A Few Things You Might Not Know About Corrosion

01.15.16   Thomas L. Bane, PE | More by this Author

A Few Things You Might Not Know About Corrosion

Galvanic corrosion in the building enclosure is a problem that designers run into on many projects. Unfortunately, it’s something that is not well understood by the industry. Sure, many have heard of galvanic action in one form or another: if you put dissimilar metals in water, you create a battery that corrodes the metal.

If you’ve done your research, you also know that the dissimilar metals create a galvanic couple. The galvanic couple causes the anode to corrode and protects the cathode from corrosion. After that, it’s just a matter of remembering which metals are higher on the galvanic series (cathodes at the top, anodes at the bottom). The further apart the metals are, the worse it is for the metal on the bottom.

But, most people don’t know about another part: the relative size of the different metal pieces matters.

Oversimplified, corrosion happens because the cathode is stealing electrons from the anode. If the cathode is large in comparison to the anode, the cathode wants a lot of electrons and the anode only has a few electrons. Since there is more demand for electrons than supply, the anode corrodes quickly. If the cathode is small in comparison to the anode, the supply of electrons is greater than demand, resulting in much less corrosion. This helps to explain why galvanic corrosion is a problem in some places but not others.

As the photo below demonstrates, using galvanized screws to secure a copper flashing will cause the screws to corrode quickly. The copper flashing is a large cathode which will quickly steal many electrons from the galvanized fasteners, a small anode. The result will be severe corrosion of the fasteners.

However, installing a stainless steel drip on a galvanized steel angle is not a big problem. The stainless steel flashing shown below is a small cathode and takes only a few electrons from the galvanized angle, a large anode. Corrosion is also mitigated because the gap between the flashing and the angle usually isn’t wet. Setting the flashing in a bed of sealant provides an isolator that also helps to reduce corrosion.

 For more information on sources or causes of corrosion, contact Tom Bane.

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