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Neutral Corrosion Progression

May 10, 2011

Q:

Can you please explain the rate of neutral corrosion? Specifically, do neutrals corrode linearly or in some other fashion?

Also, an engineer asked me about stray voltage. We were discussing how Novinium will provide a warranty for a cable segment if 50 percent of the neutrals are good. He said, “Okay, but because of the neutral corrosion, I am having stray voltage issues affecting livestock on dairy farms. This can kill livestock. Stray voltage is also showing up sometimes on communications circuits.” How should I respond?

A:

There are several different mechanisms for cable neutral corrosion. The mechanisms are enumerated in Section 6 of IEEE 1617-2007 “Guide for Detection, Mitigation, and Control of Concentric Neutral Corrosion in Medium-Voltage Underground Cables.” (Novinium’s Glen Bertini  was one of the participants in the ICC C7 working group that developed that document.) Section 6 identifies the following mechanisms of cable neutral corrosion:

1.   Galvanic corrosion

2.   Single metal corrosion

3.   Soil corrosion

4.   Differential aeration

5.   Stray currents

6.   Galvanic corrosion resulting specifically from tin-coated neutral wires

CorrosionOf these possible causes of neutral corrosion, soil corrosion and differential aeration are by far the most predominant and problematic. In both cases, differences in soil chemistry along the path of the cable lead to differences in potential on exposed neutrals. Current flows in a loop through the soil and along the neutral. Copper is oxidized where the electrons leave the neutral and enter the soil and where there is oxygen (or sulfur) present. The rate of corrosion is a function of the current flow and is constrained by the availability of oxygen. Current flow, in turn, is proportional to the potential difference caused by the local differences in soil chemistry and inversely proportional to the resistance of the loop. The illustration shows how this all works. Either from differential aeration or differences in the local soil chemistry, the electrochemical potential is higher at point A than it is at point B.

As the neutral corrodes, the resistance in the loop goes up, which slows the rate of corrosion. The loss of the metallic copper itself leads to an increase of resistance. Less obviously, the non-conductive corrosion by-products (i.e., copper oxides) coat the copper surface and increase the resistance between the neutral and soil. Another set of chemical processes determine how quickly the copper oxides are transported off of the native copper surface below them. This copper-oxide transport mechanism is typically very slow in direct-buried environments as the oxides are not appreciably water soluble.

At the same time the resistance is increasing, the second law of thermodynamics is at play, reducing the chemical potential difference between points A and B. The homogenization of chemical potential over time would occur whether a cable was present or not. Nature abhors chemical potential differences, so chemical species migrate through the soil toward equilibrium—that is, zero chemical potential difference.

Corrosion of bare concentric neutrals is highest when the neutrals are new and the soil was disturbed when the cable was installed. As some corrosion occurs and the second law reduces the chemical potential, the rate of corrosion decreases over time. In practice, if the neutral has at least partially survived for several decades, the rate of continuing degradation is trivial.

The other causes of neutral corrosion are much less prevalent. With the possible exception of stray currents impressed upon neutrals by active cathodic protection systems of neighboring structures (e.g., gas pipelines), all other causes are similarly mitigated by the partial corrosion of the neutral and the equilibration of chemical potential due to the inexorable second law of thermodynamics.

If cows or other livestock are getting electrocuted, the neutrals are entirely destroyed either locally or systematically. If the corrosion is systematic, the cables must be replaced. The source of the systematic corrosion should also be identified and eliminated—it’s not a natural phenomenon; it’s man-made.

In the real world, most concentric neutral corrosion is incredibly local. A foot or two of neutral become corroded. It turns out this problem is easy to diagnose and easy to repair. Diagnostic techniques are described in the aforementioned IEEE 1617-2007. A step-by-step and state-of-the-art procedure is available in TDR Diagnosis and RF Locating. The publication Neutral Corrosion Repair makes fixing local corrosion a piece of cake. Once a local corrosion site has been pinpointed, chemistry can be employed to protect the location of the identified chemical potential difference. A suitably sized magnesium anode that has a chemical potential well above that of copper is installed as a sacrificial anode. The anode size can be adjusted to prevent neutral corrosion for the desired life span.

In over 20 years of rejuvenation experience, there have been few warranty claims in general and even fewer neutral corrosion issues specifically. This provides direct evidence that any postinjection progression of neutral corrosion that does occur is of little practical significance. At Novinium, we have had no failures and no warranty claims that involved an increase of neutral corrosion after the cable was treated.