Can I add an O-ring to the base of a pencil zinc?

I have been getting a small ring of salt (from salt water, I think) around the outside of the "nut" end of the pencil zinc installed in the heat exchanger. There is no sign of water leakage in the area, so I'm thinking that the pencil zinc is leaking a tiny bit of salt water that evaporates as the heat exchanger is hot while motoring. Interestingly, while the boat sits at the dock there is no sign of water leakage or even moisture in that small salt ring, even though the pencil zinc is on the underside of the heat exchanger.

Although I'm puzzled by this, I'm considering adding a small O-ring onto the pencil zinc when I replace it next. I tighten the pencil zinc reasonably tight, but don't really want to tighten it more so I can remove it as needed. The threads would still connect to the heat exchanger, but the O-ring would add slight insulation at the "nut" end. I am wondering if this might detract from the "anode" effectiveness of the pencil zinc, or would that be insignificant, as long as the threads connect?

I would appreciate any thoughts, advice.
Frank

No problem for me. I usually put a bit of oil on the zinc threads. Mine have often grown a beard of salt just because , as you say, I don't torque them home because access is so lousy and because the potential minor drip of salt water would harm nothing.

The pencil zinc threads are NPT (tapered thread). The seal is formed by the "interference fit" between the zinc and the heat exchanger fittings. If it's dripping, you can tighten it more, or just live with a small drip. An o-ring placed external to the NPT threads won't do anything.

Maybe add a dab of silicone with your finger to seal it up.

I used a light coat of this, no more oozing.

Permatex High-Temperature Thread Sealant 59235.​

Thanks for all your replies! It's nice to know that others have also experienced this. Interesting because this has only appeared in the last couple of years, so I'm not sure what has changed or is causing it. But your suggestions look useful and should solve this for me.
Thanks again!
Frank

I don't think I'd use a thread sealant. A sacrificial zinc needs good electrical contact with the metal it is protecting. If the sealant acts as an insulator, it might defeat the whole purpose of the zinc.

Kenneth K said:

I don't think I'd use a thread sealant. A sacrificial zinc needs good electrical contact with the metal it is protecting. If the sealant acts as an insulator, it might defeat the whole purpose of the zinc.

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That's why I think a small dab of silicone smeared on the outside (and not on the threads) would do the job and not affect anything.

A thin layer of pipe thread sealant is fine. If firmly tightened there's still plenty of metal to metal contact, and sealant just fills in the gaps where there isn't contact anyway.

I was curious and used a meter to confirm it, and saw no measurable difference in resistance across a fitting vs. across two joints with sealant applied.

Kenneth K said:

I don't think I'd use a thread sealant.

Click to expand...

A "light coat" will not interfere with inductance IMHO.

Is it possible these start leaking because the zinc is being eaten away (as it is supposed to be, being sacrificial) by galvanic action and so the tight fits degrades?

I really wouldn't use a layer of any sealant -- any reduction in conductivity will decrease the efficacy of the zinc, and the whole point of that is to save a $12k+ (engine replacement and installation cost, probably a low estimate) expense with a few-dollar zinc. The NPT threading should help make good contact too... maybe even if you put sealant on. But if it does, it would be by squeezing the sealant out from between the mating surfaces. Remember: we use Lanacote or other dielectric greases to _prevent_ galvanic action (i.e. reduce conductivity) when we put stainless screws into (e.g.) the aluminum mast. Doing that with your zinc seems like a way to guarantee more corrosion of your expensive engine.

And just finding good conductivity after application may not be the whole story: you'd get that if there were almost any patches of good contact. However, once the zinc starts to corrode, some of those patches may get bad... it may be helpful/important to have a lot of good contact area, and a resistance measurement will pribably not easily distinguish between some good contact and a lot of it , since in both cases the resistance may be so low (and so also maybe dominated by the meter connections etc) as to be near the limit of the meter.

I am reminded of "Click and Clack"'s comments about braking a car via downshifting (engine braking): "a great way to use a $3,000 drive-train to do the job of a $50 brake pad."

But I don't have pencil zincs on my engine, so this may be off base.

N.A. said:

Is it possible these start leaking because the zinc is being eaten away (as it is supposed to be, being sacrificial) by galvanic action and so the tight fits degrades?

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Installed properly it should not leak until it is removed. The zinc "pencil" is secured in a brass plug which has male pipe threads. That plug screws into the heat exchanger same as any other pipe fitting. The zinc works through contact to the plug and then through the heat exchanger itself.

Frank Langer said:

I have been getting a small ring of salt (from salt water, I think) around the outside of the "nut" end of the pencil zinc installed in the heat exchanger. There is no sign of water leakage in the area, so I'm thinking that the pencil zinc is leaking a tiny bit of salt water that evaporates as the heat exchanger is hot while Interestingly, while the boat sits at the dock there is no sign of water leakage or even moisture in that small salt ring, even though the pencil zinc is on the underside of the heat exchanger.

Although I'm puzzled by this, I'm considering adding a small O-ring onto the pencil zinc when I replace it next. I tighten the pencil zinc reasonably tight, but don't really want to tighten it more so I can remove it as needed. The threads would still connect to the heat exchanger, but the O-ring would add slight insulation at the "nut" end. I am wondering if this might detract from the "anode" effectiveness of the pencil zinc, or would that be insignificant, as long as the threads connect?

I would appreciate any thoughts, advice.
Frank

Click to expand...

Kind of a long shot here, but a while back a friend said he had a pencil leak (not a medical issue, lol) and couldn't tighten it up enough. Upon examination I found that his pencil zinc was too long, so after cutting off 1/4 inch all was good! May not be your issue, but could be useful info for someone!

Steve Jerromes said:

Upon examination I found that his pencil zinc was too long, so after cutting off 1/4 inch all was good!

Click to expand...

Exactly the problem we found and solved when we first became owners of our boat and found this little issue on our Universal M25XP.

And if you've tried to saw off a zinc with a hacksaw without a vice to hold it, welcome to my world.