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Thread: Cathodic Protection

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    Default 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.

    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!

  2. #2
    Junior Member Aquaman's Avatar
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    Default Re: Cathodic Protection

    Quote Originally Posted by SimpleGreen View Post
    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.

    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!
    You only need to attach zinc (anode) bars to the exterior of your coated shelter (cathode). One for each side would suffice.

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    Default Re: Cathodic Protection

    Quote Originally Posted by Aquaman View Post
    You only need to attach zinc (anode) bars to the exterior of your coated shelter (cathode). One for each side would suffice.
    Any insight on the size of anode? wire lead or straps?

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    Senior Member ProfessionalLocator's Avatar
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    Default Re: Cathodic Protection

    Quote Originally Posted by SimpleGreen View Post
    Any insight on the size of anode? wire lead or straps?
    From Wikipedia

    "The design of a galvanic anode cathodic protection system should consider many factors, including the type of structure, the resistivity of the electrolyte (soil or water) it will operate in, the type of coating and the service life.

    The primary calculation is how much anode material will be required to protect the structure for the required time. Too little material may provide protection for a while, but need to be replaced regularly. Too much material would provide protection at an unnecessary cost. The mass in kg is given by equation (5).[13]


    Mass = (Current Required x Design Life x 8760) (Utilisation Factor x Anode Capacity)

    (5)

    The design life is in years (1 year = 8760 hours).
    The utilisation factor (UF) of the anode is a constant value, depending on the shape of the anode and how it is attached, which signifies how much of the anode can be consumed before it ceases to be effective. A value of 0.8 indicates that 80% of the anode can be consumed, before it should be replaced. A long slender stand off anode (installed on legs to keep the anode away from the structure) has a UF value of 0.9, whereas the UF of a short, flush mounted anode is 0.8.[13]
    Anode capacity is an indication of how much material is consumed as current flows over time. The value for zinc in seawater is 780 Ah/kg but aluminium is 2000 Ah/kg,[13] which means that, in theory, aluminium can produce much more current than zinc before being depleted and this is one of the factors to consider when choosing a particular material."

    Zinc should work for you at a reasonable price. These anodes are also called Sacreficial Anodes as they corrode away instead of what they are protecting. Think of an electric arc welder where metal is transferred from the rod to a surface. The flow of electrons cause the anode to shed it's mass into the soil.
    So the longer the design life the larger the anode needed. This can also be done by adding more anodes rather than one or two large anodes.

    To make this work your metal structure must be electrically insulated from the soil surrounding it. So it needs a good protective coating. I worked with anodes a bit years ago on pipe encased electric transmission lines that had been installed in the 1930s'. 45 years later they were holding up fine. They were coated with a thick tar like material, about 3/8" thick coating.

    A metal line was welded to the pipe and the anode was on the other end. We used a weld then called CadWeld which was basically thermite poured into a graphite form that fit over the wire and was ignited with a simple wheel and flint gas welder igniter. The thermite burned and the metal mixed in with it melted and itself to the wire and pipe.

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    Default Re: Cathodic Protection

    IMO have a local engineering company design it for you best 500 bucks you will ever spent

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