When iron isn't stronger.

At the University of Manchester Corrosion Protection Center in England, a group has been studying cast-iron piping buried in soil (used for such things as water mains). Their work has shown that gray iron (the garden-variety cast iron used extensively until ductile iron came on the scene in the 1960s) and the far-stronger ductile cast iron suffer somewhat equally from bacterial-enhanced corrosion. However, their performance, once degraded, can differ notably.

One rason, explains group leader Roger King, is that the graphite contained in the two cast irons difers morphologically. In gray iron, the graphite exists as flakes, whereas the graphite in the imbedded flake matrix of graphite often traps and binds corrosion poducts in place. If this piping is undisturbed, it may continue to function, even after heavily corroded, because its graphite falkes serve as natural braces. The speheroidal structure of the graphite in the ductile iron offers no comparable support. Therefore, when ductile iron corrodes, it just crumbles and falls apart, he says. A second reason ductile iron tends not to hold up as well under comparable biocorrosion attack, King says, is that because of their superior strength, ductile-iron pipes tend to be made thinner. And if both irons are equally suspectible to corrosion, obviously the thinner metal fails first.