Thank you for your question regarding steel pile grounding and galvanic corrosion. It is our pleasure to help.
Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially to another when both metals are in electrical contact and immersed in an electrolyte. Dissimilar metals and alloys have different electrode potentials and when two or more come into contact in an electrolyte, a galvanic coupling is set up. Scientists have indexed materials based on the nobilities of the metals. This index is called the “Anodic Index” and provides a Voltage for each material. A voltage differential greater than 0.15 Volts is considered corrosive. Copper has a Anodic Index of 0.35 and steel is 0.85. This is a 0.50 volt differential, and can certainly be a source of galvanic corrosion as the earth will surely act as an electrolyte. But there are other concerns in regards to corrosion beyond simple nobility issues, and that is induced AC currents.
Galvanic corrosion in steel objects is generally enhanced by the addition of harmful Alternating-Current (AC) voltages. In the case of an electrical substation, the electromagnetic fields are far greater and thus will induce greater voltages into any steel object nearby. These induced AC voltages can’t be avoided in a substation, and are generally more corrosive than the DC electrochemical voltages present during normal galvanic corrosion. What we can do is provide an effective means for the electric fields to dissipate in to the earth. A properly grounded steel object in a high-voltage environment should suffer from less galvanic corrosion than an unbounded steel object, as the copper ground grid will provide an alternative and better path to earth for the corrosive Alternating-Currents (AC).
Please remember that it is quite common to see all sorts of steel objects bonded to the copper ground grid in a substation. The steel transformers, the steel fence, and steel uprights, just to name a few. Exothermic welding is the corrosion control choice when bonding two dissimilar metals. The exothermic welding process provides a permanent molecular bond, which is inherently corrosion resistant (but may not address the long term galvanic corrosion issues). Most manufacturer’s of exothermic welding products guarantee the steel-to-copper bond to be corrosion free.
That said, you really don’t have a choice: regardless of any galvanic corrosion concerns, mandatory Human Safety laws require you to bond the steel pilings into the ground grid. You may NOT isolate any metallic object in a high-voltage environment. Please refer to Title 29 Part 1910.269 of the Code of Federal Regulations (29 CFR 1910.269).
Isolated steel pilings in a high-voltage environment could generate significant differences in potential that could have serious Human Safety implications. Not only could these differences in potential increase arc-flash hazards, but they could generate life-threatening Step & Touch Voltages. These isolated steel piles could also be the source of significant electrical interference caused by the differing Electromagnetic Fields (EMF).
Another advantage of grounding your steel columns is that Direct-Current (DC) rectifiers can be used to inject protective DC currents onto the steel objects via the copper grounding system. This is called “Impressed Cathodic Protection” and uses DC currents to counteract the harmful AC currents. You should contact a corrosion specialist for more information regarding DC rectifiers and corrosion protection.
We have an excellent blog regarding deep earth pilings and grounding that may answer a few more questions for you. Please see our blog:
In conclusion, you must bond the steel pilings to the ground grid, and corrosion will not be a problem with proper exothermic welds.
E&S Grounding Solutions is of course more than happy to help you out with proper ground grid design and analysis. But if not us, please get someone with the proper test equipment, computer systems, and expertise to properly analyze your substation. Bad grounding can result in life-threatening situations. Please feel free to contact our Engineering Department and one of our engineers will be happy to speak with you about your project, free of charge.
The Engineering Team at E&S Grounding Solutions
Photo Credit: E&S Grounding Solutions