Corrosion Research a Forensic Tool
November 15, 1999
Several years ago the Waterford Police Department asked Norbert Greene to examine a small, rusty, steel box. The box - its cash contents gone - had been recovered from Long Island Sound after a burglary.
"The police brought it to me and asked if I could tell how long it had been in sea water," says Greene, a professor of metallurgy in the Institute of Materials Science. They already knew when the crime had been committed. "From its appearance, I told them the box had probably been in the water between one and six months, or possibly more," Greene says.
A corrosion expert with the spirit of a sleuth, Greene began thinking. And his thoughts turned to the many guns found in bodies of water.
"Criminals often throw firearms in lakes, streams and the ocean, and these objects corrode with time," he says. If the corrosion damage to retrieved firearms could be related to the time of exposure, it could be valuable corroborating evidence, he thought. The Waterford Police and the FBI had also toyed with the idea of using corrosion as a forensic tool, but had no idea where to begin.
Greene, in collaboration with the two agencies, started a research project to find a procedure that could measure how long a firearm had been in water by studying corrosion. The FBI sent Greene 100 confiscated handguns retrieved from criminal acts to use in the project.
"Nobody had explored this before, so we tried a variety of approaches," Greene says. "Most
didn't work. We tried a lot of things and we knew most of them would fail, but many of those led us to the final techniques that we used."
After thousands of experiments over 18 months, the researchers came up with a technique primarily based on color. "Rust, when it is first formed, is a bright orange-red," Greene explains. With time, that rust, through a series of complex physiochemical changes, transforms into a dark brown-black. That's what we followed with precision color instruments."
Bare steel was placed in either fresh water or sea water and was allowed to rust. The samples were removed from the water at various intervals, rust was scraped off the surface and dried, and color measurements were made.
Using this method experts can measure exposure periods of up to five months, Greene says. After that, there are no further color changes. "But if less than that, we can predict exactly how long the metal had been in the water," he says.
The task involved much trial and error. It was months into the study, for instance, before the researchers realized the rust had to be dry, because its color changes as it dries. "We had tremendous problems reproducing the experiment until we found out all the variables," Greene adds.
This new forensic tool could have made some past questions easier to answer, he says, recalling the collapse of the Mianus Bridge section of I-95 in Greenwich 16 years ago. What still remains a mystery in that case is whether the entire bridge collapsed at once or whether parts of it fell off over a period of time. As a technical consultant on the case, Greene was asked if he could tell by looking at rusty parts of the bridge recovered from the seawater, how long they had been there. "At the time, I said there was no way to do that," he says, "but it would have been very useful to know."