Earth Grid Corrosion Issues

[acog]

Hi,

It is typical to use copper for an earthing conductor and also to bond it to the reinforcing steel of a large concrete structure such as a power transformer foundation.

Some consultants have informed me that a passivating film will form over the reinforcing steel as a result of the concrete which will drive the electro potential of the steel up. Apparently the copper conductor and the steel will then be a galvanic cell where significant farradic loss will occur and the copper conductor we are using e.g. 70mm2 Cu Strand can corrode completely within 7 years.

I have never heard of earth grid loss due to bonds to steel reinforcement. Can anyone comment on whether they have seen this happen or suggest some studies I could look at?

I don't know how to respond to the consultants currently.

Thanks in advance.

 

[MKFPE]

HELLO,

The consultant's statement is illogical:
From dictionary.com: pas·si·vate [pas-uh-veyt] verb (used with object), pas·si·vat·ed, pas·si·vat·ing. Metallurgy . to treat (a metal) to render the surface less reactive chemically
A synonym for "passive" is "inert".
So a passivating film on the steel would create an insulating layer on the surface of the steel, decoupling the corrosion cell, slowing or stopping the corrosion rather than accelerating it.
I was a consultant for about 15 years before I took this municipal job and entered the real world where I actually have to live in the town where whether or not the power is reliable depends on ..... me.
I really hate to say it, but in the last 15 years I have learned not to trust a lot of the consultants, there are some good ones but you have to look at the practical experience element, not just the book learning part, which is why I am really beginning to like this listserv, there seem to be some truly knowledgeable people here.
If the consultant makes such a statement you need to turn the table and ask them to provide specific examples of where they have seen this corrosion occur.
It is possible, and likely depends on local soil conditions (pH?), but I have never seen it personally.
For example, a couple of years ago we dug up some copper grounding conductions in a substation originally installed in 1954 to extend the grounding to new equipment being added to the station. Underground connections were made with split-bolt connectors which are not acceptable today. There was absolutely no evidence of any corrosion damage to the copper, the split bolts could be un-screwed easily.
Also, our line department drives a copper clad ground rod and bonds is to each galvanized steel helical screw anchor guy on our transmission system, (actually by measurement I have found that the helical screw anchor is a better grounding rod than the grounding rod) so we have these potential corrosion cells all over our system and we have never found any corrosion issue with these either.
It might help to know your general location and what is known about soil conditions in your area, and whether the consultant is basing his statement on observed corrosion in the same geographical area.
Regards,

 

[Kiribanda]

acog,
To my knowledge Copper is a nobel material than steel (pd=0.7 V in electrochemical series).Then how copper gets corroded and not steel?

 

[jghrist]

The National Electrical Code (NEC) requires reinforced concrete foundations that are in contact with the soil to be used as a grounding electrode. Foundations of existing buildings are excepted from the rule. If there was a corrosion potential, the steel would be the sacrificial anode, not the copper because it is "nobler" as indicated by Kiribanda.

The NEC is not a requirement for utilities. IEEE Std 80, Guide for Safety in AC 2 Substation Grounding, is, and it gives some advantages and disadvantages.

 

[rcw retired EE]

The Green Book, IEEE Standard 142-2007 "Grounding of Industrial and Commercial Power Systems" in section 4.4.5 "Electrical Grounding and Corrosion" says that, "It should be noted that steel rebar, when encased in concrete, has approximately the same potential as copper and thus will not corrode."

Copper has a potential of -0.34, iron is 0.04, zinc 0.76 and aluminum 1.70. The hmore positive potential metal in the electromotive series will be the anode, the one that corrodes. The only common metals with lower potentials than copper are silver, mercury and gold. That means unless you have buried silver or gold plated steel youer copper ground grid is not corroding due to galvanic action.

If your grids are corroding I'd like to come and steal your gold-plated buried metal!

Note that an impressed current cathodic protection system could, in some cases apply enough potential to corrode the copper. Also, copper grounds can be affected by chemical corrosion from acids or chemicals in the soil or air. That is different than electro-chemical corrosion.

 

[dpc]

As others have noted, the corrosion issue, if there is one, would probably be the steel, not the copper. The corrosion engineers I have worked with in the past have all concurred that connection of copper grid to the steel rebar in reinforced concrete should not create a corrosion issue as rcwilson says. Connection to steel directly in contact with soil, such as driven piles could be a concern, depending on the soil. I think if you have directly embedded steel, you may want to consult with a qualified corrosion specialist.

 

[acog]

Hi All,

Many thanks for your comments. I have brought similar points to the attention of our consultant and their response was along the lines of the following:


As for the steel potentials, the range is from extremely cathodic to extremely anodic. Copper can be considered relatively stable in soils giving it a potential of around -300mV with respect to CuCuSO4 and therefore if the steel is more positive than this value, the copper will corrode, anything more negative than this value, the steel will corrode.

The attached chart taken from a paper published in the Internal Journal of Electrochemical Science titled ‘Cathodic Protection of Steel in Concrete Using Conductive Polymer Overlays’ gives information on the potential variation of steel in concrete over a period of time.

The chart shows a variation of approximately -90mV to -225mV which tends to track with the measured resistivity of the concrete. However, we have measured potentials of +100mV in extremely well drained or atmospheric concrete. While this may be the case, if the corrosion of the copper is based purely on the +100mV driving potential, there are concerns with regards to complete section loss of the earthing conductor.

As stated in the original report the low surface area of the copper with regards to the high surface area of the steel in concrete is a cause for concern if the copper is anodic to the steel as the corrosion current density would be high. However the possibility of the steel being anodic to the copper can also occur if the concrete becomes saturated, reducing the amount of oxygen available to maintain the passive layer on the steel reinforcing.

As the 0.4V driving voltage (Cu is -0.3V, Steel is +0.1 as above) in the corrosion cell is at the extreme high end of possibility of the copper corroding, the earthing designer needs to be aware of the factors that lead to the corrosion of the copper and design accordingly either by modifying material used, the circuit resistance in relation to DC current or the conductor size.

We have measured the soil pH and this will not be a cause for concern at this site, we are mainly focused on the above problem.

Another side note is that this site has low soil resistivity; the consultant is of the view that this will heavily influence our rate of corrosion.

The next step for us is to go to a previous project and test the potential of the steel with a special copper sulphate anode to see what we are actually getting at one of these foundations. The application is for wind turbine foundation earthing.

If anyone can offer a succinct intelligent comment to close this out so we don't have to go to the expense of tests and trials and experience the project delays associated with unnecessary research, I will be heavily in your debt.


Thanks