Better Stainless: Analysis Could Bring Pits out of the Steel. (Science News: This Week)
Perkins, S., Science News
Stainless steel, true to its name, resists rust. Cheaper grades of the material, however, are susceptible to pit corrosion, in which small spots on the metal's surface erode at accelerated rates. In certain environments, pit corrosion can pierce several millimeters of stainless steel in just a few weeks.
A study by British researchers, reported in the Feb. 14 Nature, offers hints about how metallurgists could conquer this pox on steel and produce materials that are longer lasting and easier to clean than current low-grade stainless steel is.
Most stainless steel is an alloy of iron, chromium, and nickel. When less than 13 percent of the atoms in the steel is chromium, the material corrodes as readily as regular iron, says Mary P. Ryan of Imperial College of Science, Technology and Medicine in London.
Researchers have long known that pit corrosion occurs around small impurities--called inclusions--that contain manganese sulfide, says Ryan. Scientists once thought the inclusions at the surface dissolve into liquids and produce corrosive chemicals that eat away the surrounding metal. But that didn't make sense, Ryan notes, because manganese sulfide is a very stable molecule.
So she and her colleagues blasted specimens of a high-chromium stainless steel with a high-energy ion beam and then chemically analyzed the resulting vapor. The researchers found that the inclusions themselves were chromium-rich, but the zones surrounding them contained as little as 10 percent chromium.
The disparity in chromium concentrations develops during the material's cooling process, says Ryan. …