Better Stainless: Analysis could bring pits out of the steel

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. Sulfide-rich inclusions remain molten slightly longer than the rest of the steel and attract chromium atoms from adjacent zones.

Metallurgy is often an exercise in compromise. When stainless steel doesn’t have any sulfide inclusions, pitting doesn’t occur. However, some sulfide content makes the material easier to machine into useful objects. Therefore, Ryan says, the key to developing pit-resistant stainless steels is to correct the dearth of chromium atoms around inclusions, not to reduce manganese sulfide content.

One solution might be to speed up the material’s cooling process, which would allow less time for chromium atoms to migrate into the inclusions. Another answer might be to add a processing step that redistributes chromium into the zones where it’s low. Both methods would be cheaper than adding expensive metals, such as molybdenum or titanium, as makers of the highest-grade stainless steels now do.

There’s a gigantic market for less expensive corrosion-resistant steel, says Roger C. Newman, a materials scientist at the University of Manchester in England. Possible customers include industries that use stainless pipes and vats in making products such as food, deodorants, and shampoos.

Newman estimates that a new inexpensive stainless steel could result in a 10 percent reduction of the initial costs of such vats and pipes and an overall savings of 30 to 40 percent over the life of a manufacturing system that includes such equipment.