Galvanic corrosion of one keel on a Moody S31

[Terpeg]

My starboard keel on my Moody S31 is showing signs of galvanic corrosion, large white 'pox marks' mainly on the inside face. This is the first time this has happened in 10 years of ownership. The sacrificial anodes on the hull and sail drive have decayed very quickly ( quicker than usual) and are working. There are no obvious reasons for the sudden onset of corrosion .
This has only started to happen since installing a sterling battery charger which is hard wired to the batteries and hard wired to the boats 240v system, however the shore power has not been connected whilst afloat since installation,
I have been advised to fit an additional anode to the affected keel, however, this is dealing with the effects of the problem not curing it! Any thoughts would be greatly appreciated!

 

[VicS]

Only just seen this question.

I guess it has fallen victim of "new user syndrome"

Therefore bumping it up.

No idea of the cause

Full details of the sterling charger installation would be relevant facts perhaps.

Any other alterations or additions to the electrical system.

Boat location ??? any changes ??

What's grounded to the keel(s)

Photos of these large white pock marks ???

Why do you think it's galvanic corrosion.

You say the anodes are working! You have checked with a meter ( eg Galvatest ) and a reference electrode ??

 

[Terpeg]

Hi VicS ,
Many thanks for your reply, It is all a bit of a mystery, I have owned the boat now for 10 years , it is on the same drying harbour mooring for the last 7 years and the adjacent boats have remained unchanged for 5 years to my knowledge. Sadly I didn't take photo's of the keel before rubbing down and priming them last week. However they took the form of fairly large 5- 10 cm diameter 'blisters' with white flakey crust!! Hence the assumption its galvanic corrosion an assumption supported by other members of the yacht club.
I will need to recover the instructions for the charger from the boat, but from memory it was nothing very complicated , the two sets of output wires were connected to the two batteries, and the mains connection was connected via a fused switch in to the 240 electrical system, but i will check to be sure.

I am not aware of anything being grounded to the keel, as the keel bolts are not clearly visible , however, somewhere in the back of my mind i seem to recall someone once saying the 240 shore power is ground through the keel, hopefully someone may be able to confirm.

I have again made an assumption that the anodes are working, purely on the basis that they have eroded quite rapidly this season , i change them every year.

 

[RichardS]

Full details of the sterling charger installation would be relevant facts perhaps.

But if the boat has never been connected to shorepower since the charger was installed how could the "corrosion" be anything to do with the charger?

Richard

 

[Iliade]

Is the anode wire still electrically connected to the problem keel? (Or has it a high resistance)

 

[Terpeg]

I am not suggesting it is the charger causing the problem as it has not been connected to shore power, but this is the only change to the boat since the problem has arisen,

 

[Terpeg]

Should the anode wire be attached to keel.

 

[Iliade]

Should the anode wire be attached to keel.

According to traditional thinking, yes, though really one should use separate anodes to protect the ironwork and the cuprous parts. However, given today's better paint systems, it seems people now prefer to let their keels look after themselves.

http://www.mgduff.co.uk/leisure-craft/cathodic-protection.html

 

[Terpeg]

Many thanks for that info, perhaps I should just fit separate anodes to the keel? Would be the same as bonding it to the main anode ?

 

[Tranona]

Many thanks for that info, perhaps I should just fit separate anodes to the keel? Would be the same as bonding it to the main anode ?

Should be no need to have an anode on the keel. It is cast iron so there is no metal for it to interact with to cause galvanic action. It is probably local impurities in the casting that are causing the antifoul to lift. Clean it out and recoat.

BTW if you have a saildrive (which has its own anode) what is your hull anode for? There is nothing that needs protecting. Hull anodes are usually only required to protect shafts and propellers on conventional stern gear, which you don't have.

 

[VicS]

But if the boat has never been connected to shorepower since the charger was installed how could the "corrosion" be anything to do with the charger?

Richard

As the Op says its the only thing thats changed. It's connected to the batteries and it's connected to the shorepower system earth.
Maybe some obscure fault/ set of circumstances is causing electrolysis.

Id also be concerned, more concerned! about the increased rate of loss of zinc from the anodes esp the saildrive anodes ........ shades of Nigel Mercier's catastrophic sail drive corrosion always in the back of my mind.

 

[charles_reed]

I think I can see the way VicS mind is working - no need for 230v to be applied - you've made a circuit.
Sounds more as though the keel was having a deposit of zinc oxide - from your existing anodes.
Though a remarkably arcane subject, galvanic corrosion is pretty random when and how it strikes - I doubt if there's a real risk of the keel falling off, certainly fitting an anode to the keel appears to be a totally redundant action.

 

[Iliade]

Should be no need to have an anode on the keel. It is cast iron so there is no metal for it to interact with to cause galvanic action. It is probably local impurities in the casting that are causing the antifoul to lift. Clean it out and recoat.

BTW if you have a saildrive (which has its own anode) what is your hull anode for? There is nothing that needs protecting. Hull anodes are usually only required to protect shafts and propellers on conventional stern gear, which you don't have.

If the keel is isolated, then it will not suffer galvanic corrosion. If it is not isolated, then iron will corrode galvanically in accordance with its place in the galvanic series. *If* the original issue was indeed galvanic corrosion, and not something entirely unrelated, then the keel cannot be isolated and a cell exists.

 

[KellysEye]

On our steel boat we had a galvanic isolator on both our 12 volt and 240 volt systems. To fix corrosion the only guaranteed way to stop it coming back is sand blast to bare metal and paint on two coats of zinc rich epoxy primer. Do the painting fast when the humidity is low otherwise you risk flash rusting.

 

[vyv_cox]

I suspect you (and your advisers) may be jumping to conclusions as regards 'galvanic'. There is usually some sort of pattern to galvanic corrosion, e.g. it occurs adjacent to the second metal, or on the side facing it or something similar. Isolated blobs don't sound like that. Photos might be helpful.

 

[KellysEye]

>I suspect you (and your advisers) may be jumping to conclusions as regards 'galvanic'.

He did say "This has only started to happen since installing a sterling battery charger which is hard wired to the batteries and hard wired to the boats 240v system, however the shore power has not been connected whilst afloat since installation". Which implies stray current corrosion and thus the need for galvanic isolators for the 12v and 240 volt wiring.

 

[VicS]

>I suspect you (and your advisers) may be jumping to conclusions as regards 'galvanic'.

He did say "This has only started to happen since installing a sterling battery charger which is hard wired to the batteries and hard wired to the boats 240v system, however the shore power has not been connected whilst afloat since installation". Which implies stray current corrosion and thus the need for galvanic isolators for the 12v and 240 volt wiring.

I dont know what these mysterious "stray currents" are or where they will have come from. Galvanic currents can be an issue if the shorepower is connected but the OP says it is not.
Galvanic isolators can be fitted in the incoming shorepower earth to prevent currents from galvanic sources.

How/ where do you fit a galvanic isolator to the 12 volt system and what exactly is its purpose ?

My fear is that electrolysis is occurring driven not by the very low voltages arising from "galvanic sources" but from the 12 volt system due to some obscure fault which may or may not be related to the installation of the battery charger. A galvanic isolator wont stop current if it originates from the 12 system.

 

[GrahamM376]

If the keel is isolated, then it will not suffer galvanic corrosion. If it is not isolated, then iron will corrode galvanically in accordance with its place in the galvanic series. *If* the original issue was indeed galvanic corrosion, and not something entirely unrelated, then the keel cannot be isolated and a cell exists.

According to MOA site in answer to the OP's question, other owners have noticed more corrosion on starboard keel and it's bonded to boats earth system whereas the port one isn't.

 

[Terpeg]

Having read Charles-reed's comments it makes sense that the white pox marks are indeed deposits from the existing hull anode which on the starboard side of the hull nearest the keels and has eroded rapidly, so if this is the situation how do I break the circuit ?

 

[Tranona]

Check whether the keel is actually grounded to the electrical system as Graham suggested.

As I suggested earlier there is really no need for a hull anode as there is nothing that needs connecting to one. The saildrive has its own anode and your seacocks do not need bonding - so remove the hull anode.