Zink anodes in Seawater coolers: Will PTFE tape ruin the job?

[Swedishrigpig]

The old boys always say "Don't ever use PTFE tape (Teflon thread tape) on the threads of the bronze plug that holds the Zink anode pencil. It will isolate the contact between the Zink via the bronze holder to the casing of the cooler.
But surely the Seawater is sufficiently conductive to do the job and hence corrode the Zink rather than the cast iron cooler body with PTFE on the thread or without it?

Thanks

 

[unclesyd]

The Zinc has to be electrically connected to the part you are wishing to protect to be most effective.

 

[Compositepro]

unclesyd is is correct. To clarify, an electrolytic connection is not at all similar to a direct electrical connection. What you are suggesting means that if you place a zinc in water anywhere near the metal you are trying to protect then that is enough. Think about the electrical current path that causes/prevents galvanic corrosion.

On the other hand it is quite possible to have electrical conductivity across a thread with PTFE tape. It is also possible that it will insulate. Why would you want to use tape in such a joint?

 

[Swedishrigpig]

Usually it is an NPT thread plug that holds the anode, so it is just to seal the port.

 

[Compositepro]

I think you are confused by the fact that an NPT thread is used. It is used in this case, because it is a common thread with readily available tooling to make it, and it is self tightening so it doesn't require the use of a lock nut. But in this case there is nothing to seal. In fact, tightening screws or bolts into liquid filled cavities that are sealed can crack solid steel.

 

[stevenal]

The Teflon tape fills the voids just like any pipe thread sealant. It does nothing to prevent electrical contact, which does not occur at the voids. The sharp threads cut through the tape easily. The conductive seawater is only part of the circuit, acting like the electrolyte in a battery. The zinc to bronze to casing metallic path completes the circuit.

 

[Swedishrigpig]

Compositepro.

The outside of the plug that holds the anode, also seals the Sea Water from leaking out and flooding my engine room and sinking my ship, that is why somepeople uses teflon tape.
Usually the pressure is very low so it will seal on the threads alone.

 

[Compositepro]

If you do, indeed, need the seal then you have to be careful how much tape you use. If you use a lot and do not tighten the thread enough you can insulate the joint. You could use an ohm meter to check conductivity.

 

[Swedishrigpig]

Hmmm, good point, why didn't I think of that?
So finally, yes, (too much) teflon tape does interfere with the cathodic protection.

Gracias.

 

[unclesyd]

What you might do is use Teflon tape on first 2 or 3 threads to help stop potential leaks. This will leave the remaining threads to make electrical contact.

 

[stevenal]

Generally you should not use the tape on the lowest threads, since the lower threads will cut it leaving loose pieces of tape in your cooler. Same with compound, put it higher up on the threads. Unless you skip the wrench and finger tighten only, you will not have an insulated joint.
Even at low pressure, I would not skip the thread sealant. Tiny leaks have a way of getting worse.

 

[moltenmetal]

The currents involved can be pretty large, and you've only got what- less than 1.5 volts of driving force. You want a good, solid electrical connection with a very low resistance for maximum protection. But you do need a thread sealant in there to seal the joint. Hmm- what about using expanded graphite tape instead? Then again, it'll generate another and perhaps undesirable galvanic cell.

Wrapping only part of the thread with teflon tape is the best option I can think of. Checking the resistance of the finished assembly would be a good idea.