Overuse of cathodic protection on aluminum or steel hulls

[Thunderhead19]

Has anyone here encountered or aware of problems with overuse of Zinc, particularly on aluminum vessels. I have heard bad things, but hard facts seem to be hard to come by. My main concern is eroding the passive layer on aluminum or damaging underwater gear, outboards etc. I know some people who use a fantastic amount of zinc, and their corrosion monitors show no ill effects, but is there a limit?

[D'ARTOIS]

Help, help - zinc on alu hulls, what are you for h*****s sake saying? Who told you this "mother of all stupidities" - as an Arab would say.

I go digging, come back on this topic.

[Thunderhead19]

It's difficult for me not to take offence at your response, but I'm a good sport.
I can't believe that an experienced designer has never heard of it being done.
We do it mostly to protect our boats while in marinas. Being moored near a rusty old tug, or any boat that is not properly grounded can damage an aluminum hull if it is unprotected. There are many other factors too. For example, most bottom-paint has copper in it. Imagine putting an aluminum plate in a bath of ionic copper and grounding it.

[D'ARTOIS]

First of all Thunderhead19 what I said is not meant as a insult or offense on the contrary - I was mocking a bit.
Nontheless, - and there might the confusion lay - you probably are pointing at zincchromate and that is different cook. That is even used on aluminium as an undercoat for paint, otherwise paint would blister away from the aluminium. Zinc paint is actually something different and includes also other metals that should not been brought in contact with aluminium.

I aplogise for my wrong wording but later on I understood that you were pointing at zinc chromates; that I didn't realise fast enough, so sorry again.

About 20 years or so ago I went on board of "Schwanensee" a German AC that was completely eaten up by the formation of large blisters around the transom,
a place were all currents running through a hull are coming together, forming a large source of galvanic erosion. To cover up those blisters they had painted the hull with a zincspray that contained also copper and aluminium and that was only for use on steel.
That ship was beyond saving and the guy from Huisman who was with me said only "the torch in it."

[Thunderhead19]

I see now! I'm sorry. I'm actually referring to sacrificial blocks of high-zinc alloy, attached below the waterline, that decay in lieu of the aluminum. Zinc chromates, from what I understand, is anti-corrosion coating.

[cyclops]

I really do not know what is protecting Aluminum Mercruiser props other than shiny paint. Or the prop back from the prop shop. I don't think they put 10 different layers of protection on them.----- How are the inner surfaces of the drive hub protected ?

[MikeJohns]

Originally Posted by Thunderhead19

Has anyone here encountered or aware of problems with overuse of Zinc, particularly on aluminum vessels.

Yes often

Tooo many anodes will blow your paint off. Everywhere there is a pinhole you will get hydrogen gas forming under the paint and lifting it off classic spotty disbondment in patches where the paint is thinner. Then corrosion pits and all the problems Aluminium alloys face.

Calculate the anodes properly and do not overprotect.