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Galvanic Couple Area

Galvanic Couple Area

(OP)
Galvanic corrosion can be mitigated by having an anode with a large area relative to the cathode. My question: Are the relative areas only the areas in contact with the electrolyte, or are they the total areas? This does not seem to be addressed in the references I have. Thanks!

RE: Galvanic Couple Area

That answer depends on the conductivity of the electrolyte. If the electrolyte is REALLY conductive, i.e. saltwater, the total areas are important. In less conductive electrolytes, you might get less protection from a large anode- but that's just speculation on my part. But yes, only the portions of the cell in contact with the electrolyte actually matter. The portion of the anode or cathode sitting in air doesn't matter at all.

RE: Galvanic Couple Area

(OP)
Thanks! These posts have been excellent and directly addressed the issues I am working on. I especially appreciated the paper metengr connected (I always wonder how you have just the right references at your fingertips...).

RE: Galvanic Couple Area

Quote (mrfailure)

I always wonder how you have just the right references at your fingertips

It's simple: maintain your own library, or do a search to find what has been posted before

Now that you have downloaded the document, what will you do with it? Will you file it away so that you too can advise someone on here to look at it when they have a related question?

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

RE: Galvanic Couple Area

That paper gave me the best clue for Air-conditioning condensing units in coastal areas.

Coastal areas are brutal on AC condensing units. Typically construction includes copper tubes, aluminum coils, steel cases, electric motor driven compressors and electric controls. Its all electrically connected and grounded. Seems a great place to apply cathodic protection, but all I find is a "Corrosion Grenade" or a zinc ball take-off of the typical outboard motor anode, which I can't understand will complete the required DC circuit.

No electrolyte means no corrosion, but in coastal areas you have the salt spray covering everything then the rain wets everything. Appears the electrolyte is not constant but frequent enough to get the job done. I'm sure this is not the only mechanism, but seems reasonable.

From what I encountered:

  • Typical coastal soil is sand, one of the least conductive soils, which requires the most negative cathodic protection
  • Magnesium alloys are the most electrically negative of anodes and more pure means more negative
  • All cathodic protection requires grounding
Since coastal AC condensing units suffer from corrosion, galvanic reactions appear obvious and passive cathodic protection appears applicable. So, why can't I find applications or examples? As an engineer, I understand the desire for precision and this problem has little, but often enough hitting the side of the barn does the job. Or in other words, start it up and see if it works.

So I bought a block of 99% pure magnesium online of ten bucks, screwed on #12 copper ground wire, sealed the connection, buried the block and wrapped the other end of the ground to the copper refrigerant pipe. The block is in the ground and the condensing unit ground wire is connected to the house's ground rod for a complete circuit. The original salt spray and wind culprits should now finish the cathodic protection circuit.

Anybody see an issue?

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