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Old 09-24-2013, 06:28 PM   #1
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Cathodic Protection

Afternoon all, i was referred to your forum by a friend of mine who is a member. I was discussing a question i had about anodes/cathodes with him.

The gist of it goes like this. I live in an area of the country that is prone to tornados and my wife and I are having a custom shelter build with steel (6'W x 6'H x 12'L). I started doing some reading and trying to understand that in combination with the water/rust proof coating that will be going on what other protections can i add to ensure that i get the most longevity out of a steel structure that is buried in the ground.

Hence came to understand anodes/cathodes and the relationship they play in boats and ships.

My questions are around what is the difference between a cathode and an anode, how are they used in conjunction with each other and ultimately what is the best way to protect this investment that will have very valuable things in it (im hoping for 20 year protection in combination with the coatings plus the anode)

Anode vs Cathode?
I understand an anode as a component that is made from a less noble metal that will consume from another source negative charge and thus be the recipient of the corrosion. Where a cathode is a more noble metal that is wanting to consume positive charge.

Question when considering strapping one or the other or both to my structure, do i need to use them in combination of each other? Example, to keep rust from forming on an area that just so happens to lose its protective coating do i need to have a cathode giving off a positive charge AND a anode accepting that charge in order to keep the steel from being corroded?

Just an Anode?
If the above doesn't apply, my understanding is that im looking at just strapping a few anodes to my shelter.

If that is the case, how big of an anode, how many, what material (zinc/magnesium/aluminum) and does it matter if it is the type with metal strap leads or the type that has a lead wire cast in the anode in which case you would weld the lead wire to the device.

I assume there is a formula that dictates how many for how big of an object and the length of time you want the protection for.

Type of Soil
I did some research on the type of soil that is in my area and this is what I've come up with:
Blackland Soil

The blackland region of Texas is in a narrow band that runs north to south through the east central part of the state. The soil is thick black clay and alkaline. Under the soil is a layer of limestone that can be as close to the surface as a few inches or several feet deep. The area was once a fertile grassland but the native prairie was replaced with farming and development. Blackland soil is still considered some of the most fertile soil in the state of Texas because of its high calcium content.


Being that the soil is alkaline (high PH) does that mean it is closer to salt water do i go with a zinc/aluminum anode vs a magnesium?


REALLY appreciate the time and insight everyone!
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Old 09-24-2013, 06:44 PM   #2
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Typically in California construction we do everything possible to keep steel out of contact with soil. The closest they typically get to each other is 2" of separation when covered in concrete.

Perhaps another member lives in an area where structural steel is direct buried in soil and can assist you. Here is a copy of the chart often used to determine the best way to protect your boat in water.
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Old 09-24-2013, 09:12 PM   #3
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If it will be buried in the earth I would do what the old gas stations did with steel tanks and just coat it in tar and perhaps a membrane before burying. To corrode you need water and oxygen (air). Tar should eliminate both. Not sure how you intend to finish the inside but it would also need protection.
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Old 09-25-2013, 08:54 PM   #4
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Quote:
Originally Posted by LaBomba View Post
If it will be buried in the earth I would do what the old gas stations did with steel tanks and just coat it in tar and perhaps a membrane before burying. To corrode you need water and oxygen (air). Tar should eliminate both. Not sure how you intend to finish the inside but it would also need protection.
thats exactly what we're doing minus the membrane, that'd be hard to construct for a custom structure i think. Im thinking all else fails sorta deal.
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