Too much zinc?

[bob26]

Can you overdo the cathodic protection?

If so, what is the result?

I have mild steel bilge plates and rudder blade. In the past I have had 1kg doughnut anodes on the outside of each bilge plate and the same on one side of the rudder. I usually get quite a bit of activity on the bilge plate anodes (they last 2-3 years and produce a lot of white oxide-like coating over the zinc). The rudder blade anode has never needed replacing.

Last winter I found some quite deep corrosion pitting the bilge plates. I read somewhere that ideally a steel plate should be protected on both sides. So when fitting out for 2010 I added 1kg anodes to the insides of the bilge plates. So I have 5x1kg doughnuts down there with an additional clamp-on shaft anode to protect the bronze prop.

The result was that pretty quickly a lot of the antifouling on the steel parts seemed to shed (same International Cruiser a/f as always). This was especially true of the rudder where I got a bit more activity on the anode than usual. Naturally, I got weed and barnacle like I'd never had before on those parts which lost their a/f. I hope you can see from the pictures that the exposed steel rusted a bit as you would expect over a season but not excessively. The shedding seemed to continue throughout the year as some of the exposed steel did not get a chance to grow weed or barnacle.

Any idea what is going on here?

I'm on a half-tide mooring as far up Chichester Harbour as you can get...

 

[Pasarell]

Too much Zinc

I think your answer is probably in the last sentence of your post - your half tide mooring.
You can have too much zinc and that will result in what is called cathodic disbondment of the surrounding coatings. In your case, however, it doesn't sound like too much zinc. More likely the surface of the metal is exposed and corroding which leads to the coating being pushed off.
Did you clean and prime the metal before antifouling? Antifoulings are not designed to go directly onto bare metal.
Cathodic protection will only protect your keel when it is fully immersed in water. If the keel is exposed to air either at low tide or when the boat is lifted out in the winter the anode gives you no protection whatsoever. You will need to clean the keel and coat with a good anticorrosive primer in sufficient thickness to prevent corrosion when exposed to oxygen

 

[bob26]

Mebbe so...

Did you clean and prime the metal before antifouling? Antifoulings are not designed to go directly onto bare metal.
C

Yes I did originally a few years ago - cleaned to bare (ish) metal with stiff steel brush in angle grinder, priming with several coats of Primocon as per International guidance - but since then all that seemed necessary was to spot repair where damaged (mostly where it grounds or those corrosion pits) and overcoat the largely intact a/f. But you may well be right that this is a paint adhesion failure... with everything coming off including the primer. Maybe the extra zinc is just a coincidence then? The failure seemed to start off in vicinity of the anodes within days of launching ... barnacles and weed soon grew there. The clean bare trailing edge of the rudder was photographed at the end of the season. Thanks for your thoughts.

PS: Further thought on the pix: the best preserved a/f on the rudder is the shaft which is stainless steel, not mild steel.

 

[rotrax]

Hi, I am pretty sure that if your anodes are covered in a white substance they are not working.Perhaps the bilge plates are not bonded to earth.

 

[Bilgediver]

Hi, I am pretty sure that if your anodes are covered in a white substance they are not working.Perhaps the bilge plates are not bonded to earth.

As the anodes on the keels are only for protecting the keels then the only connection required is directly from the keel anodes to the keels. This must be a good connection electrically . Earth bonding in the boat is not required for the keels. The shaft has its own anode!

Yes too much zinc can cause gassing from the steel which will lift off the paint. This might not be the problem here.

 

[KellysEye]

The white covering on the anodes is calcium build up that will stop the anodes working. If there is a lot of pitting on the anodes a wire brush won't remove it all. Muriatic acid will get it off but you will need a lot of it. Or just replace them.

Yes you can over-anode a steel boat. The result is paint comes off aound the anodes and corrosion sets in. We have six anode points but only use four of them.

 

[coreng]

If you suffer from over protection you will have bubbles (due to hydrogen forming ar the surface of the metal), soon followed by paint blistering. You will also observe huge spreading of shells on bronze and inox. Your hull potential is too low... Should be measured between (-) 1050 mV and (-) 800 mV with a reference electrode. You are probably over (-) 1200 mV...

 

[awol]

If there is a lot of pitting on the anodes a wire brush won't remove it all.

I seem to remember that wire brushing an anode ain't a good idea. Something about wee bits of wire being embedded and passivating the anode.

 

[VicS]

Something about wee bits of wire being embedded and passivating the anode.

Almost the opposite maybe. Little bits of steel wire would lead to an increased rate of loss of zinc as the anode attempts to protect them. However I reckon a few little bits would make precious little difference. You would not leave it looking like a zinc hedgehog with steel spines presumably

 

[bob26]

Many thanks

Thanks for all the help above.

I don't usually brush the anodes to remove the white stuff - I chip it off with an old chisel. But this year the bilge plate zincs were all brand spanking new so the wire brush theory does not apply.

I suspect I have suffered both of the suggested causes - the a/f pain definitely bubbled up round the zincs as soon as we launched in Spring. But i think the flaking off evident in the pictures by the end of the season was more likely a paint adhesion failure. For one thing its worst on the rudder where the zinc was not increased... though I did get initial paint bubbling around the zinc early on in the season (not so evident in the pics because the site is now covered in barnacles, weed and rust)

The initial paint bubbling may be because there was too much zinc or the fact I replaced all the zincs on the bilge keels. This may have meant they were cleaner than normal and therefore super active to begin with. Or it may have something to do with the fact I cleaned the bilge plates back to bare steel underneath the anodes to ensure good electrical contact.

 

[zambant]

Measure

I really think you need to take some voltage measurements to see what's going on.

Post the results back here if you like.

You will need a reference cell - and a DVM - cost for cell about £25

Regards

John

 

[bob26]

I really think you need to take some voltage measurements to see what's going on.

Post the results back here if you like.

You will need a reference cell - and a DVM - cost for cell about £25

Regards

John

Measurements between what and what? And underwater?

 

[bob26]

Like this?

Yes you can over-anode a steel boat. The result is paint comes off aound the anodes and corrosion sets in. We have six anode points but only use four of them.

Is this what you'd expect that to look like?

Pic 1 was a bilge keel anode at the end of 2008.

Pic 2 the same anode after removal at the end of 2009 when I replaced all the anodes and put additional ones inside the bilge keels on the same bolt as the outside ones. That seemed to have really got them fizzing within days of launching.

 

[macd]

I'm not familar with Chichester harbour, but if the water where you're moored is brackish, it could be that you need alminium rather than zinc anodes. In the case of brackish water zinc anodes can develop a white coating of zinc oxide (not calcium) which renders them ineffective -- and they remain ineffective even after going out to sea.

I'd suggest you ask owners of nearby boats whether they've had similar issues (or stroll around the hard standing for a look-see). From my own experience in a brackish harbour, not all boats' anodes seem to succumb in the same way: some do, some don't. (I suspect, although it's only a guess, that it might depend on the rate of anode erosion versus the rate of oxide accumulation.) So the fact that a couple of neighbours have not had the same problem would not be conclusive.

More info on the MG Duff website, www.mgduff.co.uk

 

[maxi]

That far up Chi harbour, it is likley that the water is brackish rather than fully saline - in which case an aluminium anode may well be better than zinc.

Talk to MG Duff ( branch by the Chicester Bypass I think) and they could advise re salinity testing and correctly matching anodes.

 

[KellysEye]

>Is this what you'd expect that to look like?

Yes, exactly that. I would be wary of aluminium anodes on a steel boat assuming you don't stay all the time in brackish water. I'd certainly reduce anodes or anode sizes and see what happens before considering Aluminium.

One other thought, when you mix salt water and fresh water it creates electricity, effectively stray current, which causes corrosion starting with small holes around the hull. I don't know if you have a galvanic isolator but if not fit one asap.

 

[bob26]

That far up Chi harbour, it is likley that the water is brackish rather than fully saline - in which case an aluminium anode may well be better than zinc.

I had wondered whether the water was brackish given the amounts of white deposit I sometimes get. The seawater is regularly sampled and analysed at Dell Quay though mostly to monitor for e.coli (sewage) but I will try to find out what the prevailing salinity is.

I would be surprised if it were brackish as any fresh water entering the harbour hereabouts would be miniscule in comparison with the flush of fresh salt water coming up the harbour twice a day - Dell Quay dries almost completely so the salt water is completely replaced at every tide. I would think somewhere like Chichester Yacht Basin would be more likely to be brackish since it only gets a few lockfulls of salt water each tide to offset the freshwater runoff building up there.

 

[Conachair]

One other thought, when you mix salt water and fresh water it creates electricity, effectively stray current, which causes corrosion starting with small holes around the hull.

Well, initially that sounded a bit wierd but there seems to be something in it..

Though it seems to get any useful power you need to keep them apart.
http://www.fumatech.com/EN/Membrane-technology/Membrane-processes/Electrodialysis/
Can't really see how it could be a problem to steel boats in brackish water where there isn't a clear boundary between fresh and salt water.
But being wrong would not be a new experience
Nothing from google so can't be that much of s problem.