304SS pipe corrosion, underground installations 1

[ProjectEng]

I am planning to install a loop off a fire main to service a firehouse for a cooling tower. Most of the loop will be plastic bell and spigot. However, I plan to change to S40 304SS under the building and closeby to reduce possibility of leaks under the building.

The pipe will 90-up into the building from both sides and come up to a distribution header.

If I electrically isolate the underground SS pipe as it comes up in the firehouse, will I still need cathodic protection?

Thanks for your help.

 

[Fizzhead]

ProjectEng:

In general the burial of SS is not advisable without either protective coatings, cathodic protection, or both. The reason is that the galvanic potential between SS materials and existing soil components is high enough to result in the establishment of a galvanic cell, thus corroding the buried element. Two methods of fighting this cell formation are to install a dielectric barrier (i.e., a coating material) which creates a high resistance factor, or to impose a higher potential electrical cell with reversed polarity (i.e., cathodic protection).

 

[Montemayor]

ProjectEng:

Fizzhead gives you some good advice. I would not install stainless steel under that building. There is yet another, possibly more important reason for not doing this: the possibility of your area containing cinders or source(s) of chlorides. This chloride presence threatens the SS pipe with stress corrosion and will literally destroy the pipe's mechanical integrity.

I'm writing this from personal experience. My mentor once was pressed with time and failed to analyze a field where SS pipe was proposed. He acceded to the decision due to project schedule and lived to regret it. The "field" was once the site of a coal-burning power station that had, for years, deposited ashes and cinders on all the surrounding area. The SS pipe had to be removed in fractured, crystallized pieces after it failed two years later. This is one of the notorious trade-offs when you deal with the Stainless Steels and it shouldn't have happened to such a great and experienced engineer. If it was me, I would consider FRP or schedule 40 plastic pipe before SS. I may be wrong, but I don't believe cathodic "protection" will protect you from stress corrosion. And you can't inspect protective coatings when the pipe is buried - especially under a building.



Art Montemayor
Spring, TX

 

[saxon]

projecteng., Just my two cents. What normally happens to pipes under buildings after the buildings and equipment foundations settle? Also, leaks can and will occur over time, if they're buried the source is next to impossible to identify and repair.
My suggestion is to bring the u.g. up above grade just on the inside of the outside wall and run it up overhead. BTW, 304 SS will corrode just as badly as Galv. CS even when its coated and grounded w/anode packs.

By going overhead, you can reduce proj. costs by going with galv./CS instead of the SS.

Just my two cents.
saxon

 

[rmw]

Art,

I think you are right. Galvanic corrosion, and stess corrosion are two separate issues, both of which are applicable to this situation assuming the presence of any chlorides, which would provide the bad actor for the stress situation, as well as the conductivity to the soil for the galvanic couple.

I wouldn't touch SS pipe for this application.

rmw