Stainless steel generally exhibits excellent corrosion resistance, but an Achilles' heel known as pitting can cause catastrophic failure in structures made of the alloy.

Scientists in London have now shown that pitting results from a significant reduction of the ratio of chromium to iron in the immediate vicinity of particle impurities formed as the alloy is made.

Stainless steel typically contains iron, chromium, and nickel. Its corrosion resistance arises from a protective chromium-rich oxide film on the surface. The alloy is invariably contaminated with sulfide during its manufacture.

Lecturer in materials science Mary P. Ryan at Imperial College, chemistry professor David E. Williams at University College, and coworkers used nanometer-scale secondary ion mass spectrometry to determine the Cr:Fe ratio of the steel matrix surrounding inclusions of manganese sulfide [Nature, 415, 770 (2002)].

When steel comes into contact with water, the local environment around MnS inclusions is acidified by hydrolysis of dissolved metal cations. As a result, that region becomes more "aggressive" than the rest of the material and the metal continues to dissolve. Pit formation has been widely attributed to dissolution of MnS to form corrosive species such as hydrogen sulfide.

"It was known that there was a relationship between pitting corrosion and sulfide inclusions within the steel, but researchers were divided as to why these particles were so deleterious," Ryan tells C&EN. "We have shown that, as the steel is cooled, impurity particles effectively 'suck' chromium out of the steel around them, creating a tiny nutshell of steel that is not stainless."

It's this shell of nonstainless steel, rather than the inclusion, that triggers the high rate of dissolution and the resulting pit, she says. The MnS inclusion eventually dissolves in the acidic solution, and the growing pit becomes capped with a crust of sulfur.

"It is possible," Ryan says, "that heat treatments may eliminate this local chromium-depletion by enhancing diffusion of chromium back to those sites. Such treatment may result in better performing, lower risk, and possibly cheaper stainless steels."

The work is an impressive analytical achievement based on a technique that has so far found most of its applications in microelectronics, comments Roger C. Newman, professor of corrosion and protection at the University of Manchester Institute of Science & Technology, England, in the same issue of Nature (page 743).

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