Anode use in impressed current systems

[Gorpomon]

Hi,

I have just had to look into impressed current systems. I have seen several schematics, they all show an anode connected to the + terminal on a DC supply, with the material being protected connected to the - terminal.

From what I've read, the anode provides the current used in protection, and one schematic I saw referred to the DC source as a "pump".

My question is why is the anode necessary? Can't the DC source supply enough current?

 

[SJones]

The anode is simply one of the electrodes, the other being the cathode, that forms contact with the electrolyte to conduct current from the current source in the cathodic protection circuit. It is not generating supplementary current.

Steve Jones
Materials & Corrosion Engineer

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

 

[EdStainless]

The anode must be eclectically isolated from the part of hte structure being protected so that the current flows through the electrolyte. The anodes are often very corrosion resistant alloys so that they last a long time. The power supply provides all of the current. There should also be a reference cell so that you can measure the actual potential of the protected material.

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Plymouth Tube

 

[strider6]

Have a look at these docs that explain the CP system.

http://www.bushman.cc/pdf/impressed_current_system_design.pdf

http://www.cpdesigncenter.com/public/technical/cp_impressed.htm

http://www.corrosionist.com/cathodic_protection_impressed_current.htm

S

Corrosion Prevention & Corrosion Control

 

[Gorpomon]

I think what I'm actually not understanding is why the anode has to be used to complete the circuit? Can't you just hook up the leads of the DC source to each end of protected pipe? Wouldn't the pipe become a resistive element essentially? So you'd have a very simple circuit
_______
| |
| |
5V Rpipe
| |
|_______|


Wouldn't this be a better way to have current in the pipe?

 

[SJones]

The corrosion process is occurring at the surface of the pipe in contact with the electrolyte. Therefore, the cathodic protection current has to act at the pipe surface in order to suppress corrosion, i.e. the pipe has to become the negative electrode. That means something has to become the positive electrode in order to maintain neutrality. If leads are simply attached to the pipe, current will not act at the pipe surface and the pipe will not be a negative electrode (cathode).

Steve Jones
Materials & Corrosion Engineer

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

 

[EdStainless]

You are trying to manage the potential between the pipe and the surrounding environment. So your impressed current must flow in this same path.

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Plymouth Tube

 

[Gorpomon]

From my understanding, corrosion is due to two different sites on the metal surface having different potentials, creating an anode and cathode. The soil would be the electrolyte allowing the ions to transfer. I'm not understanding why you're managing the potential between the environment and the pipe, I thought it was managing the potential differences between spots on the surface of the pipe.

While I can accept the explanations that the current must act at the surface of the electrolyte and pipe, I still don't fully see its necessity.

Wouldn't the leads on each end of the pipe still have current acting at the surface, thus eliminating the cathode and anode on the surface of the pipe?

 

[unclesyd]

By controlling an impressed current on the pipe being protected you bring the entire surface of said pipe to the same potential so all differential sites on the pipe surface are essentially eliminated.

Our soils vary quite a bit in conductivity in short distances so we have to backfill the pipe trench with a conductive material to ensure that we have an even potential on the pipe surface.

 

[SJones]

You're getting there slowly. At the corroding anode, current discharges INTO the electrolyte and is collected at the cathode. The aim of CP is to make the protected item a total net cathode by having current flow ONTO it. No anode = no corrosion. If the protected item is totally a cathode, something else has to be an anode in order for the current to flow. Connecting leads onto the pipe will have current flowing THROUGH the pipe and not at the surface; there would still be surface anodes and cathodes and, therefore, corrosion at the interface with the electrolyte which would probably be exacerbated by the resistive heating of the pipe by the current flowing THROUGH it. Try reading the links in more depth.

Steve Jones
Materials & Corrosion Engineer

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

 

[Gorpomon]

I do believe I understand what's going on now, thank you so much! SJones, the INTO/ONTO explanation solidified it for me.