It's the pits

Scully.
Photo by Jack Mellott.

Rust may get off to a slow start, but under the right conditions, it can spread like an epidemic.

Research into the process of corrosion, conducted by a University of Virginia scientist and seven other colleagues, was published in the Aug. 20, 2004, issue of Science magazine.

Their results challenge the accepted wisdom of how rust develops and open the way to new approaches to combating a phenomenon that costs American business and industry billions of dollars each year. Their results will be of particular interest to chemical manufacturers and others whose industrial processes bring corrosive materials into contact with stainless steel.

Jack Hudson, professor of chemical engineering, and John Scully, professor of materials science, along with several co-researchers at the Berlin-based Fritz-Haber Institute of the Max Planck Society, explored the process of pitting corrosion in stainless steel. Corrosion in general is estimated to cost U.S. business and industry the annual equivalent of 3 percent of the gross national product. (The U.S. GNP stood at $11.5 trillion in July 2004, according to the St. Louis Federal Reserve Bank.)

The scientists conducted a series of experiments to track the development of tiny pits of corrosion on sheets of stainless steel in response to changes in the temperature or concentration of salt in solutions. They observed the effects of the experiments under a microscope as they occurred.

What they saw was not what other scientists have theorized -- that individual pits stabilize and grow independently. Instead, they saw a critical point at which things changed dramatically, from a slow-growing phase of a few, small, independent spots, to an explosive phase of frenzied activity spurred by a chemical interaction among numerous pits.

Even slight changes in the conditions can greatly increase the likelihood of the "sudden onset of pitting corrosion as a cooperative critical phenomenon resulting from interactions among metastable pits," the researchers found. Their theory showed that each new pit increased the probability of new pits in its vicinity. This caused the rapid explosion in the number of pits, a process similar to the epidemic spread of communicable disease.

Their research suggests that corrosion can be reduced and controlled by factors that minimize the interaction of the small pits. Possible approaches include changing the metal alloys used or cleaning the reaction products from the metal's surface.

A summary of the research, which appeared in the article, "Secondary Staining of Steel," in Science magazine online.

This article originally appeared in Top News Daily, Sept. 3, 2004