Hopkins scientist tests theory of how alloys crack Corrosion linked to atoms' pattern

October 10, 1991|By Douglas Birch

What's more exciting than watching metal rust?

Not much, as long as you're Karl Sieradzki, a 42-year-old materials scientist at Johns Hopkins University. Dr. Sieradzki, who is now using a powerful microscope to peer at corrosion at the atomic level, is at the center of one of the most hotly debated topics in his field.

In recent years, Dr. Sieradzki helped pioneer a theory to explain the mechanism behind "stress corrosion cracking." That's where corrosion creates microscopic cracks in a metal alloy, such as stainless steel, that is bent or pulled by structural forces.

Those tiny cracks can suddenly -- and unpredictably -- become huge cracks, shredding the modern alloys developed to withstand harsh chemicals, high temperatures and enormous stresses.

Stress corrosion cracking was blamed for the 1967 collapse of the Silver Bridge over the Ohio River between Point Pleasant, W.Va., and Kanauga, Ohio, that killed 46. It has also plagued the steam generating pipes in many U.S. nuclear power plants -- leading to expensive repairs.

And it is one of the challenges facing the designers of a planned hypersonic aircraft, and canisters for high-level radioactive waste that the federal government plans to bury under a mountain in Nevada.

Dr. Sieradzki thinks stress corrosion cracks occur partly as a result of the pattern taken by atoms in the thin layer of corrosion that coats every alloy exposed to air or water. It's a theory he helped develop with Roger C. Newman of the University of Manchester in Britain in the early 1980s.

Proponents say the theory, called "film-induced cleavage," solves some age-old mysteries. One has puzzled scientists from the time of the Civil War. Union troops lugged brass cartridge shells around with them in big boxes. After they stacked those boxes in barns, they found that the brass -- an alloy of the metals copper and zinc -- would crumble in their hands.

The problem seemed to be the corrosive effect of ammonia, produced by animal wastes. But why would brass, which is almost immune to salt water, corrode and crumble when exposed to ammonia?

Dr. Sieradzki's theory says that salt water and ammonia leave different patterns of atoms on the surface of the brass -- and those patterns determine how vulnerable the metal is to stress corrosion cracking. The same process, he says, works on most other alloys.

Other corrosion researchers, such as Robert P. Wei of Lehigh University in Pennsylvania, are not convinced. They support an older theory, called "slip dissolution," that says the cracking results when corrosion attacks uncoated surfaces exposed when a material bends and stretches.

This topic generally triggers heated discussions among materials scientists. That's because the stakes are high.

Understanding stress corrosion cracking better, materials scientists say, could give them the tools needed to devise better ways of detecting it, predicting it and preventing it in the first place.

"The type of fixes depend on what you believe the mechanism is," Dr. Wei said. "The financial significance is very, very great." Damage from all forms of corrosion is equal to about 4 percent of the gross national product, he noted -- or about $225 billion annually.

Professor Jerome Kruger of Hopkins, another materials scientist, said every metal except gold has a tendency to react with oxygen, water and other chemicals in the environment and return to its natural state -- ore. But after they're refined, metals and their alloys immediately form a layer of corrosion as small as 10 atoms thick. That layer acts as a coating that protects against further corrosion.

This natural armor isn't fool-proof. It may be breached when the metal or alloy is exposed to heat, chemicals, the stress of bearing the weight of a structure or the strain of repeated flexing.

Dr. Sieradzki said the armor on alloys is created by something called "selective dissolution." That means that certain atoms are attacked and removed by corrosive chemicals, while others are left behind.

As an example, he pointed to stainless steel, which is composed of atoms of iron, nickel, chromium and carbon. When rust attacks the steel, it plucks iron atoms from the alloy's surface while leaving atoms of other metals.

In some alloys, the atoms left behind by this process don't move around. That means the holes remain about the same size. In other alloys, the remaining atoms may shift around like billiard balls in molasses, creating a more random pattern of hole sizes.

Where the holes remain uniform, the surface becomes brittle, Dr. Sieradzki said. Where the holes are irregular and randomly spaced, the alloy is less likely to crack.

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