Another Danger with Corrosion

Phil Kinnane | June 12, 2012
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It’s long been known that a danger with corrosion is that it compromises the structures that it affects. This is particularly relevant for the naval industry where material failure leads to leaks and the like. Now, another danger is becoming apparent.

Galvanic, crevice and other types of corrosion are electrochemical in nature and give rise to small currents passing between the corroding and most noble regions, via the material and surrounding electrolyte. To combat this, you can impose an opposite electric current in a process known as cathodic protection. In many cases, this leads to an even larger current passing through the material and surrounding electrolyte.

This is where problems arise. Electric currents give rise to electric potentials, and these can be measured. If a submarine, for example, is imposing an electric current through its hull and propeller, then there is a pretty big (in terms of area) electric potential that could be used to detect the submarine, or even set off a mine. This is further complicated by the submarine’s rotating propeller, which modulates the signal.

What to do? One solution is to turn the cathodic protection process off when the submarine is in an area of reasonable danger. But this won’t work as the corroding process itself has its own electric potential signature that can be detected.

David Schaefer at the University of Duisburg-Essen, in Germany, is simulating these underwater electrical potential signatures of submarines, on the commission of the Technical Center for Ships and Naval Weapons. Using COMSOL Multiphysics, he and his team were able to simulate the signal and find that you could run cathodic protection at a certain amperage, resulting in a signal less noticeable than that during corrosion. Read more about it in the COMSOL News 2012 article “Submarines: Corrosion Protection or Enemy Detection?” (on page 67).

Underwater Electric Potential Signatures below the keel

Underwater Electric Potential Signatures below the keel, for different currents imposed through
cathodic protection. A signature is evident when corrosion is occurring (top left), which can be
optimized at 3.5 A (top middle). Overprotection results in larger signatures.

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