Copper pipe fastened with galvanized plumber’s tape or pipe strap is a well-known no-no because of galvanic reaction.
In the case of copper pipe connected to galvanized pipe the water closes the circuit of a classic galvanic reaction, but how does such a reaction occur with galvanized pipe straps screwed to wood where there is no obvious return path for the current flow?
The Big Beautiful Bill could do away with much of the Inflation Reduction Act, including the 25C, 25D and 45L tax credits.
"I have learned so much thanks to the searchable articles on the FHB website. I can confidently say that I expect to be a life-long subscriber." - M.K.
Dig into cutting-edge approaches and decades of proven solutions with total access to our experts and tradespeople.
Start Free Trial NowGet instant access to the latest developments in green building, research, and reports from the field.
Start Free Trial Now
Replies
Normally it's not a problem. You need two dissimilar metals, water and oxygen.
Good practice dictates using copper or copper coated hangers.
I believe the water is an electrolyte between the dissimilar metals. Water inside a copper pipe makes little difference. There is always water in the air, and cold water will condense a lot of it.
But it does make a difference when a copper pipe screws to a iron pipe!
http://en.wikipedia.org/wiki/Galvanic_corrosion
So, you're telling me that cold water pipes frequently get condensation on them and that's the return electrical path for the reaction? I live in Southern California where it takes sub 40°F surfaces [like a glass of ice water] for condensation to occur. I've never seen condensation on a water pipe.
BruceT
In the case of the copper pipe hung with a galvanized strap, you have a recipie for galvanic corrosion of the hanger- not corrosion of the pipe. Any time the pipe gets damp from condensation, you have an electrolyte (water) and two dissimilar metals in electrical contact with one another, sufficiently far apart from one another on the galvanic series to drive an electrochemical cell (think battery). The current flows from the the corroding zinc to the copper.
You also have another key ingredient- LOTS of oxygen from the air to drive the corrosion by accepting the electrons from the corroding zinc.
When you have potable water flowing through a copper pipe connected to a galvanized pipe, the fresh water flowing through carries in fresh (dissolved) oxygen, and carries away the corrosion products. Since the electrolyte is always there, the problem is worse than the hanger situation.
The zinc-coated hanger would be fine if you electrically insulated it from the pipe, say with a few wraps of PVC electrical tape. No electrical contact means no current path which means no electrochemical cell to eat your hanger. The same is true for the pipe situation if you use a galvanic union- and if the pipes aren't connected electrically to one another via grounding wires in the house etc.
All the references I can find to galvanic corrosion, - for instance, http://corrosion-doctors.org/Forms-galvanic/galvanic-corrosion.htm - list three requirements: dissimilar metals, electrical contact between the two metals, and both metals exposed to an electrolyte solution. I found no references to corrosion occurring in air.-Pure water is non-conductive, since it contains no electrolytes, therefor no reaction occurs.
-Normal tap water contains some dissolved salts to serve as electrolytes; the reaction is relatively slow.
-Sea water is heavily loaded with dissolved salts to serve as electrolytes; rapid corrosion occurs.Condensed water vapor would by definition be pure, so where would those necessary electrolytes come from?
BruceT
Dust and residue on either the pipe or hanger combined with the condensation will form an electrolytic solution between the two metals.
Pure water boiled and condensed in vacuum is neutral. Water condensed out of the atmosphere contains dissolved carbon dioxide- carbonic acid. Plenty conductive enough to act as an electrolyte- and a good general corrodent.
The more intermittently the electrolyte is present, the slower the corrosion will be. Never get condensate? Then you'll never get GALVANIC corrosion. You will still get atmospheric general corrosion, which occurs due to reactions not involving the 2nd metal.
The relative areas of the two metals matter also. In this particular situation, the copper pipe's area is large and the zinc-plated pipe hanger's area is small. The zinc is the anode, and the cathode-to-anode area ratio is large: this means there's a lot of cathode around, whose corrosion is being inhibited by preferential corrosion of the anode. Steel nails in a copper plate corrode much quicker than the steel around copper rivets in a steel plate, even though the anode/cathode metals are the same in both cases.
Remember the zinc-plated part uses galvanic corrosion to its advantage: it's steel, galvanically protected by a coating of an anodic metal (zinc). But both steel and zinc are anodic relative to copper.
As to the current flows: the electrons flow from the zinc to the copper as they're liberated by the corroding zinc. The positive ions flow in the electrolyte. No different than a battery with the terminals shorted together.
The chemistry ... and the current ... is between the two metals. Any time two different metals touch, the molecules themselves provide the electricity. It matter not whether there is an electrical connection with the ground, or anything else.
This is exactly how a battery works.
I've seen thousands of non-copper hangers used with copper pipe with no VISIBLE evidence of damage.
I don't know if there is anything going on elsewhere
Remember Mary Dyer, a Christian Martyr (Thank you, Puritans)
http://en.wikipedia.org/wiki/Mary_Dyer
May your whole life become a response to the truth that you've always been loved, you are loved and you always will be loved" Rob Bell, Nooma, "Bullhorn"