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Cathodic Protection

Y2KEDDIE

Sep 23, 2012
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I am some what familar with Cathodic protection as applied to metal buildings, ship hulls, etc.
Typically the structure we are trying to protect is considered the cathode, and an external anode material is connected by wire to the structure. The anode is sacrified (deteriorates) as current (Conventional) flows from the anode to the cathode. Typically othe structure should be more neg, beyond -.85V. If this can't be achieved with supplied carbon anodes a power supply is put in series to aide in process. Magnessium anodes normally don't require extrnal power supplies.

My question: in a boiler or hot water tank. the Anode is usually inside the tank as opposed to outside. Is it as simple as : one case we are protecting the outside of a tank and the other , the inside? Why does the anode need to be imersed inside as opposed to putting an anode on the outside?
 

mofy

Dec 19, 2014
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I am guessing, because the water/metal is the electrically active corrosion, so you need the anode/water to make the electrically active anode sacrifice. If the anode is outside you wont complete the circuit, electrically.
 

Y2KEDDIE

Sep 23, 2012
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Yes, I can see that. But what would be the difference whether protecting the inside or the outside (common wall) tank. We could attach to the outside to an anode . If we are replenishing electrons to the metal ; isn't the same metal on the inside as the outside? (just that the corrosion is occuring on the inside).
 

mofy

Dec 19, 2014
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Yes, but the water must carry the return current. A current loop has a to and from, it would be like attaching only one battery terminal and expecting a current to flow.Impossible! You need both connections, and that is what the immersion in the water provides.
 

Y2KEDDIE

Sep 23, 2012
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I agree. Inside the tank an anode provides current through the electrolyte to the inner lining. On the outside an anode in the ground provides current through the soil (electrolyte) to the outside of the tank. This is the best I can explain it:; hopefully I'm right.

I wish I could inderstand totally. I believe it's something similar to ion flow within a battery cell. Some how/somewhere the electrons flowing turn into Ions moving or changing charge.

I was reading an article how ships protect their hulls with attached anodes: and how the current flow from the anode to cathode varies with the velocity of the water it's submerged in. The article said while there is cathodic protection on the outside of the hull, there also is a sysem within the hull preventing rust fom the inside. Awsome stuff!

I was trying to relate all this to my friend's boiler system. He has an anode inside a glass lined tank. I believe this protects the inside fittngs (hot water heat exchanger) from corrossion. He had a fitting corode and break. He had to replace the tank and anode last year. I rigged up a milliamp meter from the anode to tank and we can see the Milliamps vary as the water circulates through the tank. After things settle down the Millivolt settles down as well. The anode from the boiler manufactuer is $700. We were hoping we could connect a magnessium anode buried outside the house to solve the delema but I now believe we would be only protecting the outside of the tank.
 

mofy

Dec 19, 2014
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Yes I think you are correct. I am only basing my statements on how batteries and circuits work so might be worthwhile double checking.
 
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