Galvanic Corrosion of Neutrals

Oh greenest of greens,

My copper neutrals look a lot like you – green! I was wondering if our practice of bonding neutrals to the rebar in our vault structures might lead to neutral corrosion.

Califorrosion

Dear Califorrosion-

Yes, copper carbonate does have similar hue to me ... a touch bluer though. And like me, 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, 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”). 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 query that would be the steel rebar.

2. One of the two metals must be cathodic (inclined to be more negative). In your query 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 there has to be oxygen present. This last assumption is usually true unless the area is an anaerobic swamp.

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

My colleagues will be presenting a webinar on the subject: “Neutral Corrosion: Causes, identification & Mitigation” on September 21, 2012. To register for this learning opportunity navigate to www.novinium.com/events.aspx.

Green wishes,

Thermo

Methanol

Dear green frog,

In Section 16 of the material safety data sheet (MSDS) for Perficio™ 011 fluid it states that methanol is listed as a Hazardous Air Pollutant (HAP). Given your green credentials it is hard to believe that you would create any significant air pollution.

Wondering near Willamette  

Dear Wondering-

The most important word in your question is “significant!” Heck, according to the EPA I can’t breathe without emitting life threatening CO₂. I’m glad I’m not a coal-fired power plant, or I would be an endangered species. Let’s start by putting methanol in context. When you press the windshield washer button on your car the colored fluid that comes out is about 30% to 50% methanol – all of that evaporates into the atmosphere. Commercially significant rejuvenation fluids include one or more methoxysilanes. When methoxysilanes react with water in the cable they create methanol. The three most common methoxysilanes utilized over the past twenty-five years and the amount of methanol produced by 100 grams of rejuvenation fluid are shown in the table nearby.

Unlike windshield washer fluid, the methanol is not released all at once, but rather the chemical reaction that creates the methanol occurs over a period of months inside the cable. If the cable is buried in soil or is submerged in a river, a lake, or the ocean, very little of the methanol produced will make it into the atmosphere. Because the methanol is diluted and dispersed in water, it is readily metabolized by microbial activity.

Let’s calculate the methanol generated per meter of cable. A larger conductor size, such as a 750 kcmil concentric conductor can hold about 96 grams of fluid per meter of cable. One formulation that might be utilized is Perficio and in this example, less than half of the 96 grams (about 45 grams) would be methoxysilane.¹ The methanol yield per meter of cable would be about 35.15% of 45 grams, or about 16 grams of methanol per meter of cable. That’s much less methanol than a single squirt of your windshield washer. And most of it will be consumed by microorganisms.

The other side of the rejuvenation coin is replacement, which has a much larger environmental footprint. Check out my March 17, 2011 post for some quantification of the environmental benefits of recycling medium voltage power cables.

It’s easy to be green,

T. Frog

¹See Step 8 of Novinium Rejuvenation Instruction 20, NRI20.

For authority on the above check out the declaration: 20-20120803_GJB_Declaration.pdf (74.28 kb)