Zinc boat anodes not working!

A. Right, first statement - I'm not sure what the coating you've got on your boat is, Or how this will behave on boats. But, passivated zinc is much more reluctant to corrode than plain zinc. I've seen a study where passivated zinc displayed "white rust", the build up of zinc corrosion products took 20 times as long to occur, and the time to "red rust" where the underlying steel corroded, took twice as long to occur. This was a salt fog spray test, some what different to your situation, and the passivation was a chromate, what yours is - I don't know. The effects of differing aeration will be very strong with a boat. Simply the medium the zinc is in effects is corrosion potential, the area well below the water line will have a lower oxygen level in the water, compared to the area at the water line, where the oxygen level will be at a max', it's corrosion potential will also be higher, thus the water line zinc would corrode to protect the below water line zinc. I don't know enough about boats to say you've got enough zinc anodes, or that their distribution will be correct, but your manufacturer probably does. Does anyone out there actually experienced in this field?

A. Dr. Clark,

By now you have already found the answer to this but just in case.... The passified zinc coating. I know when a zinc anode passifies that it no longer works or burns. I remove them and install new ones when that happens. Generally zinc anodes are not pure zinc, but have other trace elements in them such as cadmium. Anyway, the cadmium in zinc anodes keeps it burning clean, and when you wipe off the anode with your hand it reveals bright shinny metal.

On your steel hulled vessel with 44 anodes, which to me would seem adequately protected through sheer number, the anodes would protect the passivated zinc coating on your hull from deterioration. I believe that this is your question.

There is a meter you can buy that you can ground to your hull and measure the potential of the vessel to find out if in fact it is adequately protected. You drop the other end in the water which is a silver/silver chloride cell. I believe if you read 850 millivolts or above, that you are adequately protected. Too high and you will blister off coatings.

I will take the liberty here to assume that you worry to darn much, so you will probably need to buy one of these meters. You can also buy a reference electrode that can be attached to your hull permanently that has a readout in the boat. That is what you really need.

On the other hand, if you haul your boat out and see orange spots, or can see orange spots in the water, your hull is rusting and you are not adequately protected.

I have a 50 foot steel hull and have about 10 or twelve anodes installed, have had no problems.

As for the anti-fouling capability of your coating, I have serious reservations for anything that does not contain cuprous oxide. Preferably tributyltin-cuprous mix, except you cannot get the good stuff anymore.

As for overcoating, any good underwater metal epoxy should do, read the can. Regards, Dave

A. Doctor Clark,

Regarding the lack of corrosion on your boat-hull, it is for two main reasons. Firstly, you have a great deal of protection from the anodes you have fitted. Most boat owners who have zinc-coated hulls usually only fit anodes on the stern around the propshaft/rudder areas due to dissimilar metals.

Certainly Secondly, certain zincs will absorb magnesium from the water in the North Sea and form a 'barrier' layer which passivates the zinc to a high degree. Once the anodes have worn away, as per your reply from a another devotee of this site, the zinc will indeed start to go quicker. The 'mapping' of your hull by placing an electrode in the water and walking the length of you vessel and taking resistance readings on the steel work is also excellent advice you have been given. This exposes any weak areas which are prone to early corrosion.

Being a chemist, I have been involved in the coating of marine vessels with zinc for many years and have been through many of the scenarios which can be encountered. The ideal system on top of a zinc-silicate is an epoxy MIO primer (micaceous iron oxide: small overlapping leaves of metal/aluminium contained within an epoxy medium to greatly retard the migration of water vapor to the surface of the steel) and a coat of anti-fouling paint. Most cuprous and trybutyltin coatings have been banned in many countries. In some cases the copper-based anti-foulings have reacted with the primer and/or the metal itself.

!! Hi, "the good stuff", tributyl tin, is strongly discouraged by the Rotterdam Convention, and internationally banned by the International Maritime Organization because it is both highly persistent and bio-accumulative, with profound impacts in the unbelievably low parts per billion range. Whether we wish we knew then what we know now, or whether we wish we didn't know now what we didn't know then, the fact is now we know: it's horrendous stuff; don't even think about it.

More on tributyl tin:

• Tributyl tin on Wikipedia
• Thread 30418, "Developing new paints that incorporate biocides"

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Hi Dr. Clark.
You can obtain a good MIO epoxy from companies like MG Duff International, Jotun Paints, International Paints, Leighs Paint, Sigma Paints, etc.

A. There are several companies in the USA that will custom make metals/metal alloys to your specifications, and I'm sure its the same overseas.

A. If you're looking to make your own anodes, beware of not only lead as an impurity, but other metals such as iron that will detract from the current output of the zinc anode. The military specification MIL-A-18001K allows for a maximum of .006% lead , and .005% iron content. With the higher levels of these impurities, the anode will become less efficient and may passivate depending on the amount of impurities in the anode. When cathodic protection is achieved with anodes, the protection is created by galvanic corrosion from one metal to another by a natural low voltage circuit. If this system is interrupted in either the short term, or the long term, your cathode (or structure) will receive the electrical current damage. A helpful website may be www.harboranodes.com

A. The zinc galvanizing itself, not the whole zinc galvanized steel item, is what would become an anode in this situation. Because the coating is thin, if it remained unpainted it would not last long. But if it is galvanized and then painted, the galvanizing is not much exposed, and the coating will offer good protection because the steel under a scratch in the paint will be protected. But the galvanizing can't offer cathodic protection to the rest of the boat because it can't corrode.

So, galvanizing might be a fine pretreatment under paint, if done right, but it's utility as a sacrificial anode is probably pretty marginal.

A. I believe Magnesium is even more susceptible to galvanic corrosion than Zinc, so that might slow the process more (or the Magnesium bars might just get eaten up faster).

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A. I can't speak for the efficacy of any particular "impressed current system", and presumably it's possible to install them wrong. But the theory is sound--I believe they can stop electrolysis completely.

A. Hi Paul. Regardless of how well painted, a steel hull vessel in salt water will profit greatly from zinc anodes.

Regards,

A. The 'right qualities' needed for galvanic corrosion include a metallic current path as well as the liquid current path, so the copper exchanger cannot cause galvanic corrosion. The cast aluminum components will be anodic to the cast steel block though. Consequently, an anode could be beneficial. I understand that anodes must be magnesium rather than zinc to be useful with fresh water.

A. Anytime we have dissimilar metals and water we have set the stage for galvanic action to occur. Most all marine shops should be able to get zinc 'sticks' for you, these sticks can be found in various shapes and sizes, commonly used in the cooling system as a 'sacrificial metal' to help control electrolysis.

Replace a fitting or two in you plumbing with a similar fitting that also has a smaller hole for a 3/4 inch pipe plug, use the zinc stick that comes with a 3/4 pipe plug on one end. Buy spares, you can screw them on/off the special pipe plug.

Depending on the hours of service you will figure out the service life of the size sticks that you are using. Visual inspection alone is not enough, zinc can 'honeycomb', always tap the zinc to ensure it is still solid. Replace the stick if more than 50% used.

Do this on BOTH the fresh-water system and on the raw-water system. Preferably 2 places in each system.

I have further slowed the galvanic process on raw salt water plumbing by linking the pipes across the hoses with stainless cable and hose clamps, making a direct metal to metal path bridging the pipe gap formed by any hose.

A. Hi Paul. I don't think it can possibly have any adverse galvanic effect.

A. It is not really a wonder that your zinc anodes are not degrading as they are the more noble metal than the aluminium your boat is made from. This means your aluminium will corrode to protect your zinc . You should use magnesium anodes for aluminium, Zinc can used to protect steel.

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A. Must disagree with you, Marcus.

Contrary to your statement, zinc is in fact anodic to aluminium and provides protection in salt water.

In fresh water, the surface of the zinc doesn't remain clean enough and magnesium is needed to protect aluminium. But in salt water, zinc is the standard protection for aluminium.

A. Hi. Bill is right; aluminum vs. zinc is one of those funny galvanic relationships ...

Although the first step in electroplating onto aluminum is almost always an immersion into "zincate", wherein the aluminum dissolves and the zinc plates out onto it, i.e., the aluminum is sacrificial to the zinc ... in more dilute solutions, i.e., the "sea water series", zinc is sacrificial to aluminum.

I always had zinc anodes on my aluminum engines (salt water boats) and they worked fine. Maybe you are expecting anodes on the outside of the boat to protect the inside of the boat when there is no continuous fluid path between the two. Or maybe proper electrical contact is not being maintained between the aluminum and the zinc for some reason. For sacrificial anodes to protect a given area of metal, you must have both a conductive fluid path and a conductive metal path between the two.

Good luck.

Regards,