Steel hull pitting

[tkeithlu]

Wow! You've gotten a ton of detailed advice, with only a small bit of it contradictory, which is unusual! Let's summarize:

1. Bond everything; you have no chance of isolation approaches with a steel hull. That includes your propeller shaft/propeller; send me an address and I'll ship this spare brush to you in Chile.

2. I'm not sure why aluminum is preferred, but can tell you that my starting points that led me to do it were ABYC standards, my surveyor, and Nigel Calder.

3. I think you've got the idea of the silver test electrode; you just hang it over the side, connect it to one side of your multimeter and the other to your hull. Get a volt or more. Don't leave home without it.

4. You need more anodes, whether you choose zinc or aluminum.

5. Anodes of course corrode. The aluminum oxide or zinc oxide surface doesn't do the job. They have to be cleaned to bare metal in order to maintain effectiveness. Going five years without a pull and clean sounds like lost effectiveness, but that's what you've got the test electrode to tell you.

6. I think you're on path. Best of luck with it. I'm looking forward to your sharing your knowledge with others who need it.

[RaymondR]

I had similar problems and found that the hf radio, which was grounded to the steel hull, was the problem. I installed an isolater (which consists of a bunch of capicitors which provide rf earthing whist blocking DC) and this solved the problem.

[Mollymawk]

We’ve definitely got lots of useful advice here, thanks everyone! Sorry for the delay in getting back on here to follow up.

I will have a look at the books you recommend, hpeer and boatpoker (and you’re quite right to point out that galvanic corrosion is the correct term rather than electrolysis).

We've also ordered a reference electrode, to use once we’re back in the water.

tkeithlu, thanks for the offer of your spare brush for the shaft! I’ll follow up with you by private message. I definitely think we need to make sure everything is bonded.

I’m inclined to agree that we don’t have enough anodes, and we're planning to add more. It’s interesting to see that some people have less though without problems (hpeer). I’ve also read about overprotection and too much zinc causing blistering in the paint. We do have blistering in the paint… Hmm…

To clarify, we have never gone five years without cleaning / replacing the anodes. We did go around 18 months at one point, but normally we dive and check them quite regularly, and at the very least wire brush them in place, but often remove them to clean on board or replace with new ones.

hpeer: you make a good point about the fact that for the zinc epoxy to work the particles of zinc must not be electrically isolated. And I can say that it does work (it does protect the metal remarkable well when there are small chips in the paint), so presumably that is the case. (I'm not sure though why "the top coatings must be maintained perfectly if you have zinc underneath". The whole point of the zinc is to protect the metal when the top coatings are damaged.)
I’m not sure that it follows that the if the zinc is not electrically isolated the same must be true of the copper. The epoxies are very different – the zinc-rich epoxy is an epoxy paint whereas the copper is just in epoxy resin. Also, of course, the copper is on top of several other layers of paint, not directly on the steel.
I don’t see that the copper particles being electrically isolated from each other and the hull would stop the antifouling from being effective. Firstly, after applying the Coppercoat it is ‘burnished’, or sanded back to remove the top layer of epoxy and expose copper to the water. Secondly, the epoxy in the original Coppercoat is a water miscible epoxy which is supposedly somewhat permeable (however, our DIY stuff with normal epoxy resins works just as well…).

Thanks everyone for the help so far. I’ll certainly follow up with what we end up doing, though as ever with this sort of thing, it may be a while before we know what has made a difference…