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Old 25-04-2017, 08:30   #1
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dialectric grease and zincs

I am replacing the zincs on my prop shaft, max-prop, and a large round one on my stainless cutless bearing strut. Should I use a dialectric grease on the mating surfaces between the zincs and the stainless/bronze they are protecting, and/or between the mating surfaces of the zincs?? Can't find any useful information on the internet, but I'm sure someone here will know!
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Old 25-04-2017, 08:36   #2
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Re: dialectric grease and zincs

No. Dielectric grease is an insulator, exactly what you do not want. Clean surfaces only
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Old 25-04-2017, 18:12   #3
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Re: dialectric grease and zincs

I thought that dialectric grease improved the conductivity by excluding water and preventing galvanic corrosion etc. The grease is squeezed out where the surfaces meet so there is good electrical contact, even though the grease itself is not a conductor. It is used all the time on electrical connectors where conductivity, not insulation is required. Or have I got the wrong end of the stick again???
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Old 25-04-2017, 22:09   #4
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Re: dialectric grease and zincs

What you want is to keep a good electrical connection between the zinc and the protected metal for the life of the zinc.
If you bolt a zinc to metal and water gets in between then there is a chance that some zinc will corrode away there, and then loosen the grip and the electrical connection.
The way it is done on big ships is to have a rubber pad between the zinc and the boat, and to make sure that the bolt is providing the electrical connection. The head of the bolt also has to bite into the zinc (to provide the electrical connection) but must also have a seal to stop water from getting to the zinc bolt head interface.
All very complicated, but logical.
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Old 26-04-2017, 10:21   #5
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Re: dialectric grease and zincs

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Originally Posted by jamhass View Post
No. Dielectric grease is an insulator, exactly what you do not want. Clean surfaces only
Beat me to it. If you isolate it, it will not be sacrificial.
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Old 26-04-2017, 11:55   #6
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Re: dialectric grease and zincs

The above posts claiming grease will insulate the zinc are incorrect. The reason is simple - grease is a thickened liquid. When you tighten a zinc the grease is displaced, it has no ability to stop metal to metal contact. what the grease does do is fill in the microscopic gaps between the metals and thus stop corrosion, which WILL destroy electrical conductivity. This is not theory, I've worked with this stuff for a long time. Prove it to yourself with an electrical meter. Install your zincs with grease - dielectric rating is not important, water resistant rating the important thing. Measure the resistance between the installed zinc and the shaft/strut. You will find there is no difference in resistance installed with or without grease. But the grease ensures a much more reliable electrical connection over time.
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Old 26-04-2017, 12:01   #7
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Re: dialectric grease and zincs

What he said ^^^

I use dielectric grease on every electrical connection, I'd be in bad shape if even 1/1000 of them didn't get good contact. Air is also an insulator but it also is displaced when you put two conductors together.

I also use dielectric grease on every rubber hose connection, every O-ring, and to lube rubber parts like raw water impellers. Handy stuff.
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Old 26-04-2017, 12:27   #8
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Re: dialectric grease and zincs

While it seem counter-intuitive, using dielectric grease is a good thing with zincs. Think of it this way... If a little dielectric grease is enough to isolate the zinc from whatever you are attaching it to, you don't have anywhere NEAR a good enough connection!
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Old 26-04-2017, 13:13   #9
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Re: dialectric grease and zincs

I don't see the need for any dialectic grease on zincs. We aren't trying to save the zincs -- their purpose is to be sacrificial. They need to have a good contact with the metal they are protecting. Installing tightly on clean surfaces have always worked fine for the life of a zinc for me.
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Old 26-04-2017, 13:32   #10
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Re: dialectric grease and zincs

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Originally Posted by Pauls View Post
The above posts claiming grease will insulate the zinc are incorrect. The reason is simple - grease is a thickened liquid. When you tighten a zinc the grease is displaced, it has no ability to stop metal to metal contact. what the grease does do is fill in the microscopic gaps between the metals and thus stop corrosion, which WILL destroy electrical conductivity. This is not theory, I've worked with this stuff for a long time. Prove it to yourself with an electrical meter. Install your zincs with grease - dielectric rating is not important, water resistant rating the important thing. Measure the resistance between the installed zinc and the shaft/strut. You will find there is no difference in resistance installed with or without grease. But the grease ensures a much more reliable electrical connection over time.

^^ What he said. In fact, I have done testing of zincs (Practical Sailor) that confirmed that a good anti-corrosion grease simply makes everything more dependable.

I will go a little farther, though, and say that dielectric grease is ONLY better for certain very sensitive electrical connections. For all common uses--all 12v wiring on a boat and anodes--waterproof grease (such as Green Grease, Lanicote, or No-Ox-Id) is the better choice. Dielectric grease has only a very few, specialized applications, since it has only weak anti-corrosion properties.
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Old 26-04-2017, 18:52   #11
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Re: dialectric grease and zincs

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^^ What he said. In fact, I have done testing of zincs (Practical Sailor) that confirmed that a good anti-corrosion grease simply makes everything more dependable.

I will go a little farther, though, and say that dielectric grease is ONLY better for certain very sensitive electrical connections. For all common uses--all 12v wiring on a boat and anodes--waterproof grease (such as Green Grease, Lanicote, or No-Ox-Id) is the better choice. Dielectric grease has only a very few, specialized applications, since it has only weak anti-corrosion properties.
I assume just keeping electrical parts dry and protected from air is most of the battle on preventing corrosion. Silicone is fairly harmless to most plastics and rubbers, good for most of them in fact, and not as messy as most greases.

I'm not sure how long silicone grease lasts underwater...? I usually use some kind of thread sealant/thread locker on threaded fasteners. Even if it is low-strength, it fills and seals threads so you can get them apart again.
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Old 26-04-2017, 19:04   #12
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Re: dialectric grease and zincs

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I assume just keeping electrical parts dry and protected from air is most of the battle on preventing corrosion. Silicone is fairly harmless to most plastics and rubbers, good for most of them in fact, and not as messy as most greases.

I'm not sure how long silicone grease lasts underwater...? I usually use some kind of thread sealant/thread locker on threaded fasteners. Even if it is low-strength, it fills and seals threads so you can get them apart again.
Practical Sailor recently published some wash-off and corrosion testing stuff.

Silicone grease did not do particularly well. It is great for high temperature oxidation stability and high electrical resistance (in the absence of additives). Compatibility with most elastomers, but not silicone rubber or silicone caulk. But corrosion protection and wash off are unimpressive.
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Old 26-04-2017, 19:42   #13
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Re: dialectric grease and zincs

I won't argue against the folks saying the grease will squeeze out and contact will still be made, but in that case what is the logic in favoring dielectric (electrically insulating) grease on a connection that's supposed to conduct?

Because you like that it contains the word 'electric' ?
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Old 27-04-2017, 07:46   #14
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Re: dialectric grease and zincs

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I won't argue against the folks saying the grease will squeeze out and contact will still be made, but in that case what is the logic in favoring dielectric (electrically insulating) grease on a connection that's supposed to conduct?

Because you like that it contains the word 'electric' ?
On most electrical applications you could short things out by using conductive grease. In this case, it really doesn't matter. Keeping the electrical contact healthy is all that matters, and that is best accomplished by keeping moisture out.
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Old 27-04-2017, 09:36   #15
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Re: dialectric grease and zincs

Quote:
Originally Posted by Pauls View Post
The above posts claiming grease will insulate the zinc are incorrect. The reason is simple - grease is a thickened liquid. When you tighten a zinc the grease is displaced, it has no ability to stop metal to metal contact. what the grease does do is fill in the microscopic gaps between the metals and thus stop corrosion, which WILL destroy electrical conductivity. This is not theory, I've worked with this stuff for a long time. Prove it to yourself with an electrical meter. Install your zincs with grease - dielectric rating is not important, water resistant rating the important thing. Measure the resistance between the installed zinc and the shaft/strut. You will find there is no difference in resistance installed with or without grease. But the grease ensures a much more reliable electrical connection over time.

There are greases made for conductivity. dielectric is not and was the grease in question.
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