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.