Corrosion problems w/ stainless steel w/ buried pipe

[ODtape]

I am in the process of developing standards for my company for 3-6” fittings used in buried pvc pressurized pipe applications carrying reclaimed water at ambient temperature. I have been looking into the possibility of using stainless steel fittings in new additions to this system, but have concerns over corrosion. One of the primary design goals is long term reliability. Soil conditions vary from site to site, the worst conditions being a moderately corrosive clay soil in a coastal region. My concerns center around a couple questions:

1) Am I mislead to believe that 304SS will have good corrosion resistance in a buried application? Would 316SS have better corrosion resistance? There seems to be disagreement over this idea, with someone having mentioned that microscopic galvanic corrosion is a problem inherent to stainless steel. My primary attraction to stainless steel is that it does not rely on coatings to prevent corrosion. I would specify ductile iron fittings with FBE coating, but such a coating will likely be subject to physical damage after it is installed. Then, I assume, it would only be a matter of time before the fitting fails from pitting corrosion. Also, note that the existing portions of the system feeding the new additions are a hodgepodge of old cast iron, ACP and newer PVC. Some corrosion product from the old C.I. and galvanized iron piping is present in the system. The system is always pressurized with water, but sees normal flow rates several times a week.

2) New portions of the system will often connect to existing ductile iron flange fittings (note that these fittings are not as likely to sustain damage to their protective coatings as I mentioned above). I am proposing to connect to these flanges using a stainless steel flange with a dielectric isolation flange assembly (including bolt isolators). Would this provide effective protection from galvanic corrosion in a buried application? I am worried that the soil, with a high water content, and moderate salt content would provide a conductive path between the stainless flange and the DI flange, hence allowing galvanic corrosion. Has anyone analyzed a similar situation?

 

[MJCronin]

constantlove (?)

You should also be concerned about something called "MIC" or Microbiologically Influenced Corrosion in a buried stainless steel system containing water. Carbon Steel systems can also be affected

As I understand, the problem first became significant when stagnant SS piping in Nuclear Power Plants began pitting and leaking.

MIC is, essentially, corrosion (pitting) by a colony of different kinds of bacteria in stagnant conditions.

NACE has several papers and books about this problem......lots of info on the web.use GOOGLE and search using "MIC" and "corrosion" as search terms......

Good Luck.....let us know how you make out



MJC

 

[Metalguy]

I would NOT attempt to use 304/316 in this case. OD Cl pitting is almost certain, unless you REALLY protect it.

Why don't you simply stay with glued-on PVC fittings?

If the soil or water in the pipe is pretty conductive, you can forget about insulating flanges etc. doing much good.

 

[MedicineEng]

I agree with Metalguy. Is better to stick with the PVC pipe. It is not a real good idea to have galvanized and SS joined together. Insulating flanges will only work when the only connection point is the flange itself. By putting the pipe in a soil that has water or any other conductive material, for sure galvanic corrosion will happen.
Regarding the difference of 304 and 316, 316 is more resistant to corrosion, but still not an option in your case.

PR

 

[ODtape]

Thanks for the responses. It sounds like stainless steel is probably not going to be a good solution. From a corrosion standpoint PVC would certainly be the best choice. However, at the pressures I am working with (up to 200 psi), PVC fittings do not have adequate strength at sizes 6" and larger. For 3" and 4" sizes, poured concrete thrust blocks are a necessary provision, which with my application has distinct drawbacks.

 

[Bensley]

The best answer to your material problem is GRE pipe used to be called WAVIN?
This is a glass reinforced epoxy pipe with no flange bolting only "O" rings and keys to lock the pipes together, pressure is not a problem. The pipe runs have to be accurate as the pipes come pre-lengths for elbows, risers. Pulsation is a problem but suitable anchors are designed. This is a buried pipe once installed correctly will last everyones lifetime including the plants?

 

[Metalguy]

I don't have a reference book handy, but you might look up CPVC and see if it would be strong enough. It's harder to glue it well vs PVC, but still pretty easy compared with other options.