How much zinc is enough? - SailNet Community
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post #1 of 37 Old 10-06-2011 Thread Starter
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How much zinc is enough?

Hi,

I have a Freedom 36 on which I have 1" SS shaft protruding about 2.5 Ft out of the hull on which I have put two collar zinc anodes and one on the tip of my MaxProp propeller. Many people in the Marina also hang a clamp on "Grouper" zinc anode due to stray currents causing galvanic corrosion. I an planning on doing that as well, but are two anodes on the shaft and one on the tip of the propeller enough?

I have hauled the boat for painting today and found that my entire shaft was covered with barnacles and small mussels. Is there anything that can be done to prevent that since it is not advisable to paint the shaft with antifouling paint?

Can the shaft be pained with anything to protect it?
Petar
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post #2 of 37 Old 10-06-2011
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Sounds like you have enough zincs as long as you change them at least once a year. I have a single shaft zinc and the zinc on my maxi prop and I change them twice a year. Zincs don't stop mussels and barnacles as you discovered. There are special antifouling paints for motor boat outdrives that might work on the shaft, but I don't think I know anyone who does that. You wouldn't want any of it on the shaft where you put your zinc, because you want metal to metal contact. I just scrape the shaft and prop off twice a year when I replace the zincs. It's just one of those things you have to do.

Ray
S.V. Nikko
1983 Fraser 41
La Conner, WA


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post #3 of 37 Old 10-06-2011
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One of side effects of using zincs to control galvanic corrosion is that it attacts barnacle growth. One product you can try is Pettit Zinc Coat Barnacle Barrier. I've used it and it helps, but is certainly not the complete answer.
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post #4 of 37 Old 10-07-2011
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Quote:
Originally Posted by Petar View Post
Many people in the Marina also hang a clamp on "Grouper" zinc anode due to stray currents causing galvanic corrosion.
If you didn't notice any corrosion on your running gear when you hauled, there likely no need for an additional zinc hung over the side. And unless that grouper zinc is in direct electrical contact with the item(s) you want to protect, it isn't doing anything anyway. Most boaters are pretty ignorant about the causes of corrosion and the "hot marina" is largely a myth. I'd be surprised if those other berthers in your marina actually have a valid reason for their grouper use.

Last edited by Fstbttms; 10-07-2011 at 11:19 AM.
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post #5 of 37 Old 10-10-2011
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From what I've read it is possible to over do it with zincs. There has to be the correct amount of sacrificial anode to neutralize electrolysis. Too much will create current where it didn't exist before.
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post #6 of 37 Old 10-15-2011
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Yeah, you can over do it with anodes. They will react with the copper in the bottom paint.

Heck--- on my old Hunter 30, I USED NOTHING!!! AND NOTHING WENT WRONG after 25 years of sailing! The best way to determine if an anode is required is to measure the various wetted metal voltages against a half cell.

I now own a Silverton 40' aftcabin. What I found was everybody gave advice about anodes....I gave up on zinc, I now use aluminum anode. The first couple of years I owned the boat I copied placing zincs all over the bottom as did the previous owners. What a waste! I knew there was a better way to solve this problem.

I cruised Ebay for a while looking for a silver-silver chloride half cell and was lucky enough to purchase one for $40!!! What a steal! The thing new was priced over $600 as a part of some test gear to corrosion testing. Anyway, I now use a single aluminum anode plate mounted on the stern and that is it!!! No shaft doughnuts (which never seemed to stay anyway), nothing on the rudders or anywhere else.

I also made my own prop shaft brushes using oil impregnated sintered bronze bearing to bond the shafts. They work although there are better methods to bond the shafts such as something like a wire brush or even a carbon electrode.

After doing those things above, I measured the voltages between each piece of exposed wetted metal and the half cell. I am well within the safe range for corrosion prevention.

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post #7 of 37 Old 10-15-2011
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Quote:
Many people in the Marina also hang a clamp on "Grouper" zinc anode due to stray currents causing galvanic corrosion.
Stray current does not cause galvanic corrosion. Stray current corrosion and galvanic corrosion are two entirely different things.

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Yeah, you can over do it with anodes. They will react with the copper in the bottom paint. Foggy
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From what I've read it is possible to over do it with zincs. There has to be the correct amount of sacrificial anode to neutralize electrolysis. Too much will create current where it didn't exist before.
You cannot "over do it with anodes" No matter how many anodes you affix (if they are all of a kind) the voltage potential does not change. You can only over protect by using improper anodes (zinc, aluminum-indium, magnesium etc.) for the water the boat is in.

Electrolysis has nothing to do with it. Electrolysis is a chemical change in an electrolyte due to the passage of current.

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Yeah, you can over do it with anodes. They will react with the copper in the bottom paint.
Simple physics ..... there can be no electrolytic reaction without contact and anodes should not be in contact with bottom paint.

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They work although there are better methods to bond the shafts such as something like a wire brush or even a carbon electrode.
A carbon electrode will most certainly cause galvanic corrosion in stainless steel.

Petar : There is more mythology on this topic than anything else in boating and it an extremely complex issue. Either hire an ABYC Certified Corrosion Technologist or take the corrosion course yourself but take all dock walker advice with a grain of salt.

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Last edited by boatpoker; 10-15-2011 at 09:16 PM. Reason: spelling
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post #8 of 37 Old 10-16-2011
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Boatpoker--

Guess I will eat my bottom paint and chew the carbon.

In past years I saw islands of bottom paint gone from around through hulls after hauling. I never gave it much technical thought and made the assumption that it was the result of excessive zincs. But of course that would be impossible because copper is more noble than zinc. The problem could have been caused maybe by the bonded metal through hull at 0 volts reacting with the -0.4 copper paint. After giving this topic more thought, I agree that adding more zinc or for that matter aluminum alloy designed for anode applications in salt water will cause no harm.

Carbon brushes. My brushes that ride on my SS shafts are made from machined oilite bearings (oil impregnated sintered bronze). They do work because I have measure their performance. But there is a risk of an oil film developing on the shafts that could reduce the effectiveness of the bond between the two metals just as transmission oil does; that is the reason for shaft bonding.

I do remember seeing what I believed was carbon used for this purpose in commercial shaft bonding products. And yes, carbon is in the most noble or cathodic range on the metal corrosion charts whereas SS in in the mid section between the most and least noble range. But my guess is that teh rotating shaft would provide a wiping action that would polish the bond between the carbon and SS. My shafts are 1 1/2" in diameter. It would take many lifetimes to weaken such mass. So there may be a reason for making a trade off between potential corrosion and providing a better shaft bond.

And bottom paint tastes terrible!

Foggy

Last edited by foggysail; 10-16-2011 at 02:25 PM.
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post #9 of 37 Old 10-16-2011
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Of course it is possible to over-protect a boat with zinc. When creating a galvanic cell (such as when protecting an underwater metal part with an anode) the water around the part becomes alkaline. Many anti fouling paints are softened and blistered by alkali. Hydrogen bubbles may also form under the paint surface, causing it to lift off. The more anodes you use, the worse the effects. I have seen many, many examples of anti fouling paint being damaged like this.
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post #10 of 37 Old 10-16-2011
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And on a wooden boat the issue of too many zincs causes the wood to decay around any fasteners near the zinc.

Brian
Living aboard in Victoria Harbour
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