Corrosion Protection for Underground Compressed Air Pipe

Do you know anything about the soil? Moisture levels, salinity, biological activity?
Look at the guidlines for gas lines and water lines.  They are coated, often with multilayer coatings.  The also often have cathodic protection systems also.  Or at least ways to measure if there is any active corrosion.

How dry will the air be?  I would also worry about internal condensation.

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Corrosion never sleeps, but it can be slowed down.

I don't have information regarding soil conditions. I have checked several guildlines and standards regarding underground piping. I have ruled out HDPE because of the compressed air. I have read several posts saying "do not use galv. steel underground". Im intrested to know the reason behind that?

Thanks for your post

Kevin

If I was laying  a new buried compressed air line, I'd go with the "yellow-jacket" steel pipe that the gas companies use. Talk to the local gas utility, they may have some useful information for you.

The Zn in hte galvanize is consumed rather quickly and then you are left with no protection.
Gas lines usually have a three layer poly jacket on them.

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Corrosion never sleeps, but it can be slowed down.

As a gas utility guy I can say that 3-layer poly is not a common coating for most distribution applications. It is more likely to be used where the need for extremely high reliability (such as offshore) or resistance to abrasion (such pulling pipe through a bore) warrants the high cost. The extruded polyethylene (also sometimes called yellow jacket) coating is common and is much less expensive, although also less reliable. Fusion bonded epoxy (FBE) is common on some projects, particularly large, high pressure pipe where the extra expense is warranted.  Pritec (wrapped poly over an extremely tough mastic (perhaps a butyl???))is also a great coating that is sometimes used. For almost any coating you select you should stringly consider supplementing it with cathodic protection by using sacrifical anodes, assuming the soil resistivity and other characteristics don't require use of impressed current.  NACE has a lot of technical guidance available for your situations like yours.

Top coating the galvanized coating should include making sure that the top coat is compatible with the zinc.  Not all coatings will adhere without special primers and surface prep, although it may be more of an issue for coatings typically used above ground.  Although galvanized pipe below ground would not be my first pick, the galvanizing will provide some temporary resistance to corrosion where the coating gets damaged.  You can always retrofit the pipe with C.P. anodes later.

Galvanized CS underground will not protect the line. Have you ever unearthed an old buried galv elec conduit?

"yellow jacket" is a good coating, but don't bury the line w/o adding cathodic protection. If you go w/sacrificial anodes, make sure you isolate the pipe. If not, current wil, be drained and the CP reads will not show compliance.

Otherwise, you could retrofit an unisolated CA line, but you'll need a bigger CP system (recifier powered).