Corrosion with Stainless Steel in Buried Pipe Application 1

[ODtape]

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?

 

[bimr]

Try a google search. The "Nickle Development Institute" has information on the web that will answer your query.

 

[stanier]

The term stainless was coinerd to reflect the fact that the material would rust/corrode less fast than carboin steel. You will find that =staniless steel will corrode in moist buried situations. You would be much better off with DICL FBE coated. Thats the standard for most water authorities.

Sharing knowledge is a way to immortality

 

[coatitall]

316SS will corrode at a lesser rate that 304SS, but as the above poster stated, it will still corrode regardless.

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.

 

[cvg]

try also posting this in the Materials Engineers>Corrosion Engineering forum

 

[BRIS]

As noted by cvg you may get a more technical response from the corrosion forum. From a practical point of view I am very puzzled as to why you are considering stainless steel fittings to make connections to an existing PVC system. Why not use PVC?

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.

 

[ODtape]

As you can see by the date on the original post, it has been a couple years since I posted these questions. At the time I was attempting to educate myself on the subject of buried pipe systems. For a brief technical overview of stainless steel corrosion in buried applications you might try looking here:

http://www.euro-inox.org/pdf/map/CorrResist_SoilsConcrete_EN.pdf

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.

 

[BRIS]

I didn't note that your original post was so old and read 3-6" to be 3" to 6" not 900mm. (a problem with this site is that it is international but contributors from the US assume that we are all familiar with US standards and terms). The need to de-rate PVC for surge, temperature, fatigue etc is often overlooked and can result in the allowable working pressure being much less than the rated pressure. I would agree that DI was your best choice. Polyurethane coatings have an advantage that the can be easily repaired, polyethylene sleeving or tape wrapping in adverse environments are simple low cost remedies. Generally the only flanges I would bury would be mechanical joints using sheradised or iron bolts. In adverse conditions I would wrap. Flanges leak and the only flanges that should ever need to be buried are those on direct buried valves. In this case the flanges should be wrapped (after pressure testing).

It is nice to learn what solution you adopted - it is shame that more contributors do not feed back the results.

 

[stanier]

there are technical papers at

http://www.nidi.org

that cover buried stainless steel. 304 will definitely corrode.

 

[ODtape]

BRIS, it looks like my last post needs some clarification. In my original post, I WAS refering to sizes 3" through 6" (75mm - 150mm). C900 is the name of a specification published by the American Water Works Associatation. It has been the standard specification for 4" - 12" PVC water main pipe in the US. The pipe is often refered to by this specification.

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.