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post #11 of 32 Old 11-12-2016
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Re: Sad looking zinc's

Aluminum is just fine in salt water. Actually it is better than zinc for pretty much all usage and applications in salt water. It is only a small bit more galvanically active than zinc (50mV), but the increased current capacity gives them 50% more lifespan.

Pretty much all sterndrive and outboard manufacturers require aluminum anodes for salt water now.

Magnesium is the material not to use in salt water.

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post #12 of 32 Old 11-12-2016
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Re: Sad looking zinc's

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Originally Posted by colemj View Post
Aluminum is just fine in salt water. Actually it is better than zinc for pretty much all usage and applications in salt water. It is only a small bit more galvanically active than zinc (50mV), but the increased current capacity gives them 50% more lifespan.

Pretty much all sterndrive and outboard manufacturers require aluminum anodes for salt water now.

Magnesium is the material not to use in salt water.

Mark
No one said aluminum wasn't good in salt water, just that it depletes faster due to being more active (basic electrophysics, more current=more electrons=more depletion). All measurements in the realm of cathodic protection are in (small) millivolts. A small difference in millivolts can make a huge difference in rate of depletion. The mfg's don't care how fast your anodes deplete it's about CYA.

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Last edited by boatpoker; 11-12-2016 at 10:00 AM.
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post #13 of 32 Old 11-12-2016 Thread Starter
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Re: Sad looking zinc's

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Originally Posted by boatpoker View Post
Given that one boat is ok and the other shows quite a bit of anode wasteage, I'd be looking for a little bit of stray current corrosion, bilge pump floats are a very common cause. is there any other corrosion evident ? Is the propeller turning slightly pink along the blade tips ? is the shaft pitted (even minutely) ?

I'd suggest against the use of aluminum anodes in salt water as they are more galvanically active and will disappear even faster than zinc.
That makes sense.

So what other things can cause stray current corrosion?

I'm assuming anything that is connected to the battery that somehow puts voltage in the water?

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Re: Sad looking zinc's

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No one said aluminum wasn't good in salt water, just that it depletes faster due to being more active (basic electrophysics, more current=more electrons=more depletion). All measurements in the realm of cathodic protection are in (small) millivolts. A small difference in millivolts can make a huge difference in rate of depletion. The mfg's don't care how fast your anodes deplete it's about CYA.
You are wrong about the depletion rate. Yes, aluminum anodes are more active than zinc as far as the difference in galvanic scale (50mV difference, compared to 1300mV difference between zinc and magnesium). However, that is only one of the two properties to consider. The other equally important property is current capacity, and aluminum has ~3x greater current capacity than zinc. That means that the aluminum will last ~3x longer than an equal mass zinc. Since anodes are sold by size rather than mass, and aluminum weighs less than zinc, an aluminum anode will last ~30-50% longer than an equal sized zinc after the difference in mass is taken into account.

Using batteries as an analog, the first point covers the nominal voltage of the battery (say 12V), while the second point covers the capacity of the battery (say 220Ah). In this analogy, the nominal voltage of the two are practically the same, while the capacity is 50% more with the aluminum.

I'm not making this up - you can check my work...

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post #15 of 32 Old 11-12-2016
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Unhappy 'Sad Zinc's' might lead to other Sadness

Looks like your marina has some serious stray current going on. Presumably there are rules and posted signs prohibiting swimming around the boats?
Divers doing cleaning and repair will likely be temporarily unplugging yours and the boat on either side from shore power while they are working. If you have a diver working (paid or unpaid) you need to get his/her understanding of the potential risk they face diving in fresh water with a local history of current leakage.
(Your bio indicates that you are mooring in fresh water, so my response is based on our experience here in fresh water.)

When you have documented anode wastage, there is a problem. Period. And potentially some risk to swimmers.

Here is an article concerning this.
Electric Shock Drowning (ESD) Explained - Seaworthy Magazine - BoatUS
Kevin has visited our YC and does explanatory presentations around the NW about this danger.

So, there are two parts to the "problem" -- loss of your underwater metal parts on your boat, and, direct risk to anyone in the water around your boat.

Given the national attention this issue has received in the marina industry in the last few years, any marina that ignores current leakage into their waters is approaching a storm of liability, as well.

Our YC marina has spent significant $ on bringing our whole wiring system up to code and just finished changing out all the 30 amp outlets to GFCI breakers. Not cheap, but Due Diligence requires attention.

Not to change the subject slightly, but the changeover to the sensitive new breakers also revealed several boats with bad interior AC wiring or corroded appliances that needed to be repaired and made safe! Bit of an "eye opener" for several of our members, it was.

All the best, and stay safe.
Loren
(Pt Capt at a 150 slip marina)
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post #16 of 32 Old 11-12-2016
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Re: 'Sad Zinc's' might lead to other Sadness

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Originally Posted by svHyLyte View Post
As the zinc's erode, the irregular wasting creates an imbalance of the weight distribution about the longitudinal axis of the shaft. As the shaft rotates, the imbalance causes the shaft to "whip" or wobble which leads to rapid deterioration of the cutlass bearing and your packing gland packing material.
Give me a break.


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Originally Posted by boatpoker View Post
I'd suggest against the use of aluminum anodes in salt water as they... will disappear even faster than zinc.
That is simply untrue.


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Originally Posted by olson34 View Post
Looks like your marina has some serious stray current going on.
Considering that the OP's other boat did not suffer excessive anode depletion, I'd say the best bet is that the issue lies aboard the boat in question, not with some general, marina-wide problem.

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Re: 'Sad Zinc's' might lead to other Sadness

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Originally Posted by Fstbttms View Post

Considering that the OP's other boat did not suffer excessive anode depletion, I'd say the best bet is that the issue lies aboard the boat in question, not with some general, marina-wide problem.
it could also be the slip it was in or the boat that it was next to it. since you can't put both boats in the exact same location. Are the boats equipped with galvanic isolators?
I have found the aluminum anodes to work better and last longer, cheaper also. I check my boat every time the anode is changed with a reference electrode

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Re: Sad looking zinc's

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Originally Posted by colemj View Post
You are wrong about the depletion rate. Yes, aluminum anodes are more active than zinc as far as the difference in galvanic scale (50mV difference, compared to 1300mV difference between zinc and magnesium). However, that is only one of the two properties to consider. The other equally important property is current capacity, and aluminum has ~3x greater current capacity than zinc. That means that the aluminum will last ~3x longer than an equal mass zinc. Since anodes are sold by size rather than mass, and aluminum weighs less than zinc, an aluminum anode will last ~30-50% longer than an equal sized zinc after the difference in mass is taken into account.

Using batteries as an analog, the first point covers the nominal voltage of the battery (say 12V), while the second point covers the capacity of the battery (say 220Ah). In this analogy, the nominal voltage of the two are practically the same, while the capacity is 50% more with the aluminum.

I'm not making this up - you can check my work...

Mark
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Lightbulb Re: 'Sad Zinc's' might lead to other Sadness

Quote:
Originally Posted by Fstbttms View Post
Considering that the OP's other boat did not suffer excessive anode depletion, I'd say the best bet is that the issue lies aboard the boat in question, not with some general, marina-wide problem.
I should have been clearer. You are right to focus first on the OP's boat (and/or a nearby boat). Still, stray current is an under-recognized hazard in most marinas, and a caution seems to be in order.
Regards,
Loren
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Re: Sad looking zinc's

The vast majority (every corrosion survey I have done) of stray current corrosion cases are caused by the boat suffering the problem however it can be caused by other vessels nearby.

I'm outahere. As usual with this issue there are many opinions posted of little value by untrained, uneducated people.
Certified Corrosion Analyst.

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