Protection of Internal Surfaces with CP

[zdas04]

I've been looking hard at Oxygen corrosion for the last few weeks and I'm beginning to understand that internal Oxygen corrosion is a cathodic reaction just like the external reactions that people normally use CP to protect.

What I've been unable to determine whether an impressed-current CP system will protect against internal cathodic reactions like oxygen corrosion or MIC. Does anyone have an opinion?

David

 

[SJones]

It's a cathodically driven reaction (it's still the anode that is dissolving). Internal ICCP is used in seawater systems for instance. What about inside tanks - NACE has a standard for it? Studies have shown that the electrochemical conditions established by application of CP can interfere, in a beneficial way, with the MIC process.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[unclesyd]

Cathodic protection is used for tank internals both for corrosion and MIC.

We tried this approach early on in the MIC wars to mitigate corrosion to SS heat exchangers. Our only measurable response came with using Lead anodes. Our particular flora/fauna was extremely sensitive to the Pb ion. This project did help with the MIC but was quashed by the environmental people.

We tried sacrificial anodes and impressed current in a large waste water storage tank. The project didn't pan out as our product composition never got to a steady state.

http://www.npl.co.uk/lmm/docs/cathodic_protection.pdf#search="cathodic protection in tanks"

 

[mshimko]

CP WILL help, IF you can "project current" to the surfaces you want to protect.

On painted surfaces, such as underwater hulls and most tanks, the steel is painted, and CP is only needed to protect steel exposed by failed pait.

In most cases the internal surfaces of piping systems is unpainted. The "CP current" will tend to "short out", and its difficult to "project" current any distance down small diameter piping and tubing. In the cases of heat exchangers, its unlikely CP provides any benefit past a few inches of the tubes (of course, its often the first inch or two that is most likely to fail, also).

 

[zdas04]

Thanks for that answer. I know that the magnitude of current impressed on externally-coated pipe is pretty small, do you have any feel for what kind of protection might be provided on the interior surface of smallish pipe (say 4-inch through 10-inch nominal) with this minimal current?

David

 

[mshimko]

Easy first cut answer (VERY CRUDE):

- what matters isn't the CURRENT, but the CURRENT DENSITY.
- the exterior of the pipe is probably painted, so all you need to protect is paint holidays and areas of paint damage.
- the inside of the pipe is bare.
- the equation for current density is "current / area".
- Increase the denominator (area) by 10^3 or 10^4, and you'll have the affect on current density.