Electrolytic Corrosion of Steel Pipes

Yes.

Batteries also create DC current. Now if I remember correctly one makes a battery by putting one piece of metal into an electrolyte and ....

Why steel and not plastic pipes?

Steel pipes are available pre-rusted, so there's less time to wait for failure.

Yes, and the likelihood is very high unless the services of a competent corrosion engineer are used. In addition you should also consider hiring a competent engineering firm (could be the same) to advise you about the rigorous requirements for laying conductive pipes within the right of way of overhead transmission lines.

Electrolytic or galvanic corrosion is caused whenever there is a Potential difference.

The average potential of an AC line is ideally Zero, however, it can also build up a DC potential. The DC potential on long lines can be caused by the capacitive properties of the wire or pipe. An electrical storm or solar flares can also build up large and dangerous potential differences on long cables or pipes.

Any steel pipes along the HT lines has the potential to magnetically pick up the electromagnetic radiation from the lines. Small differences in the + and - potentials will add up over time.

It can also be caused by dissimilar metals in contact.

Suggest you read:

http://en.wikipedia.org/wiki/Galvanic_corrosion

http://en.wikipedia.org/wiki/Cathodic_protection

http://www.geomag.bgs.ac.uk/education/gic.html

You understand i/2 of the answer. Corrosion is generally the result of DC potential. The AC voltage you have operating close to you steel pipe requires rectification to yield a DC component and there are many natural ways to achieve this. Also, natural electro-chemical processes are always present. You have every reason to expect corrosion.

I believe the corrosion you are referring to was a problem in the London Underground Railway (at least I think it was the London Underground) where they use a hot third rail to carry the power to the trains and the normal earthed rails for the return. Since the return rails were connected to the metal tube that the trains ran through you ended up with a small potential between the tube and surrounding soil due to the resistance in the return path for the power. This potential then accelerated the rate the metal tunnel tube corroded and thus cause premature failure of the tunnel walls. The solution was to add a fourth rail that was also insulated from the tunnel tube to carry the return current. Since there was no contact between any of the conductors and the metal tube of the tunnel the small voltage was no longer present and the corrosion rate dramatically reduced.

Now I'm guessing that you aren't going to have any current AC/DC or otherwise running through your steel pipes themselves in which case you shouldn't have the same problem as they did with the underground railway.

Having said that, any metal buried in the ground is going to corrode and trying to eliminate induced currents in it is almost impossible so you will more than likely have corrosion problems unless you find some way of preventing the soil from coming into contact with the metal pipe. Unfortunately this too is almost impossible to achieve and if any part of the pipe does come in contact with the soil then the corrosion at that point will likely be dramatically accelerated.

The Hong Kong Mass Transit Railway had a similar problem. The solution there was to reverse the polarity of the DC supply, which eliminated the tendency of the reinforcing bars in all the concrete structures supporting the railway to corrode.

I apologize for my ignorance. The following suggestion comes from a gut or simple-minded notion. Which is that - if currents are going to be accumulated on sections of the pipe - why can't a load be introduced to discharge this energy? Perhaps the loads can be of varied in sensitivity & applied according to measured values which can be conducted after installation is in place. Also, it occurs to me that there must be some variations in the conductive & chemical values found in the available material (i.e. soils) that would surround the pipes. I don't know if this has been mentioned. Thanks Carlos

It is very difficult to help these people when they give no useful information at all.

<...IF it is carrying gas, oil, or other hazardous liquid...>

Quite. GA for highlighting it. The original poster hasn't shared this vital snippet - it is, after all, in the electrical engineering section and such trivia are clearly irrelevant (not); Electrical Engineers aren't interested in process fluids, are they <rhetorical question>?

Non-conductive pipes are used for some fluids, which would solve the original poster's problems if applicable. The kindness of a response by the original poster to lots of queries above is clearly needed to progress the thread, and is currently missing.

So, from Dodman's Lane, a pox on this topic.

_{<unsubscribes>}

In order to have induced current in any conductor there has to be AC present as there is not any transformer affect created by nor available with DC power.

The only time you might see DC current flowing in/on a pipe would be from ground fault leakage in an adjacent DC power system such as a train wherein the fault current follows the pipe due to it's lower impedance than the surrounding soil and/or as a parallel path for the DC voltage to return to the source.

Yes you will have transformer affect from the overhead power line with significant induced current flowing in the pipe(s) and if you do not install proper cathodic protection on the pipe(s), it/they will oxidize and prematurely fail.

Cathodic protection pipe protection systems inject DC voltage directly onto the steel piping to disrupt/defeat/stop AC induced current flowing in the pipe(s).

OOPS!

Bad day at bedrock coupled with brain burp.

Correction: As already mentioned you can get DC currnet flow in the pipe(s) due to "battery" affect caused by soil composition, PH, and moisture.

Sorry about that.

Apologies, but ...

Re: (Post 14) "Cathodic protection pipe protection systems inject DC voltage directly onto the steel piping to disrupt/defeat/stop AC induced current flowing in the pipe."
and
(Post 15) "you can get DC currnet flow in the pipe(s) due to "battery" affect caused by soil composition, PH, and moisture."

CP is NOT used to prevent AC currents from flowing in the pipe ["Period"] ...and, DC current flow in pipelines is NOT due to "battery" effect ... (what you are attempting to make reference to is the galvanic corrosion due to "local cell activity" which occurs on any carbon steel placed into any type of electrolyte environment).

Just trying to prevent a cancerous "jumping on the bandwagon" here ....

Preventing galvanic corrosion

There are several ways of reducing and preventing this form of corrosion.

.Cathodic protection uses one or more sacrificial anodes made of a metal which is more active than the protected metal. Metals commonly used for sacrificial anodes include zinc, magnesium, and aluminium. This is commonplace in water heaters. Failure to regularly replace sacrificial anodes in water heaters severely diminishes the lifetime of the tank.

Finally, an electrical power supply may be connected to oppose the corrosive galvanic current. (see impressed current cathodic protection)

"So far so good" ... (you're getting there).

I try my best not to offer "profound advice" unless I feel pretty confident that what I have to offer is both germane to the specific topic and scenario, as well as being correct.

Having completed 8 courses with NACE over the years, I feel that I am pretty well versed in both sacrificial and ICCP (impressed current cathodic protection) systems.

What I stated in post#16 is true-and-correct, when read verbatim.

CP is not used "to disrupt/defeat/stop AC induced current flowing in the pipe(s)". It cannot elicit that effect (Period).

CP is applied (more ofter than not) to carbon steel structures in order to drive the entirety of the protected structure to a potential that is more electronegative than the ("otherwise"/"native") most electronegative site ("local-cell") on that inhomogeneous structure.

By eliminating the inhomogeneity, the tendency for uniform corrosion to occur (between adjacent / local cells) throughout the structure is eliminated.

Again, "apologies" for feeling the need to 'step-on-your-post' ... but, "correction" is oftentimes called-for ... (to prevent a snowballing-cascade of similarly incorrect responses obscuring / overshadowing the facts). This happens all too frequently, thus deteriorating the otherwise beneficent nature of (this and other) forums.

"Cheers" ~

Thanks for stepping in Tom.

No need to apologize, I need all the help I can get.

The goal should be to provide the needed and correct information to the OP so I would hope those that are better versed in a subject will do exactly what you are doing.

Thank you very much for your time and for sharing your knowledge on this subject.

The proposed steel pipes (DN 450) will be for the conveyance of raw water.

It is clear that galvanic corrosion will take place due to the soil which acts as an electrolyte and it's degree of intensity will depend on the properties of the soil: moisture content, oxygen content, etc. Cathodic protection can be provided to reduce it's affect.

My question was whether the overhead 33 kV line (AC) will contribute to electrolytic corrosion. From the responses received, my interpretation is that the likelihood of such corrosion is there provided the current is direct (DC). Alternating current (AC) can result in DC potentials but it does not present any significant cause of concern.

You might want to consider the conclusions drawn in this particular paper:

"DISCUSSION OF CRITERIA TO ASSESS THE ALTERNATING CURRENT CORROSION RISK OF CATHODICALLY PROTECTED PIPELINES"

http://www.apia.net.au/wp-content/uploads/2010/06/Paper-26-Buchler.pdf

I'm not sure if this applies to your particular situation since there are too many variables in your particular case that are known to you alone.

[For the OP] ~ A "tough read", for anybody lacking a broad background in corrosion / electrochemistry... . Care must be taken when reading such papers "out-of-context" (meaning, without proper background).

Pages 1-8 of the linked paper *seem* to incline the reader to believe that simply "cranking-up the CP level" will (by ensuring a low J_ac/J_dc ratio) insure protection against ac corrosion...(?!?)

Fortunately, at least one sentence at the bottom of pg 8: "...related questions regarding coating disbondment...should be considered", begins to acknowledge this critical issue.

A paper from 1973 "Effect of Overprotection on Pipeline Coatings" (W.R.McCaffery, Australia) enlightened the industry to problems arising from too high of a CP potential.

The (linked-paper's) Closing remark: "While significant progress in understanding the processes taking place during ac-corrosion has been gained, there is still a lack of a universally applicable criterion for the determination of the ac-corrosion taking place on the pipeline" completes the "Introduction's" remark: "Despite all of these investigations, the involved mechanisms are still not understood."

The paper's data does seem to introduce a ("different / deeper") insight into the dissolution of reduced iron (ions) into elevated pH soils. Previous understanding (my own) was that carbon steels (unlike zinc, magnesium, lead, and numerous other elements) are *not* amphoteric; i.e., they do not succumb to corrosion at both elevated and decreased pH's. While CS does deteriorate faster in lower pH environments, it is actually protected (to a degree) at higher pH's (as evidenced by iron's Pourbaix diagram):

As previously noted, MANY factors must be taken under consideration, before determining whether a problem (with corrosion due to ac currents induced by overhead lines) actually exists ... and, IF so, which course of remedial action to take.

Again, wishing "Good-Luck" on the project ~