Steel hull/Coppercoat/Stray current - ouch!

[FurryWritingDesk]

We had our boat hauled out last week, as part of the toe rail needed replacing. Since then, we seem to have embarked on a rather unpleasant voyage of discovery. A bit of background is probably helpful.

The boat was re-Coppercoated last year. Anodes were replaced. She'd been in the same berth for about 18 months, but the level of growth wasn't too bad and the anodes were in ok nick, but sensible to have them replaced.

Last summer, a big MFV (steel hulled) was moved onto our pontoon. It is a liveaboard and is permanently plugged in, and attached to the water supply. We're both moored side on to the pontoon, with our bow pointing towards the MFV. And yes, we do have a galvanic isolator for our boat. We only have her plugged in to shore power when we're on her, and leave her with all systems off.

Last week, the boat came out with a huge amount of growth on her, firmly attached. All the anodes were almost completely eaten. But worst of all, we have severe pitting on the hull, most notably on the bow.

There appear to be some areas of cathodic calcification (I've learnt some new words in the last week).

I would love it if anyone has any thoughts or suggestions as to what we can do or, indeed, what we could have done differently.

We seem to be facing the prospect of having her blasted back to bare steel and starting again, as well as finding a new home for her. None of which is either cheap or particularly convenient.

 

[sailorman]

Looks like you have manufactures a battery

 

[BlueChip]

Difficult to see how that can be galvanic action from the MFV as you unplug from the mains when not on board.
I wonder if you have some sort of power leakage on your boat that's causing this.

 

[JumbleDuck]

Could the MFV be leaking enough electricity into the water to create a significant potential gradient around your boat? It might be interesting to stick a couple of multimeter probes into the water, as far apart as you can manage, and see if there is a potential difference between them. Try AC and DC.

 

[KellysEye]

If calcium is covering all the rusty areas then it doesn't looks like stray current corrosion that doesn't have calcium on it, the metal is just rusty. Out of interest did you apply a coat or more of epoxy to isolate the copper from the steel? If you didn't then it's likely to be galvanic corrosion caused by self-induced current created by electrical potential of two dissimilar metals in contact with an electrolyte. As with anodes there is a calcium build up.

 

[FurryWritingDesk]

re: the coppercoat - we put a layer of epoxy onto the old coppercoat, then gave 4 layers of the new coppercoat. It didn't go straight onto steel. The old coppercoat had been on for about 9 years and had done its job properly.
The bits of hull that are being exposed are showing copper, rather than steel - we think this implies that the coppercoat is being sacrificed where the anodes have decayed so far that they can't do their job.

 

[KellysEye]

>The bits of hull that are being exposed are showing copper, rather than steel

OK the calcium build up is quite impressive and the decayed anodes wont help, I wonder if just needs more anodes, or replace them more often. It's certainly an electrical problem if the copper coat is being sacrificed.

 

[vyv_cox]

It has been reported many times on these pages, by many people including the manufacturers, that Coppercoat does not conduct electricity. The copper grains are totally surrounded by epoxy that isolates every one. There can be no galvanic reaction between Coppercoat and anything and I believe it is even stated to be OK for aluminium saildrives.

 

[Pasarell]

I think we need more information because there could be several factors in play.
What was the original anticorrosive scheme applied before the first Coppercoat, and when?
Has there been any damage / defect to the original coatings before this last recoat?
What surface preparation was carried out before the barrier coat of epoxy last year?
What was the barrier coat? VOC solvent? Solvent free? Water based? Did it contain any metal pigment? Zinc? Aluminium?
How was the barrier coat applied and at what film thickness? How did you measure thickness?
Who applied the Coppercoat last year and how?
Where is the boat moored?
What is the water like? Salinity? Pollution?
Have you added any new electrical equipment or had any electrical problems on board?
Have you added any new skin fittings?
Did you change the number or size of anodes when you renewed them?
Sorry about the long list of questions but my first reaction is that there's more than one event taking place although they could be compounding each other.

 

[john_morris_uk]

It has been reported many times on these pages, by many people including the manufacturers, that Coppercoat does not conduct electricity. The copper grains are totally surrounded by epoxy that isolates every one. There can be no galvanic reaction between Coppercoat and anything and I believe it is even stated to be OK for aluminium saildrives.

Agreed. I don't know why people keep thinking that it will affect galvanic action.

 

[maby]

>The bits of hull that are being exposed are showing copper, rather than steel

OK the calcium build up is quite impressive and the decayed anodes wont help, I wonder if just needs more anodes, or replace them more often. It's certainly an electrical problem if the copper coat is being sacrificed.

Copper is on the wrong side of iron in the galvanic series to be sacrificed in preference to the iron. http://en.wikipedia.org/wiki/Galvanic_series

 

[Pasarell]

I didn't understand the OP comment about copper either, but anyway this is almost certainly not a galvanic reaction (and I only say almost to avoid ruling anything out at this stage!) but it could be electrolytic - at least partly. Either from his own boat or from some external factor. The cathodic chalking certainly suggests that although the deposition patterns are unusual. Could also be some coating disbondment resulting from anode density. Could be lots of things, or a combination of several

 

[FurryWritingDesk]

I think we need more information because there could be several factors in play.
What was the original anticorrosive scheme applied before the first Coppercoat, and when?
The first coppercoat was applied in 2006, before we owned her - no idea what was used prior to that.
Has there been any damage / defect to the original coatings before this last recoat?
No, the first coppercoat was lovely, but getting towards the time expired state
What surface preparation was carried out before the barrier coat of epoxy last year?
Jetwash and manual scrub. My husband still whinges about it.
What was the barrier coat? VOC solvent? Solvent free? Water based? Did it contain any metal pigment? Zinc? Aluminium?
Epoxy as supplied by the Coppercoat people
How was the barrier coat applied and at what film thickness? How did you measure thickness?
Can't answer this one, done by the yard and Mr FWD
Who applied the Coppercoat last year and how?
See above - done by the yard and Mr FWD over a 5 day period. Plenty of time left for it to cure properly.
Where is the boat moored?
Inner harbour at Ramsgate
What is the water like? Salinity? Pollution?
Salt water, and the inner harbour flushes albeit there's a lock.
Have you added any new electrical equipment or had any electrical problems on board?
No new electrical kit, the only thing we've done is to remove, service and refit the Ampair windie.
Have you added any new skin fittings?
Nope, all as is.
Did you change the number or size of anodes when you renewed them?
Replaced like for like.
Sorry about the long list of questions but my first reaction is that there's more than one event taking place although they could be compounding each other.
I concur - it seems really odd, and the guys in the yard are perplexed as well. I'm hugely grateful for all the insights people are sharing here and it does seem to be a combination of things. Looking to get an electrical survey done whilst the poor girl is out to see if there is anything "wrong" with her electrical set up.

 

[JumbleDuck]

Agreed. I don't know why people keep thinking that it will affect galvanic action.

The copper surely cannot be completely encapsulated, can it, or the coppercoat wouldn't work. Isn't the idea that the copper particles next to the surface are exposed to the sea? In which case, I'd have thought that you would get a lower-resistance route than usual through the sea, as electricity could spent some of its time going through copper. It might not taeke part in galvanic action directly, but it might extend the usual line-of-site rule about where it could take place.

 

[VicS]

The copper surely cannot be completely encapsulated, can it, or the coppercoat wouldn't work. Isn't the idea that the copper particles next to the surface are exposed to the sea? In which case, I'd have thought that you would get a lower-resistance route than usual through the sea, as electricity could spent some of its time going through copper. It might not taeke part in galvanic action directly, but it might extend the usual line-of-site rule about where it could take place.

Each tiny copper particle on the surface may well be exposed to seawater but they are insulated from each other and from the substrate. So the coating does not form a conducting layer either as a whole or between the water and a metal hull.

 

[Marsupial]

I don't have a Megger but a simple test with a digital Meter on my GRP Copper coated hull showed infinite resistance even when the probes were a few milimeters apart. So its hard to conclude that copper coat has any effect on conductivity except as an insulator.

 

[JumbleDuck]

Each tiny copper particle on the surface may well be exposed to seawater but they are insulated from each other and from the substrate. So the coating does not form a conducting layer either as a whole or between the water and a metal hull.

"... electricity could spent some of its time going through copper ..."

I was thinking of electricity hopping from copper grain to copper grain through seawater.

 

[KellysEye]

>>The bits of hull that are being exposed are showing copper, rather than steel

Looked at another way copper isn't being exposed the whole hull is covered in copper, but parts of it being covered in calcium. The question is why, I looked up 'what causes calcium to build up on a yacht hull' and there is no answer with corrosion, anodes, stray current etc in it everything is saying it does because if a boat sits in salt water a long time calcium will build up and then mostly offering to clean it.

 

[VicS]

>>The bits of hull that are being exposed are showing copper, rather than steel

Looked at another way copper isn't being exposed the whole hull is covered in copper, but parts of it being covered in calcium. The question is why, I looked up 'what causes calcium to build up on a yacht hull' and there is no answer with corrosion, anodes, stray current etc in it everything is saying it does because if a boat sits in salt water a long time calcium will build up and then mostly offering to clean it.

Calcium carbonate can be deposited.

I dont know if it occurs with sacrificial cathodic protection to any great extent but it can with impressed current systems, The reactions which occur at the cathode result in an increase in pH locally. The increased pH leads to formation of a calcium carbonate deposit from the dissolved calcium bicarbonate.

I don't think this is what has happened in this case, however. The carbonate deposit would be local to the anodes not spread across large areas.

If the deposit is the result of some electrolytic action I would be looking at the possibility of and onboard electrical fault, but I have no idea what it might be. possibly a fault on another nearby vessel, but that would probably only cause any problem if both were connected to shorepower for prolonged periods. A GI wont offer protection against this sort of thing... they are only effective at preventing the effects of galvanic action from dissimilar metals.

 

[Pasarell]

Yes, you can get cathodic chalking with sacrificial anodes but usually only when the system has gone out of balance. It is far more common with IC systems than sacrificial.

I think (and not possible to be certain from this remote position) the problem is a combination of an old and tired anticorrosive coating being stressed with a new epoxy scheme on top, probably combined with an electrical fault on the boat. As VicS says the fault could be on another boat but is much more likely to be on the OP boat itself.
If the original anticorrosive has any weakness then an electrical fault will exacerbate it and help that original coat to disbond from the steel. Once disbonded water will get to the steel and corrosion sets in. An electrical fault will promote pitting in the steel rather than a more even corrosion pattern.
We know the anticorrosive had been on the hull since at least 2006 and possibly longer. We also don't know the type and quantity applied. If it was new, good quality, and applied properly to well blasted steel in 2006 then I would expect the previous owner to have told the current owner or to have it logged in the papers for a steel boat. I'm guessing but would suggest it predates 2006 and may not have been done to the best standard. HP washing and scrubbing last year may have opened up any weak points or burst blisters and the new barrier coat would only mask that temporarily.
New anodes could also help to show up weak points, especially if the anticorrosive was getting a bit thin. However if this was the case you would normally only see it in the from of blisters around the anode that, when opened, would show clean shiny steel underneath before it quickly rusted.
Before looking to the boat next door I would suggest a careful audit of electrical systems on board to make sure there are no leakages to the hull. A worn sheath on a cable is the most likely cause, where the item being powered possibly still works but it is also now connected to the hull.
Obviously the affected areas need to be recoated and that probably means the whole underwater hull. Ideally blast to Sa2.5 followed by a high performance anticorrosive scheme applied to the correct film thickness. If blasting is not possible then at least grind the surface back to achieve St2 followed by a good conventional anticorrosive. Whichever system you use make sure the surface is salt free before coating. If the corroded areas are pitted you should probably also have some plate thickness measurements done at this stage.
I hope you manage to find the cause(s) and it would be interesting if you posted back here with any conclusions. Good luck with the rectification work.