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Galvanic Erosion


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12 hours ago, ditchcrawler said:

Saw these this week,  what does the team think

DSCF7333small.jpg

Perhaps the owners suffer from canal sickness.  After all in the 60's many cars used to be "earthed" to prevent travel sickness :P

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10 hours ago, Tony Brooks said:

If the first one is used to connect the hull to the piling and thus the soil and water I think that it will be setting up a cell so corrosion is inevitable. At least the piling is galvanised so with a bit of luck that will be eaten first but then what?

No idea about the second photo without knowing what's on the other end of the cable. maybe its a centre anode which is probably OK or maybe its another ground connection completing a cell circuit and that is not. Anyway I can not see much signs of heat damage to the paint at the boat end so maybe its just glued on, in which case its insulated and a waste of time, effort and money - whatever the owner thinks it is supposed to do.

I agree with Tony. In the first pic, the earth lead looks like it is there to connect to the boat so making a circuit between earth ground (the bank) and boat ground - exactly what you are trying not to do. Any difference in potential between the bank and the boat will cause a galvanic cell and some metal disappearing from somewhere. I certainly would not do this.

Again the second one is bad if it is used to complete a circuit to shore but ok if just part of the boat. This wire could just be ensuring something on the boat is earthed well. One example is if you were to put a radio transmitter on board that required a 'very' good earth (all transmitters need an earth plane but some more than others). In another life, I did a lot of Ham Radio stuff and if I was putting a big 40M or 80M vertical on the boat, I would be looking to get a good ground plane by connecting i) to the boat hull - an ideal ground plane for this wavelength/freq and ii) the water. If i was not convinced the hull was earthed to water I may try a wire like this as a temporary measure. The hull is likely to be earthed to water via the rudder/propshaft but I have yet to try it.

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The croc clip might be to earth a generator when running on the bank side, or it may be to earth the gennie when refilling with petrol to reduce the chance of sparks/fire.

The second picture may not be a cable, but maybe a length of orange pipe passing through a hole in the side of the boat, but I can not think of any reason for having such a pipe.

Edited by Chewbacka
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Back on my trying to accrue more information about this plague- galvanic corrosion. Am I correct that as it requires a electrolyte in the narrow boat world- that's the canal water- it only occurs at below or just above the waterline? The reason for this question is that as I`m contemplating DIY Zinga and wonder in the value of doing it as high as the rubbing strafe. 

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11 hours ago, jddevel said:

Back on my trying to accrue more information about this plague- galvanic corrosion. Am I correct that as it requires a electrolyte in the narrow boat world- that's the canal water- it only occurs at below or just above the waterline? The reason for this question is that as I`m contemplating DIY Zinga and wonder in the value of doing it as high as the rubbing strafe. 

You are correct that water is the electrolyte for canal boats and galvanic corrosion can only occur where a battery of 2 metals and electrolyte are present.

My10 year old boat was treated with Zinga and 2 part blacking  from new.

The Zinga goes up to the rubbing strafe and the 2 pack to the gunwales. 

Recently taken out of the water for inspection and advised that the blacking is still good for at least another 2 years.

Well worth doing it  my opinion.

Edited by cuthound
Spillung
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On 19/04/2017 at 10:51, Flyboy said:

I have a 58' nb and have 8 anodes. Front,  back and 2 on the sides. The side anodes are low profile so don't project beyond the base plate as the sides angle inwards at the bottom. Sides anodes certainly won't make any noticeable difference to the performance of the boat.

I had the same setup but when I had her surveyed last year I was told to remove the ones off the side as too many anodes turn the boat into a giant battery.  I removed one off each side.

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4 hours ago, Psycloud said:

I had the same setup but when I had her surveyed last year I was told to remove the ones off the side as too many anodes turn the boat into a giant battery.  I removed one off each side.

Did the surveyor explain why that was bad? If you have too many anodes it can cause the paint/blacking to fall off locally to the anode but I would not expect that to happen on a 58ft boat.  Duffs suggest the anodes only have an effect for about 6 ft all round, that 12 ft overall and 12 x 4 = 48, not 58.

The reason we fit them is to turn the boat and anodes into a battery so the anodes gets eaten away in preference to the steel.

I just can't see the justification in what he said on the face of it.

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2 minutes ago, Tony Brooks said:

Did the surveyor explain why that was bad? If you have too many anodes it can cause the paint/blacking to fall off locally to the anode but I would not expect that to happen on a 58ft boat.  Duffs suggest the anodes only have an effect for about 6 ft all round, that 12 ft overall and 12 x 4 = 48, not 58.

The reason we fit them is to turn the boat and anodes into a battery so the anodes gets eaten away in preference to the steel.

I just can't see the justification in what he said on the face of it.

That's all he said I'm afraid.  It was Tony Tucker who said it.

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I guess it was assumed too many anodes may cause overprotection causing hydrogen bubbles on hull surface which can cause disbonding of blacking. There is a word for it..... polarisation iirc.

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On 10/05/2017 at 17:53, Tony Brooks said:

Did the surveyor explain why that was bad? If you have too many anodes it can cause the paint/blacking to fall off locally to the anode but I would not expect that to happen on a 58ft boat.  Duffs suggest the anodes only have an effect for about 6 ft all round, that 12 ft overall and 12 x 4 = 48, not 58.

The reason we fit them is to turn the boat and anodes into a battery so the anodes gets eaten away in preference to the steel.

I just can't see the justification in what he said on the face of it.

 

I'm 'shocked' that a surveyor would advocate the removal of anodes.

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8 hours ago, Mikexx said:

 

I'm 'shocked' that a surveyor would advocate the removal of anodes.

I am sure that he had what Mark99 described in mind but it seems odd for a surveyor to say that in the circumstances quoted UNLESS he saw evidence of local of it. But if he did he shoudl be duty bound to explain it to his client properly.

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Polarisation and coating disbondment is a well documented feature of too much protective current. However its normally associated with impressed current systems where a forced "pumped" current is put through the metal via a transformer rectifier ( the potentials are can be much higher than can be achieved via sacrificial anodes naturally occouring potentials). The anode being a groundbed in those cases.

Perhaps in a very good electroyte like seawater / brackish its possible to get overprotection from too many sac. anodes.

 

Edited by mark99
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4 hours ago, mark99 said:

Polarisation and coating disbondment is a well documented feature of too much protective current. However its normally associated with impressed current systems where a forced "pumped" current is put through the metal via a transformer rectifier ( the potentials are can be much higher than can be achieved via sacrificial anodes naturally occouring potentials). The anode being a groundbed in those cases.

Perhaps in a very good electroyte like seawater / brackish its possible to get overprotection from too many sac. anodes.

 

Magnesium anodes (as used on most canal boats) do have too much 'voltage' if used in sea water.  But it is not a case of too many anodes, rather it's the wrong metal.  So if you had Mg on a salty water boat I can see him telling you to change them all.

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On 12/05/2017 at 08:01, mark99 said:

Polarisation and coating disbondment is a well documented feature of too much protective current. However its normally associated with impressed current systems where a forced "pumped" current is put through the metal via a transformer rectifier ( the potentials are can be much higher than can be achieved via sacrificial anodes naturally occouring potentials). The anode being a groundbed in those cases.

Perhaps in a very good electroyte like seawater / brackish its possible to get overprotection from too many sac. anodes.

 

Your post mirrors the consensus in an earlier topic.

 

 

 

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  • 2 weeks later...

Reading the topic regarding earth return and the explanation for how and why the subject of corrosion again reared its head. Is it possible - probably stupid idea and again showing my ignorance but is it possible that static electricity plays any part?

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3 hours ago, jddevel said:

Reading the topic regarding earth return and the explanation for how and why the subject of corrosion again reared its head. Is it possible - probably stupid idea and again showing my ignorance but is it possible that static electricity plays any part?

 

Static electricity is all about surface charge and the currents involved are negligible when compare to those required for galvanic corrosion.

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