Blown Antifouling around Steel Boat through hulls

[BigART]

Is is possible for zinc anodes to be hyperactive?

About 2 months ago, I fitted 4 anodes that I had purchased in South Africa, they were little larger than the usual ones but not by much. On relaunch, the hull voltage was around 0.96 volts, no problem. A few weeks later, I noticed that there was paint bubbling around some of the welded steel pipe through-hulls. I have attached some underwater pictures.

http://www.doitcruise.info/ForumQ_Cockpit_drain_small.JPG
http://www.doitcruise.info/ForumQ_head_pipes_small.JPG

In the eight years we have owned the boat, I have never seen anything like this.

I remeasured the voltage and it had crept up to 1.0 volts so I think this may be the cause. I have now removed an two anodes and the voltage is back down to 0.96 volts.

In the eight years I have owned the boat, I have never had this problem. Has anyone seen this behaviour before? So is it possible for an anode to be too active, too pure or some unusual alloy may be?

Regards

Angus

 

[saltylegs]

Is there more exposed/bare steel underwater than usual as that can cause the anodes to be more "active"

 

[VicS]

Is is possible for zinc anodes to be hyperactive?

About 2 months ago, I fitted 4 anodes that I had purchased in South Africa, they were little larger than the usual ones but not by much. On relaunch, the hull voltage was around 0.96 volts, no problem. A few weeks later, I noticed that there was paint bubbling around some of the welded steel pipe through-hulls. I have attached some underwater pictures.

So is it possible for an anode to be too active, too pure or some unusual alloy may be?

Regards

Angus

Over protection can cause paint blistering ... but I assume you have not increased the number of anodes. It would be pretty borderline for a small increase in anode surface area to have that effect. Its interesting though that removing a couple have returned your reading to normal ,,, was that done using a silver /silver chloride electrode or what ?

Zinc anodes are normally about 99.4% Zn or better so unlikely to be due to abnormally pure zinc. Any chance that you fitted aluminium anodes ? I think if you had fitted magnesium anodes a high rate of loss would be apparent.

Aluminium and magnesium anodes are much lighter I think theres a good chance you might have noticed.

Not much help sorry ... Just thinking out loud

 

[Pasarell]

Too many anodes, or too high a current in an ICCP system, can cause disbondment of the paint coating, Known as cathodic disbondment. Hydrogen is formed at the steel / coating interface and the paint blisters, usually in concentric circles around the vulnerable points such as welds. If you cut off a blister when the boat is out of the water the steel underneath will be shiny bright - and then rapidly flash rust as it is no longer protected.
To overcome the problem either reduce the anodes as you have done or significantly increase the primer dry film thickness around the affected areas.

 

[KellysEye]

>Too many anodes

Agree, too many or too big. On our steel boat we had five anode fittings but only fiitted four.

It could also be stray current corrosion, have you got a galvanic isolator fitted? We had one on both the 12volt and 240 volt systems.

 

[BigART]

Sorry about the slow reply, I have been out of touch for a few days (St Lucia to Antigua). Thanks for all your helpful replies folks.

The measurements were carried out against a piece of silver wire which has always worked well in the past and the readings are within a couple of millivolts of a commercial silver/silver chloride test cell.

I am not aware of any more exposed steel than usual, and the anodes are almost certainly zinc, definitely not Mg or Al. We never connect to shore power AC and only rarely go into marinas so pretty sure there is not a problem there. The throughhulls are welded steel pipes and maybe there is a little more exposed steel inside the tubes but it is hard to tell.

When we purchased the boat, she had 16 anodes, yup sixteen and the antifouling was being blown off all the time. i have since reduced the anodes to 5 and have not had a problem for five years - until now.

There are now signs of rust in a couple of the throughulls so I think the solution is the haul out, repaint around the bad patches using plenty of primer as recommended by Pasarell and buy/fit anodes of the same size as previously - if I can get hold of them...

Thanks again.

Angus