Keel anodes causing antifouling to fall offon SO39I

[ellij]

After owing my SO39i for 15 years and removing and filling blisters from the leading edge of the keel every haulout I fitted 2 anodes to the keel. (one on each side)
I dug out the blisters and primed the area around the blisters with Primacoat.
I then filled the craters with a mixture of antifouling and micro balloons. I then antifouled with the usual Awlcraft No5 and fitted a teardrop anode on each side of the keel, by drilling and tapping 2 M6 stainless screws for each anode.
3 months later there is a circle around each anode (around 300mm) where all but the original antifouling has disappeared.
The anodes are showing reasonable signs of activity.
Has anybody got any idea what could be causing the antifouling to let go?

[Charlie-Bravo]

I am not a chemist, but I suspect you may have created a zinc iron battery, and the copper in the anti foul is also adding to the battery effect.

Q’s .  is the remaining coating in the circle created really original anti foul, or could it be the original hull epoxy paint?
         is the keel connected to your ground circuit? if so ….. why.
         could the craters at the front of the keel start from mechanical damage to the keels paint coating? as opposed to electrolytic                  corrosion.

Cast iron keels in salt water never seemed a great move to my mind, and once the rust starts it takes quite a lot of grinding and epoxy to stop it coming back. With a jolly good paint job the salt water shouldn’t get to the keel, what precautions did you take after drilling the keel to fit the anodes to prevent salt water ingress to the bolted joint.

You mention anode activity , so this does add to the battery theory, perhaps you have attached the anodes with good intent, but might of been better off (except in the wallet) having the keel shot blasted or ground back and painted, and leaving the keel electrically isolated.

Hope you get to the bottom of it, and report your findings.

CB

[zaphod]

I can't say I have ever heard of putting zincs on an iron keel. This is my first iron keel, but I have been told that rust blossoms on the keel are just a fact of life, and you just have to fix them as they come up.

It sounds to me like you are creating a new problem with the zinc on the keel.

[ellij]

Thanks CB.
I have only inspected this by snorkeling at this stage and yes it is possibly back to the original epoxy, but it is a blue color similar to the antifouling.
The keel isn't electrically connected to the boat.
Yes the original damage was likely to be caused by mechanical damage as the leading edge must hit all sorts of debris during a boats life.
I didn't take any special precautions apart from some Tef-Gel on the treads when attaching the anodes, just screwed them on over the top of the new antifouling.
I think you are probably correct in saying that I shouldn't had done this, but I was advised to do this by a boat yard professional who said he had done this on several similar boats and it had cured the problem.
Cheers

[Charlie-Bravo]

When repairing your blisters, and any rust blooms for that matter, after grinding back / shot blasting it is important to get your primer on quickly, preferably within half an hour, and in good weather. This might seem a bit fussy but it does make a difference to the effectiveness of the repair, if left overnight the rust will have started even if you can’t see it, you then paint over it and will most likely see it again next haul out.
Not that my little rust patches are all cured forever, but they are getting less in number and smaller.

CB

[ellij]

Thanks
It looks like I should take the anodes off asap and at least get some antifouling on temporarily until the next haulout.
Cheers and thanks for the help!

[sawduster]

Sounds like cathodic disbondment from overprotection - www.cruisingworld.com/how-to/prevent-halos-bottom-paint/