Sacrificial zinc anodes

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.

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

Regards,

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
Letter 30418, "Developing new paints that incorporate biocides"

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
July 2013

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.

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Ed. note: For additional thoughts on ship anodes not corroding properly, see also Letter 51262 "Zinc boat anodes not doing their job!"