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  #11  
Old 08-08-2012
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Re: Too much zinc?

Sorry, Rackham, but Le Chatelier's Principle doesn't apply for this oxidation/reduction reaction. Wikipedia is right on the mark in its explanation...

"The design of a galvanic anode cathodic protection system should consider many factors, including the type of structure, the resistivity of the electrolyte (soil or water) it will operate in, the type of coating and the service life.

The primary calculation is how much anode material will be required to protect the structure for the required time. Too little material may provide protection for a while, but need to be replaced regularly. Too much material would provide protection at an unnecessary cost. The mass in kg is given by equation (5).

Mass = (Current Required x Design Life x 8760) ÷ (Utilisation Factor x Anode Capacity) (5)

The design life is in years (1 year = 8760 hours).
The utilisation factor (UF) of the anode is a constant value, depending on the shape of the anode and how it is attached, which signifies how much of the anode can be consumed before it ceases to be effective. A value of 0.8 indicates that 80% of the anode can be consumed, before it should be replaced. A long slender stand off anode (installed on legs to keep the anode away from the structure) has a UF value of 0.9, whereas the UF of a short, flush mounted anode is 0.8.[13]
Anode capacity is an indication of how much material is consumed as current flows over time. The value for zinc in seawater is 780 Ah/kg but aluminium is 2000 Ah/kg,[13] which means that, in theory, aluminium can produce much more current than zinc before being depleted and this is one of the factors to consider when choosing a particular material.

The amount of current required corresponds directly to the surface area of the metal exposed to the soil or water, so the application of a coating drastically reduces the mass of anode material required. The better the coating, the less anode material is needed.

Once the mass of material is known, the particular type of anode is chosen. Differently shaped anodes will have a different resistance to earth, which governs how much current can be produced, so the resistance of the anode is calculated to ensure that sufficient current will be available. If the resistance of the anode is too high, either a differently shaped or sized anode is chosen, or a greater quantity of anodes must be used.[13]

The arrangement of the anodes is then planned so as to provide an even distribution of current over the whole structure. For example, if a particular design shows that a pipeline 10 kilometres (6.0 mi) long needs 10 anodes, then approximately one anode per kilometere would be more effective than putting all 10 anodes at one end or in the centre."
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Old 08-08-2012
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Yes, too much zinc is as bad as too little. It's about keeping the corrosion voltage within the safest range, EG for fiberglass -750 to -1000mV.
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Old 08-09-2012
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Re: Too much zinc?

The "corrosion voltage" is a function of the material chosen for cathodic protection and NOT how much of the material is in place. The amount of zinc present is of absolutely no importance. Whether you have a tenth of an ounce or a hundred pounds makes no difference.

Again...the reaction is galvanic and this description from boatzincs dot com explains the only way "overprotection" occurs:

"Overprotection usually occurs from one the following conditions:

• Using the wrong type of anodes (e.g., magnesium anodes in saltwater);[zinc or aluminum or magnesium are the most common materials used for anodes and each has a different galvanic voltage. Just as a lead-acid battery's nominal cell voltage is ALWAYS about 2.1 volts whether you have a lawn mower battery or a 4D, the voltage developed by a zinc battery in a protection scheme for bronze or iron/steel in saltwater will ALWAYS develop the same voltage.]

• A defective impressed-current corrosion controller;

• Stray DC currents originating from defective wiring or equipment within
your vessel.

The most common source of stray currents is a defective bilge pump, float
switch or wire insulation -- each of which carry +12 VDC submerged in bilge
water. Make sure these electrical devices (and wire splices!) are inspected
and tested for electrical leakage."
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Old 08-09-2012
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Yikes. You need to read their whole site to get yourself educated instead of cutting and pasting selected pieces that seem to fit your understanding of the issues. http://www.boatzincs.com/corrosion-quiz.html
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Old 08-10-2012
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Re: Too much zinc?

Reading comprehension and logic not two of your stronger skills I guess. Sorry. I can see I am wasting my time trying to correct mistaken understanding of the facts. You can have the thread back to spread misinformation now.
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Re: Too much zinc?

Quote:
Originally Posted by fryewe View Post
Reading comprehension and logic not two of your stronger skills I guess. Sorry. I can see I am wasting my time trying to correct mistaken understanding of the facts. You can have the thread back to spread misinformation now.
Well, it is what it is. Here's quote from the site you referred to as supporting your argument.

"Overprotection can create conditions that damage underwater hull coatings, aluminum alloy metals and wooden hulls.

• Steel and fiberglass hulls -- decreased effectiveness of anti-fouling
paints and barrier coatings when made more negative than -1100 mV.

• Aluminum hulls and outdrives -- highly susceptible to alkali corrosion of
its metal, and hydrogen blistering of its paint coatings, when made more
negative than -1200 mV.

• Wooden hulls -- destruction of wood fibers (alkali delignification) occurs
around metal fittings made more negative than -650 mV."


Whether or not I have any logical cognitive abilities and using your very own reference material, I did find that it appears too much zinc protection may not be as good as you think. This topic has been raised in every forum, SA, CF and others. The answer still remains the same, but you do to your boat what you think is fit. Happy trails.
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Re: Too much zinc?

OK. I'll give this one more shot, Sea Hunter and I'm not trying to get in a p**ing contest. Read what you posted above carefully and you will see that the excerpt you posted from boatzincs dot com in fact doesn't mention zinc at all but only the voltage of concern for overprotection.

The OP asks if you can have too much ZINC...not if you can overprotect.

What the boatzincs dot com website makes clear and I tried to point out with my bracketed comments above in my previous post is that the PROTECTION VOLTAGE IS A FUNCTION OF THE MATERIAL CHOSEN TO PROVIDE PROTECTION and NOT the amount of it you use. The voltage that results from the very low voltage battery that results from the anode's use to protect the hull and other components has NOTHING to do with how much of the "stuff" is used.

The points the boatzincs dot com website is trying to make are USE THE RIGHT MATERIAL and avoid stray currents.

I don't know the history of posts on SA or CF or other sites concerning this topic but the physics and chemistry are undeniable...if zinc is the right choice for protecting your equipment or aluminum is the right choice or some other material is the right choice...and develops the appropriate galvanic voltage...having more than is needed for protection will increase cost and weight but has no effect on protection.

Hope this clarifies things for anyone who is reading this.
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Old 08-10-2012
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Re: Too much zinc?

The only problem I've seen with too much zinc on a steel boat, is tiny blisters forming under the epoxy, on otherwise clean, corrosion free steel.
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The quantity of zinc controls amount of voltage depending on the field one's boat generates. Too much zinc changes to value of the voltage lower, not enough higher. This is why too much is not so good, but better than not enough. The issue of amount of zinc or voltage is mute as they go hand in hand. What we're looking for is the correct balance. In water where fields are generated by stray electricity a galvanic controller may be required as the optimum voltage point cannot be maintained with zincs alone. There was a steel boat in our yard this week they beaded. Under the epoxy fairing the boat had pin holes produced by the effect of too much zinc. On fg sailboats cutlass supports burn up and excessive zinc can cause issues with rigging. On powerboats legs have virtually disappeared in one season from galvanic action. I've seen with my own eyes the effect of too little and too much zinc over the years and heard the endless arguments and frankly, just do what you want to do because that's what we're all going to do anyways. I'm going boating.
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Old 08-17-2012
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The debate goes on!

It's amazing that a simple issue like this has such a diversity of opinions. I really didn't expect it when I started this thread.
I called Boatzincs.com and asked them. They said that you can only have too much Zinc on wooden boats because it effects the binder in the wood (who would have thunk!), as discussed on their website.
What happened to Rackham? I would have liked to hear his response to fryewe. This issue has obviously been discussed for a long time elsewhere, and has proponents on both sides. If the too much zinc theory is a wives tale, it's long lived one! I mean, this isn't exactly a subjective issue. After all....we are not discussing whether the Mona Lisa is the best painting of all time. It's basic science (probably well explained by fryewe). You can either have too much zinc or you can't...right?
BTW....any opinions on the gizmo Boatzincs.com is selling to measure the currents in the water using a multi meter Corrosion Reference Electrode. I've tested the water for current using Don Casey's methods but didn't know that tool existed.

Last edited by L124C; 08-17-2012 at 03:18 PM.
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