Exterior coating of steel penstock

[21121956]

Hello everybody:

Steel penstocks of welded construction for hydro electric power plants, when embedded, the surfaces in contact with the concrete are left in rough condition and without protection (without any painting applied).

But for buried steel penstocks it is normally mandatory to apply special coatings which will provide corrosion protection to the exterior of this steel structure.

My insight is that the concrete acts as a impermeable body surrounding the pipe and there is no danger that subsurface water gets in contact with the pipe, provoking corrosion.

In the other hand, when earth is the media of contact, just quite the opposite occurs with the pipe, and for that reason it is required to paint properly the exterior surface of the pipe.

So, I would like to know your professional opinion on this subject. Thanks in advance.

El que no puede andar, se sienta.

 

[MJCronin]

Corrosion of steel is basically the same as the corrosion of old men ........ just oxidation.

If you stop the source/amount of oxygen incontact with the steel, the corrosion stops.

Halting that flow of oxygen can be:

- Covering the steel with a paint coating or metalic plating

- Puting the steel in an environment depleted of oxygen (e.g. a blanket of nitrogen)

The concrete "coating" deprives the surface of oxygen, while most common soils.....do not.

Steel will "rust" (oxidize) in water until there is no more disolved oxegen in the water...... the process then stops.

Steel placed in water with a continuous source of oxygen (e.g a flowing stream)will continue to rust

IMHO.....

 

[21121956]

Hello everybody:

Thanks MJCronin for your well structured opinion.

El que no puede andar, se sienta.

 

[SJones]

http://www.corrosion-club.com/concretecorrosion.htm

Steve Jones
Materials & Corrosion Engineer

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

 

[stanier]

Cathodic protection using impressed current or sacrificial anodes is also used to protect penstocks.

You may find that using a stainless teel could be a cost effective solution if life cycle costs are taken into account.

Many penstocks in sewage treatment have cast iron frames coated in bitumen. But these have ony lasted up to 100 years so probably not much good to you. Because the technology has been around for a few hundred years do not ignore it.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[21121956]

Hello everybody:

Thanks for your answers to my post.
SJones, I understand the corrosive mechanism described in the article; but, I think that it is very difficult (not saying impossible) to avoid that corrosive species can enter the mix if "contaminated" mix ingredients are used (water, aggregates, additives).

So, in such a case, one can always expect the occurrence of corrosion in metal surfaces embedded in concrete, and for that reason it will be necessary to apply a coating to the exterior of the pipe, no matter if it will be concreted or simply buried.

stanier, our penstock will be 3,00 m in diameter and 540 m long, so, at first sight, to think in construct it on stainless steel seems to me out of possibility.

From your post I can see that you could accept to apply some coating (Coal-Tar Epoxy, Epoxy Polyamide, etc.) in order to expect a good life span for this pipe.

El que no puede andar, se sienta.

 

[stanier]

that is not a particularly large penstock. Do the life cycle cost benefit analysis and check out a suitable grade of ss.

Modern sewage treatment plants in Australia use ss or aluminium penstocks. In fact our fabricators export around the world.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[bkeil]

There is a great reference for steel Penstocks and their design, ASCE MOP 79. A new version will be available in the next two months if you can wait that long.

To your original post, buried steel pipe is commonly lined and coated with cement mortar (at least in North America), the protection comes from the passivisation of the steel surface, the cement is not a barrier to water as some would believe, cement mortar / concrete are porous. Instead they create an environment where corrosion can not occur due to the high Ph. The most common coatings for above ground applications are epoxy or polyurethane systems, it is also common where exposed to sun light to top coat these with an aliphatic polyurethane.

The corrosion of an uncoated above ground steel pipe will depend greatly on the environment you are placing it in, but to get the longest life of the pipeline, a coating is highly recommended.

 

[cvg]

1) hire a corrosion engineer to make appropriate recommendations based on actual data from your soil, water and steel
2) recommendations will probably be to provide both cathodic protection and coatings

Corrpro would be one example of a company that could provide these services

http://www.corrpro.com/content/168/pipeline-corrosion-prevention.aspx

http://www.corrpro.com/mm/files/Cathodic_Protection_Typical_Specification.pdf