Corrosion with Stainless Steel in Buried Pipe Application
Corrosion with Stainless Steel in Buried Pipe Application
(OP)
I recently posted this question in the piping and fluid mechanics category. I am guessing that some readers of this thread may have some answers or insight, so I am posting it again here.
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 native clay soil near 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, as another member 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 will be 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, because of their physical location 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 adapter 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 ductile iron flange, hence allowing galvanic corrosion. Has anyone analyzed a similar situation?
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 native clay soil near 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, as another member 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 will be 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, because of their physical location 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 adapter 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 ductile iron flange, hence allowing galvanic corrosion. Has anyone analyzed a similar situation?





RE: Corrosion with Stainless Steel in Buried Pipe Application
RE: Corrosion with Stainless Steel in Buried Pipe Application
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RE: Corrosion with Stainless Steel in Buried Pipe Application
The use of isolation flanges will not prevent corrosion. These flanges mainly serve their purpose in cathodic protection (CP) systems, where you need to isolate a segment of pipe from an induced current. As long as you have soil, water, and steel without a CP system and/or a coating you will have corrosion. The rate of corrosion will depend on the corrosive nature of the soil, water table, ions present in the soil/water, etc.
RE: Corrosion with Stainless Steel in Buried Pipe Application
RE: Corrosion with Stainless Steel in Buried Pipe Application
Are you proposing to manufacture these SS fittings - do you have a very rich client?
Are you mainly concerned with internal or external corrosion ?
Stainless steel relies on the presence of an invisible chrome oxide layer (ceramic) to prevent corrosion. It needs very careful handling to protect the coating. It is susceptible to corrosion from chlorides, scratches and pits are a source of corrosion as is any contamination of the surface by ferric tools. If you really want to use steel then mild steel with a proprietary coating system (e.g polyurethane)will be cheaper and as reliable if not more so. If you are really concerned about subsequent damage then apply a tape wrapping.
RE: Corrosion with Stainless Steel in Buried Pipe Application
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Why not use PVC? Due to their asymetrical geometry, fittings will have areas of concentrated hoop stress, along with internal shear stresses / bending moments when pressurized. Because of pressure cycling, bending forces eventaully cause something analogous to work-hardening in the PVC. Eventaully, cracks will develop in these areas and cause the fitting to fail. This failure phenomenon is well documented in the report DESIGNING, OPERATING AND MAINTAINING PIPING SYSTEMS USING PVC FITTINGS by Keller-Bliesner Engineering. This report is usually available upong request from PVC fitting manufacturers such as Spears. Note that C900 PVC pipe used in waterworks mainlines is usually installed with ductile iron fittings.
If anyone is interested, stainless steel fittings are available with both standard tapered pipe threads and Victaulic grooved end connections. I ended up going with epoxy coated ductile iron fittings for my application. Manufacturers of waterworks fittings are increasingly making epoxy coating an available option. The primary problem that I see with this is the likelyhood of the coating being damaged during transport and installation, mainly due to the weight of the larger fittings and the typical handling methods used by installers.
Another interesting phenomenon is the increasing use of stainless steel fasteners for flanges, etc. in these buried pipe systems. Some corrosion enineers have suggested that due to the geometry, stresses and the way that they fit together, stainless steel fasters are especially prone to corrosion.
RE: Corrosion with Stainless Steel in Buried Pipe Application
It is nice to learn what solution you adopted - it is shame that more contributors do not feed back the results.
RE: Corrosion with Stainless Steel in Buried Pipe Application
RE: Corrosion with Stainless Steel in Buried Pipe Application
I agree that flanges are prone to developing leaks, and I generally avoid using them in buried systems, except where necessary.
There were a couple reasons why I did not choose PVC fittings for my particular application. First, almost none are available with pressure ratings as high as the pipe that I am using (DR 14 and DR 13.5). Second, the systems that I am designing for have frequent pressure cycling. As I alluded in the previous post, PVC fittings are prone to failure from pressure cycling. One manufacturer of iron fittings quotes some illustrative findings from the article FATIGUE TESTING OF PVC PIPE FITTINGS, Journal of Vinyl Technology, June, 1992, Vol. 14, No. 2, (D.B. Edwards, B. Lehman and R. Cohen): In tests, 6” schedule 40 PVC tees withstood less than 15,000 cycles of 100 psi every 2 seconds. Under the same test parameters, 6” PVC pipe will withstand 1.5 million cycles.