Corrosion on steel hull around zincs

Need advice!
Last year I bought a 38 ft steel yacht. She has been on the hard since last November. With the snow finally gone I wanted to put on a new coat of antifouling today. I discovered several places that not only there were blisters in the old antifouling, but rust beneath, definitely not only from this year. See pictures. The PO was obviously a great believer of covering rust or no rust with antifouling, hoping that what he/me don't see will go away
The affected areas are all around zincs - about halv of them. I would have thought that the areas around the zincs should be the best protected ones?
I will wire brush the affected areas and build up a new coat with epoxy primer and 2-part PU paint. But I would really rather avoid having to do it again in a year or two.Has anybody some advice why this happened and how to avoid it in the future?
Regards
Walter

 

Seeing how they are localized, their shape, the shape of the paint build-up in the area with obvious hand touch-up, and the condition of the existing zincs; my money would be that they are the heat affected zones left by the removal of the old zinc fasteners. Was the boat just out of the yard with new bottom paint and zincs when you bought it?

 

The electical field may be too strong...the paint system is being burnt.

The anode should not touch the steel hull...only the steel tabs on the anode should physically be in contact with the hull. Try moving the anode away from the hull by bending the tabs. .

Use a good epoxy primer system when you repaint. when water touches the steel the affects of the anode burn on your paint system are magnified.

 

Jehardiman, Michael; Thank you both for your input! The boat was build in 1999 and, as far as I know, has not been repainted (new antifouling, though). There are some very minor rustspots, but in general the paint coat is in pristine condition. Michael: How far away from the hull would you put the zincs?
Regards
Walter

 

I dont know the correct distance. A few mm. No contact with the paint system is what is important.

Many installations use a plastic sheet to insulate the anode from the paint system

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It really doesn't look over zinc'd right now, and the zincs/studs look too fresh for a '99 hull so they have been replaced. <<shrug>> Anyway, as Michael says you will need to clean the metal bright, feather back the paint edge and repaint all places where the paint has lifted. Make sure you use a good epoxy system to prevent porus conductivity.

 

Walter in reply to your PM:

Following on from the other posts..

No problems really, pretty run of the mill localised coating failure requiring a bit of patching.

Just check the interior to make sure it's nothing coincidental that started inside. Clean to bare metal and epoxy a couple of coats before antifouling.

I recommend epoxying and antifoulong the back of the anodes and then after the anode nuts are tightened epoxy the studs and nuts too.

 

Mike, thank you for your quick reply!

That's a relief to hear!

I will get to it next week - providing the yard doesn't object ...
Walter

 

Painting the back of the anodes should do the trick...I never thought of it.

 

Too many zincs?

Thank you all for good advice! In the meantime I cleaned the affected areas and started to put on epoxy primer.
All in all there were 21 zinc anodes on the hull. This seems like some serious overkill to me. I found Mikes recommendations here:
http://www.boatdesign.net/forums/materials/blisters-around-zincs-steel-boat-23941.html
According to this there should be a distance of 6m between the anodes. It's about 2 meters on my boat - see picture (sorry for the bad quality).
Since it will be rather late in the season before I have her in the water I don't plan to have her out of the water before 2016. I am very tempted to remove halv of the zincs. What do the experts think?
Walter

 

The correct way to anode a boat is with a voltmeter. Its a science. You cant just add a couple " extra" just to be safe.

Perhaps 6m is too much. Who knows ?

In fresh water your boat may burn with zinc anodes.

In sea water it may be correct.

Only a voltmeter will tell you.

Google anode, voltmeter and there is plenty of literature on the net.

Kasten Marine was always a good site for metal boat info. http://www.kastenmarine.com/articles.htm

...the "metal boat society" is also worth a look

 

Walter

I'd remove the anodes closest to the stem and those on the LE of the keel and those first abaft the keel.

Bearing in mind the total mass of zinc should be calculated at least to be close to the required mass as I detailed in your link.

 

Do not use SS studs. There shouldn't be more than 0.15v difference in the anodic index of hull and studs.
Otherwise the studs will corrode the anode fixing legs much faster than the hull.

 

Would corrode the local zinc off the anode legs but not the base steel strap while the anode has any zinc remaining.

If you paint the studs with epoxy you can use anything really, SS is common although I spec steel studs on a steel boat. Paint the andode straps and the studs and nuts after installation with epoxy. I also recommend painting the backs on the anodes with epoxy\antifoul.

 

Stainless studs are Ok...mostly for practice reasons.

The key to good electrical anode bonding is the washer. Regulations state "steel serrated washer".

I always use thread lubricant on studs. I always use a torque wrench. Ive not heard any argument against thread lubricant.

Most of the boats I service use steel...not stainless, studs and fasteners.

Purchase a box of steel serrated washers and Replace this washer at each anode inspection.


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