nearly any metal, said Bernhard Wessling, Ph.D., president and managing partner of Ormecon Chemie GmbH & Co in Ammersbek, Germany. It is already being used for projects in several Asian and European countries, including Japan, Korea, Italy, Germany and France, according to Wessling, who also has had discussions with some U.S. manufacturers.

Conventional coatings act as temporary barriers to environmental assaults. By contrast, polyaniline reacts with iron, steel and other metals to create what Wessling describes as an "organic metal [that could] last forever."

Rust and corrosion occur when a metal donates some of its electrons to oxygen, causing the formation of impurities that weaken the structure. This can be delayed by painting a metal or plating it with zinc or another, more reactive metal. Zinc, which is a good electron donor, reacts with oxygen, leaving the metal underneath unaffected. But there is a limit to how long paint and zinc coatings can last.

Polyaniline works differently. Instead of creating a physical barrier, it is a catalyst that mediates the reaction that leads to rust. The polymer accepts electrons from the metal and, in turn, donates them to oxygen. This two-step reaction forms a layer of pure iron oxide that halts corrosion.

Under controlled laboratory conditions, polyaniline prevented rust 10,000 times more effectively than zinc, reported Wessling. In field tests, it proved three to 10 times more effective. Still, says Wessling, that's enough to outlast the usefulness of most products.

The polymer coating compares favorably to zinc in other ways as well. It is not a heavy metal, which, if not disposed of properly, can enter the food chain and affect human health. It is also cheaper than zinc, according to Wessling.