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CS/SS galvanic corrosion in a wet and dry environmentHelpful Member! 

wannabepe (Structural) (OP)
23 Feb 12 9:22
Good afternoon all. I am trying to solve the following problem:
We insert an electrical harness through the shell of a potable water tank to operate a submerged 110V electric motor. We penetrate the tank wall with a carbon steel coupling and use a stainless steel sealing gland on the interior to keep the water from leaking back out. The potential problem, of course is galvanic corrosion at the point where the carbon meets the stainless. We have used PVC at this junction but it failed either due to low temperature exposure or (we suspect) falling ice.

We have tossed around a few other possible solutions:

First, a flanged and gasketed carbon to stainless connection with plastic insulators at the flange bolts.

Second, a dielectric coupling which of course means a substantially weaker threaded connection, susceptible to shear.

I have come up with the following three possibilities (or a combination if necessary) and would appreciate a professional opinion:
1.    Weld a carbon steel coupling to a stainless steel coupling with 309L filler metal.
2.    Paint the carbon surface of the exterior of said carbon/stainless weldment OR the carbon steel side of a threaded joint. Footnote 1-If the joint were threaded, would it make sense to use a stainless nipple into a carbon coupling, thus a larger anode, smaller cathode? Footnote 2-This assumes that the dry interior of the piping (dry side of the sealing gland) will not pose a corrosion problem in the absence of water.
3.    Install a sacrificial zinc anode in close proximity to the carbon/stainless junction, be it a welded or threaded joint.
Once we Install the electric motor and fill the tank, we do not want corrosion problems occurring inside the tank since it will be inaccessible for observation and maintenance.
 
Any input from the professionals would be greatly appreciated.

Thanks. Wannabe
 
Helpful Member!  moltenmetal (Chemical)
24 Feb 12 7:29
Unless this is expected to be 30+ year durability, your carbon steel tank has zero corrosion allowance, or the potable water is either hot or otherwise more corrosive than normal, the likelihood is that galvanic corrosion in this case will not be a big problem.  The noble material (stainless cathode) is small in area relative to the carbon steel shell (anode) which will be corroding.  (You get problems when you have the reverse situation- think steel nails in copper plate- copper nails in a steel plate is much less of a worry).  Your NPT threaded joint is going to be covered in thread sealant, making the contact resistance quite high, and the stainless itself isn't very conductive, limiting the current flow.

The other option of course is to use a carbon steel gland, assuming the gland is a compression fitting- they are available in carbon steel.
wannabepe (Structural) (OP)
24 Feb 12 9:03
Thank you moltenmetal for your response. It sounds like less of a corrosion issue than I thought. If we can expect 10 years or more I think that would acceptable.

I'm hesitant about using the carbon steel gland however, as it may corrode at some point potentially causing a leak and damaging/shorting the electric harness. This thinking may sound counter to the carbon steel coupling penetrating the shell, but we typically use a heavy wall (3000#) fitting there so it's not an issue. We want to make this as durable and maintenance-free as possible.

Would you agree with my thinking on the dry side of the gland (Footnote 2) that the absence of water will be a less likely environment to cause corrosion? This would be the unpainted interior of the fitting; a DRY carbon to stainless connection. I say unpainted because the fitting is small (3/4" - 1") making it difficult to surface prep and paint. We could opt for a larger carbon coupling, then bush down with stainless. This could make painting possible.

This would enlarge the noble material, but in the grand scheme of things, not by much if I use your your logic. (It's a pretty small nail)

Thanks again!
moltenmetal (Chemical)
24 Feb 12 12:50
Agree about the carbon steel gland- I'd use stainless as well unless the water is hot and full of chloride such that cracking would be a risk over time.

I suspect the worry would be in the crevices of the coupling, but if it's a 3000# coupling it will have lots of meat to sacrifice to corrosion before you have a problem.

Others might be able to give you a better calibrated guess about the lifetime.
wannabepe (Structural) (OP)
24 Feb 12 13:14
Yes, that's the intent with the heavy coupling; corrosion allowance. Also a good heat sink against distortion during welding.

Thank you again my friend. Very helpful.
Metalguy (Materials)
16 Apr 12 1:47
Coat the wetted surface of the SS, not the CS.  A threaded connection with the threads coated with sealant usually has near zero resistance across it, as there will still be metal-to-metal contact.

It wouldn't be capable of carrying much current, but would be enough for the typical corrosion currents involved.

Galvanic corrosion usually occurs very close to the joint area, on the anodic (CS) side, of course.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein  

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