When Metals Collide

When Metals Collide Code Applications July 2020
Published On
Jul 15, 2020

Cathodic action and dissimilar metal interaction are phenomena with a long history of recognition in the laws of physics as well as within the National Electrical Code. I first ran across this issue back in the 1970s and ‘80s when I would get phone calls about the destruction of steel water pipes connected to copper water lines; the calls reported would get the steel water pipes were deteriorating. I traced these failures to the direct connection of steel piping to copper water piping systems.

The dissimilar metal corrosion was an electrochemical process that causes destruction such as a reduction of the thickness and strength of one of the metals. I also saw installations where metal, underground, natural-gas-piping systems developed leaks due to direct connections to above-ground metal gas piping in buildings.

For example, one installation involved a large resort in Sedona that was being sold. I was evaluating the electrical and mechanical systems at the resort for the potential new owners. The natural gas utility company delivered gas to a distribution building. The gas-piping system from the gas distribution building to each of the 10 resort buildings was installed underground and was owned by the resort.

Walking the resort grounds, I noticed a strong smell of natural gas (actually, the odorizer added to the natural gas) and noticed what looked like small ant hills in a straight line on the earth. I looked closer and determined these small hills were eruptions of natural gas leaking from the underground metal gas-piping system. After following these gas lines to the individual buildings, I determined that the metal gas lines in the building were directly connected to the underground metal gas piping without a dielectric union separation. The entire underground gas-piping system required replacement. Fortunately, no one was smoking in the areas of the leaks or the whole place might have looked like Yellowstone National Park with all the eruptions.

In the case of the water piping, the metal that was being destroyed was the steel water pipe, which was connected to the copper water pipe, causing water discoloration and steel-pipe deterioration. Wikipedia defines bimetallic or galvanic corrosion: “Dissimilar metals and alloys have different electrode potentials, and when two or more come into contact in an electrolyte, one metal acts as an anode (the steel pipe) and the other as a cathode (the copper pipe). The electro-potential difference between the reactions at the two electrodes is the driving force for an accelerated attack on the anode metal (the steel pipe), which dissolves into the electrolyte. This leads to the metal at the anode (the steel pipe) corroding more quickly than it otherwise would and corrosion at the cathode being inhibited. The presence of an electrolyte and an electrical conducting path between the metals is essential for galvanic corrosion to occur.”

As the steel-pipe water system deteriorates, this metal collects on the inside of the copper pipe. Where the underground gas-piping system was deteriorating, the metal from the steel-piping system was migrating to the earth so the gas pipe was the anode and the earth was the cathode.

The answer to these phenomena is to provide a dielectric union as a separator between the copper- and the steel-piping system or to provide a cathodic-protection system. This isolation or cathodic-protection system reduces or eliminates the deterioration. The dielectric union is a term used to describe a two-part fitting of dissimilar metals (copper on one side and iron fitting on the other side with an insulator between the two metals), which are electrically isolated from each other to prevent galvanic corrosion.

Anywhere there is a difference of potential with direct current, the flow of metal from the anode to the cathode can be stopped using a dielectric union or by installing a cathodic-protection system. The flow of DC current that can cause deterioration of these metals can occur from a grounding electrode system connection, from water running through a water-piping system, and many other causes. If the difference in voltage between the two points can be eliminated, then there will be no DC current and no metal piping deterioration.

Section 250.6(E) of the NEC states: “Where isolation of objectionable DC ground currents from cathodic protection current is required, a listed AC coupling/DC isolating device shall be permitted in the equipment grounding conductor path to provide an effective return path for AC ground-fault current while blocking DC current.”

This discussion will continue next month with a review of how stainless steel is affected by these issues.

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and Mark.C.Ode@ul.com.

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