Electrolytic Corrosion-where to start?

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Kurrawong_Kid

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When my boat was recently lifted for mid season scrub, it was noticed that the stripper had dropped off the P bracket and where it had been there was significant corrosion. In addition the main anode had worn away considerably and the shaft anodes less so. The boat was launched in February with two shore power heaters on a time clock. In previous years I have done the same with no corrosion noted,
I have made 2 modifications since launching. I have fitted a water tank (12v.) gauge of the impedance type (from Force 4) and extended a 240v. ring main. Gould either of these have set off the corrosion?
Or might it be from "hot" water in the marina?
The boat is fitted with a sterling battery charger and sterling galvanic isolator. Any advice on how to start to track down the source of the corrosion would be very welcome.
 
Kurrawong_Kid said:
When my boat was recently lifted for mid season scrub, it was noticed that the stripper had dropped off the P bracket and where it had been there was significant corrosion. In addition the main anode had worn away considerably and the shaft anodes less so. The boat was launched in February with two shore power heaters on a time clock. In previous years I have done the same with no corrosion noted,
I have made 2 modifications since launching. I have fitted a water tank (12v.) gauge of the impedance type (from Force 4) and extended a 240v. ring main. Gould either of these have set off the corrosion?
Or might it be from "hot" water in the marina?
The boat is fitted with a sterling battery charger and sterling galvanic isolator. Any advice on how to start to track down the source of the corrosion would be very welcome.
Click to expand...

unless you have done something incredibly stupid extending the 240volt ring main should not be a problem.

This is presumably the tank gauge
830033.JPG
Hopefully if installed correctly that wont cause any problems either but I have not been able to find any information about it, even on the Zig Electronics website.

You galvanic isolator will protect your anodes from corrosion caused by low voltages from galvanic sources. It wont protect you from electrolysis driven by a 12 volt source or from mains voltage sources.

You don't say what you hull anode is intended to protect. What is it bonded to. Not to bronze skin fittings hopefully.
 
Check the bilge pump wiring and any other wires that could be in bilge water, wires to lights on mast if keel stepped, etc for bare wires and/or loose connections.
I agree with VicS that the AC wiring you added shouldn't be the problem.

I am curious about the anode as well.
 
Thanks all for your replies. Yes it is that type of water gauge. The main anode is a sausage shaped Duff and is bonded to all skin fittings. As far as I can tell the anode has done its job and the skin fittings seem not to have been effected. The yard manager suggested that the bonding to the P bracket may be defective and that is why there has been corrosion at its extremity-I've not yet been able to dive beneath the cockpit floor to check his theory, but that does not explain the corrosion in the first place.
I will look at the bilge pump as the first point of suspicion.
 
Kurrawong_Kid said:
Thanks all for your replies. Yes it is that type of water gauge. The main anode is a sausage shaped Duff and is bonded to all skin fittings. As far as I can tell the anode has done its job and the skin fittings seem not to have been effected. The yard manager suggested that the bonding to the P bracket may be defective and that is why there has been corrosion at its extremity-I've not yet been able to dive beneath the cockpit floor to check his theory, but that does not explain the corrosion in the first place.
I will look at the bilge pump as the first point of suspicion.
Click to expand...

Skin fittings hopefully made of bronze or DZR brass are corrosion resistant and should not need the protection of an anode.
Indeed one of the recommendations of the MAIB investigation into the near loss of the fishing vessel "Random Harvest" a few years ago was that they should not be connected to the anodes at all.
It will certainly lead to an accelerated rate of loss of the anodes even if nothing more serious..

If the "P" bracket is bonded to the same system the bronze or DZR skin fittings may be the cause of the corrosion esp if it is an aluminium alloy.

But. What metal was the stripper made of. If it was stainless steel or a copper alloy ( eg bronze or brass) and was fastened to an aluminium alloy "P" bracket corrosion of the "P" bracket would have been inevitable

Lessons should also been learnt from the recent experiences of a forum member in which a connection between a bilge pump and a skin fitting resulted in electrolytic corrosion of the skin fitting leading to the sinking of his boat. (and because it was deemed to be due to corrosion his insurance company refused to pay out for his loss!)
 
Thanks VicS. The P bracket is certainly not aluminium. I suspect the skin fittings are all bonded because the yacht is of 1984 vintage (Oyster Heritage) and was probably manufactured to comply with the USA standards of the day.
I'll take note of your advice.
My main task is to find what is causing the wasting of the anode and the corrosion, which has not happened before. I do wonder whether I have picked it up from other boats moored alongside the marina pontoon.
I shall go next weekend, armed with my meter to see if|I can find any obvious leakages.
 
Your galvanic isolator should protect you from currents from low voltage galvanic sources. For example it will prevent your anodes attempting to protect all the marina piling or a steel boat moored nearby.

It won't however offer any protection against electrolysis driven by higher voltages. Even from say faulty 12 volt wiring on a nearby boat connected to the shorepower system.

There can also be a problem caused by 240 volt equipment with switch mode power supplies such as some modern battery chargers. It is discussed in This article

There is only one certain way of avoiding problems associated with shore-power. That is to unplug it!

Faulty 12v wiring on your own boat could be disastrous.
 
2 problems?

Hello everyone,

You might have 2 problems and I doubt meter investigation will prove useful. But wait and see.

The first problem is related to the stripper. Corrosion was located here, according to you. It should have been protected by the shaft anode. Faulty alloy of the stripper?

The second one concerns the worned main anode. How much was worned and what equipments are connected to it? Is the hull GRP and is the shaft connected to main anode (my guess is GRP and not connected)?

Anyway, this is why equipment potential should always be checked after new mounting or anodes change, just like it is on bigger boats. It prevent damages before they occurs.
 
I would remove all through hull bonding wires. Bonding these was common practice in the past but is not now. In the days before marinas and shorepower it was ok but now all it does is invite corrosion to skin fittings that are all of one alloy unnecessarily.

I believe VicS is incorrect about the galvanic isolator. It will protect you from your own stray current but not that of others. It is designed to block up to 1.2 volts of your stray current through a diode array but will do nothing for others.

The only certain way to prevent problems with shorepower - leave the marina. It can have an effect on your underwater metals if you are not plugged in if close to other boats with AC/DC leaks.

More problems are caused by DC in your boat than AC.
 
I believe VicS is incorrect about the galvanic isolator. It will protect you from your own stray current but not that of others. It is designed to block up to 1.2 volts of your stray current through a diode array but will do nothing for others.
Click to expand...

Its is current arising from galvanic sources involving metal under water parts of a boat, the shorepower earth conductor and something else, perhaps parts of another boat, connected to the shorepower earth and also in the water.

For example the anodes on one boat, the shorepower earth circuit and perhaps the hull of a steel boat nearby. Without a galvanic isolator in the circuit the anodes will be at risk.

Possibly also anodes, shorepower earth and nearby steel piling could cause the same problem.

Aluminium hulls of course of particular concern to those who own them.

Not really your own stray current nor someone else's for that matter but a current arising from two dissimilar metals in the water that are connected via the shorepower earth.
 
If any of you underwater metal components are an anode to a neighbour's cathode a galvanic isolator will not help. Your zincs will disappear quickly followed by other underwater metal.
 
mitiempo said:
If any of you underwater metal components are an anode to a neighbour's cathode a galvanic isolator will not help. Your zincs will disappear quickly followed by other underwater metal.
Click to expand...

Of course it will help

That is just what it does do by interupting the circuit which is otherwise completed via the shorepower earth.

No circuit, no current, no corrosion !
 
If a galvanic isolator was that perfect of a solution all you would have to do is install one and there would be no risk. But it is not. If zincs are dissolving quickly there is current in the water, from another boat's AC, DC, or your own DC. It is a partial solution but not the total one.
 
mitiempo said:
If a galvanic isolator was that perfect of a solution all you would have to do is install one and there would be no risk. But it is not. If zincs are dissolving quickly there is current in the water, from another boat's AC, DC, or your own DC. It is a partial solution but not the total one.
Click to expand...
If I don't find any obvious problem with either the 12v. or 240v. wiring, leading to leaks, I suspect this may well have been the problem. Will ask neighbours in the marina to examine their stern gear carefully when they next lift out.
 
mitiempo said:
If a galvanic isolator was that perfect of a solution all you would have to do is install one and there would be no risk. But it is not. If zincs are dissolving quickly there is current in the water, from another boat's AC, DC, or your own DC. It is a partial solution but not the total one.
Click to expand...

It stops currents from galvanic sources, ie dissimilar metals. These produce a EMF of less than 1.2 volts, which the galvanic isolator will block.

Currents arising from faults on 12 or 24 volt DC sytems or from 240volt AC mains circuits are a different matter altogether and the only solution is to clear the faults.
Electrolysis due to a fault on 12 volt system is likely to cause very rapid attack of the anode whatever metal it is. It could be a bronze or DZR skin fitting as it was in the case I mentioned at the end of post #7

A galvanic isolator will do what it says on the tin ... prevent galvanic corrosion that might result from being connected to a shorepower system.
It wont prevent electrolysis caused by faults on electrical installations. I have not suggested that it will.
 
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