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Galvanic Corrosion of Neutrals

August 16, 2012

Galvanic Corrosion of Neutrals

Q:

My copper neutrals look green. Could our practice of bonding neutrals to the rebar in our vault structures lead to neutral corrosion?

 

A:

Yes, copper carbonate does have a bluish-green hue, but copper carbonate is your friend. The copper carbonate patina protects the native copper underneath from corrosion. It’s also true that bonding two metals can cause corrosion under the right circumstances. In fact, this type of corrosion, called galvanic corrosion, is the first mechanism mentioned in IEEE 1617™ (Paragraph 6.1 of “IEEE Guide for Detection, Mitigation, and Control of Concentric Neutral Corrosion in Medium-Voltage Underground Cables”).

IEEE 1617

For galvanic corrosion to occur, five conditions are required.

1.  One of the two metals must be more anodic (inclined to be less negative). In your case, that would be the steel rebar.

2.  One of the two metals must be cathodic (inclined to be more negative). In your case, that would be the copper neutral.

3.  There must be a metallic connection. That would be the bonding hardware together with the neutrals and rebar.

4.  There must be an environment for ions to flow. Wet soil provides such an environment. Wet concrete works, too, but not nearly as well.

5.  Finally, oxygen must be present. This last assumption is usually true unless the area is an anaerobic swamp.

The bottom line in the answer to your question is that the steel rebar is the anode and the copper is the cathode. Any galvanic corrosion that occurs will occur to the detriment of the more anodic rebar. The rate of galvanic corrosion on the rebar will likely be quite slow, however, because concrete, even submerged concrete, does not allow for the rapid transport of ions.