Stainless steel's resistance to rust makes it one of the most useful and widely employed metallic products. It is not infallible, however. The material can succumb to so-called pitting corrosion, in which highly localized defects weaken the steel, often with disastrous consequences for applications such as industrial pipes and tanks. Now researchers writing in the journal Nature think they know why this happens. Their findings may help pioneer techniques for minimizing corrosion in the future.
Atoms contained within the stainless steel alloy, particularly chromium atoms, supply its rust-resisting power by reacting with oxygen to form a protective film. Previous research into pitting corrosion had placed the blame on sulfur-rich impurities in the alloy. In the new work, Mary P. Ryan of Imperial College of Science, Technology and Medicine in London, England, and colleagues demonstrated that the impurities do cause the corrosion, but indirectly. Borrowing a technique from microelectronics, the team examined the metal around sulfur-rich impurities and found that those regions had significantly less chromium than the rest of the steel. As Roger C. Newman of the University of Manchester Institute of Science and Technology explains in an accompanying commentary, "The dip in chromium content around the inclusion weakens the alloy's protective oxide layer, making it vulnerable to corrosion."
The chromium-depleted pocketsless than 400 nanometers across but responsible for many catastrophic failures, according to the reportform during the steel fabrication process. Because the sulfur impurities solidify at a lower temperature than does the steel, they remain molten for a period after the metal has solidified. These molten spots suck the chromium from the surrounding steel. "We've worked out the sequence of events that cause [pitting corrosion], we know what causes this Achilles' heel, and we can use this information to work out how to fix it," Ryan says. Indeed, Newman suggests that "unconventional heat treatments, designed to diffuse chromium back into the depleted zones," could improve the corrosion resistance of stainless steel.