Copper-aluminum Connections

I am a lineman for the power company and constantly making electrical connections between copper and aluminum. There is a theory at my job that aluminum must go on top of copper. Is this true? And if so why? What is the difference between being above or below. I looked up galvanic corrosion and understand the process in which the anode ( copper I believe) breaks down to the cathode ( aluminum) with a presence of an electrolyte. But what I am unable to find is the reason for them having to be in a certain order. Thank you very much for your help I look forward to reading your response.
- Dan (age 21)
Guilderland NY

By "on top" I'm assuming you mean actually higher up, not farther from some support structure.  I can't think of any simple reason why that would matter, since the gravitational forces involved are very weak compared to the local electrical and chemical effects. I can think of one reason why it might conceivably matter. More water will accumulate on the lower piece. Maybe for some reason having more aluminum wet is worse than having more copper wet. Do your colleagues have any account of why it should matter?


Although that wasn't much help, perhaps having this posted will draw some more knowledgeable comments from other readers.

Mike W.

(published on 10/31/2011)

They say that the connection breaks down significanty faster if the copper is ok top. example on a wye system there is a neutral wire running from pole to pole for which we use aluminum. When making a ground attachment we have copper. So the tail of the copper wire can either be bonded above or below the aluminum neutral. I cannot research any hard evidence as to why is matters .
- Dan (age 21)

Whoops- In the first answer, I had the two metals switched- fixed now. 

Any electrochemists out there who can help?

Mike W.

(published on 11/02/2011)

The general subject matter of the question on "copper/aluminum corrosion" is explained in the BURNDY publication, "Connector Theory and Application." The following discussion addresses the specific question asked.The so-called ABC rule - Aluminum 'Bove Copper (which can also be Anode 'Bove Cathode, but no one remembers which is which) was created as a rule of thumb referring specifically to the physical location of dissimilar metals in outdoor locations (e.g. overhead distribution lines). For this discussion, keep in mind aluminum is very anodic to copper, and to some degree all other metals commonly used in and around our power delivery system. In an outdoor application, rain drops will pick up ionic salts from a cathodic material. If the cathodic material is physically located above an anodic material, the miniature cathodes contained in the droplets will then drip down onto the anode. Each droplet contains all of the necessary components to create small galvanic cells (or electrolytic cells), which are very corrosive to the anode. Thus, the cathodic droplets will "consume" the anode over time as the anodic material corrodes away.To put it in terms of the connection described in the question, with the copper wire located on top, rain will wash copper salts onto the aluminum, corroding the aluminum connector very quickly. Even with an oxide inhibitor, loss of aluminum material to constant galvanic attack will degrade the connection's clamping force and contact points. The result is most often a premature burn down, though in applications under tension a physical break in the weakened aluminum material may also occur. When the aluminum is placed on top, the galvanic cell is switched around and both materials will last MUCH longer.In actuality, the connection will still corrode as it is difficult to keep water from settling at the aluminum-copper interface. Of course, the corrosion rate depends on exposure to, frequency of, and even temperature of the wet conditions. This is why it is recommended that like materials (i.e. Cu-to-Cu, Al-to-Al) be used whenever possible. However, in applications where it is absolutely unavoidable to join the dissimilar metals (and those cases do occur), the ABC rule works to add greater longevity to the life of the connection. Gary Di TroiaIMPLO Project ManagerBURNDY LLC
- Gary Di Troia
Atlanta, GA USA

Many thanks for this excellent description!

Mike W.

(published on 04/27/2015)