Copper Valves in Carbon Steel Pipe 2

[Montana1]

I have a question that revolves around the galvanic corrosion mechanism. If I use threaded bronze valves in a carbon steel boiler feedwater circuit, will there be a concern with galvanic corrosion in the threaded joint? (Temperature is 400F at 250 psi.)

 

[strider6]

Yes, you'll have corrosion due to galvanic corrosion. One solution could be a dieletric joint, isolating joint.

http://www.corrosionist.com/Galvanic_Corrosion.htm

http://www.eci-ndt.com/gallery_a.htm

http://www.corrview.com/tech_p_10.htm

S

Metal Corrosion

 

[PEDARRIN2]

Correct me if I am incorrect, but I think it would be ok.

Since carbon steel is more anodic on the galvanic series than coppor alloys, the corrosion would tend to be in the carbon steel.

But since the carbon steel will have the greater surface area in the system, won't that protect it?

We commonly put small sections of copper piping on carbon steel sprinkler lines and don't see corrosion. We do not do the opposite, though which would be a failure waiting to happen.

 

[EdStainless]

Well, on one hand this is feedwater so the conductivity is very low so galvanic corrosion will be minimal.
On the other this is at 400F and galvanic corrosion rates are exponential with temp.

Do you want to use bronze valves at 400F? Even without the CS pipe I would think that there is a risk of dealloying.

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Plymouth Tube

 

[Montana1]

Thank you all for your valuable comments. I appreciate the documentation on copper valves threaded to CS pipe. I tend to agree with EdStainless; since the boiler feedwater has oxygen scavengers in it as well as corrosion inhibitor, the conductivity would be very low. These bronze valves are actually rated for 250 psi at 400F. The boiler really operates at 150 psi at 366F, and these are used as level glass valves, so there is minimal water flow. My guess is they would be seeing 250F, so the concern of dealloying would be minimal. Thanks again for the knowledge.

 

[stanweld]

There have been a number of instances of dealloying of copper alloys in these types of services. Copper can plate onto the carbon steel pipe surfaces leading to galvanic corrosion and severley shortened service life.

 

[metengr]

Montana1;
You better go back and check ASME B&PV Code , Section I, copper and copper alloys are limited for service -

SB-61 alloys are 550 deg F
SB-62 alloys are 406 deg F

I recognize that your application is for a boiler feedwater circuit, but personally, I would not recommend in this application because of allowable stress concerns and creep.

 

[EdStainless]

Good point Stan. The corrosion can go either way. Once you get some pure Cu plated onto the CS you will get some local corrosion around the spots.

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Plymouth Tube

 

[strider6]

In my opinion you should avoid such connection since i think that we can discuss a lot on area effect, conductivity of the water... but since there is a potential galvanic corrosion and since your service is water @ 400F and 250 psi, i'll define it a critical service and in my opinion it's necessary to avoid any possibility even remote of corrosion 'cause the risk associated with a failure is too high.

This is a paper found on the net regarding a case similar to that we are discussing:

"The leak on December 5, 2002, was located where the carbon steel pipe threaded into a bronze check valve. The leakage was attributed to galvanic corrosion at threaded onnections that contained dissimilar metals.
Based on this, an extent of condition review and ngineering evaluation was completed which was centered around threaded connections with dissimilar metals."

http://www.nrc.gov/NRR/OVERSIGHT/ASSESS/REPORTS/nmp_2002006.pdf

S.

Metal Corrosion