Dissimilar metals

[WB01]

I currently have a situation that involves HDPE to 316 SST underground flanged connections. The HDPE pipe was supplied with ductile iron back-up rings. 316ss bolts were used to bolt through the DI rings into the ss flanges. The Owner is concerned with the possible reaction between the two metals over time. The soil in this area is sandy and possibly wet in some areas. Does anyone have any case studies or experience with this type of situation?

Thanks.

 

[11echo]

Normally I don't like to bury flanges, but if you have to do so why not look into using an insulating flange kit? That'll eleminate any direct metal to metal contact.

 

[bimr]

Order a backing ring fabricated from stainless steel.

The back-up ring is required for polyethylene flange joints in order to develop uniform pressure around the entire sealing surface. Without a back-up ring, a polyethylene flange will leak
between the bolts. Standard back-up rings are convoluted ductile iron with AWWA C207 150 lb drilling. Other available back-up ring materials are steel, primer-coated steel; epoxy coated
steel, stainless steel or fiberglass. In below ground service, coatings and cathodic protection may be needed to protect metal back-up rings from corrosion. One edge of the back-up ring
bore must be radiused or chamfered. This edge fits against the back of the sealing surface flange.

 

[rconner]

Not going to profess specific expertise here -- however, I believe since about the time of Sir Humphry Davy, or shortly thereafter it has been known that electrically coupling a rather large area of cathodic metal (let's say bare stainless steel) to a smaller, highly stressed anodic metal (let's say iron, or carbon steel) all in the presence of an electrolyte (let's say corrosive or conductive soil) is generally not a good idea. I don't know, however, enough of your conditions and application to know this is what you have.
About the only other thing I will say is that just saying (e.g. bare) all "stainless steel" as the answer for some buried service may not satisfy at least some current corrosion engineers. As appareently all stainless steel has not been corrosion free in such applications, some insist that it be coated, wrapped, and cathodically protected just like bare carbon steel.

 

[gerhardl]

The connection and application is known and used in Scandinavia, but with precautions as described by bimr. No iron or ductile iron to be buried without proper certified coating, also in boltholes, and undamaged (epoxy or better). Offer anodes of sink and earthing used in some cases.