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Shore Power/Galvanic Corrosion


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1 minute ago, Bargebuilder said:

Hi Tony,

You are wise to question my logic, but:

Anodes on a steel vessel should be spaced such that all the steel is protected and in my experience this is usually the case.

The anode is more noble, so if the galvanic circuit is between the vessel and the pile, the galvanic action will be dominated by loss of aluminium or magnesium over steel, all the time that either is in the locality, even if the steel was uncoated, which of course, it wouldn't be.

The weld between the stud and the anode would not have a significant effect because the current will be too tiny: Tiny increases in resistance would only be a significant factor at high currents. 

I would be horrified if there was a large potential difference between the pontoon and the boats hull. 

I was trying to keep this simple and I still insist that a few hours use of shore power could not possibly justify the cost of a gi or an si in terms of potential damage that might be done to the steel of the hull.

 

I agree that OP's the stated use in the majority of cases the amount of damage would be so small as to be insignificant but:

1. We are talking inland narrowboats here, not seagoing boats or properly designed and constructed barges. In the majority of cases narrowboats do not have anodes placed to protect all the steel. Typically they will have one either side on the front swim and the same at the back.  A few higher end boats may fit middle anodes in recesses in the hull side. This means a large area of the hull is not protected. Few have anodes placed every 4 metres along the hull. To do anything else risks either having anodes on the base plate ripped off or if on the side hang ups in narrow locks.

2. I would suggest that at least half of all narrowboats do not have any form of coating on the base plate. Even if they were any scrape to bare metal would concentrate any damage in that one area.

I still do not see how an anode protects against corrosion caused by current flows between the hull and surrounding water, structures and ground. That current forms no part of the cell that the anode forms with the hull and fittings.

My problem is that a poor shoreside installation might results in several volts difference between the hull and the surrounding water, structures, and ground as could a significant fault on an adjacent boat that uses the OPs hull as part of the current path. Somewhere on here in the dim past this was found to the cause of corrosion. I also understand close by industrial scale power use can also produce higher voltage on the company earth line so although the OP would probably not suffer any damage by his stated use one can not be sure. I think £115 or whatever would be worth paying as an insurance against hull damage but its up to the individual to decide.

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1 hour ago, Tony Brooks said:

I still do not see how an anode protects against corrosion caused by current flows between the hull and surrounding water, structures and ground. That current forms no part of the cell that the anode forms with the hull and fittings.

I'm not surprised that you can't see it Tony because it's rubbish. Anodes have nothing whatsoever to do with stray current corrosion which is what we're talking about here. 

2 hours ago, Bargebuilder said:

I would be horrified if there was a large potential difference between the pontoon and the boats hull. 

Go measure a few hulls in marinas and prepare to be horrified then. It's not in the slightest bit unusual for the protective conductor to be sitting at many volts above true earth. 

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8 minutes ago, Tony Brooks said:

Thank you Tony (Wotever). I was beginning to think that I was gong nuts and did not have the faintest clue about the subject.

Not you :)

Obviously I agree with everyone else that for the OP either a GI or IT is totally unnecessary, however he should definitely bond the earth. 

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Anodes protect against corrosion not galvanic corrosion, although the principle of both is similar in that metal is moved from one type of metal to another according to their position on the galvanic scale.

They do so by being part of a battery, the anode (cluster in the name) is the aluminium anode, the electrolyte is the canal water, and the cathode is the steel of the boat.

A tiny current flows through this battery, removing material from the anode and depositing it on the cathode and thus protecting the cathode from corrosion.

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Just in case people havnt seen it...

 

First and 3rd photos have pits of over 4mm deep, one leaving 1mm of steel left.(20 year old hull with no GI)

2nd and 4th show extensive corrosion with 2mm plus pit (No GI on boat,15 years old)

 

20170824_101714-816x459.jpg

20170824_123438-816x459.jpg

20170824_101643-816x459.jpg

20170824_123428-816x459.jpg

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Some of the posts here talk about the OP's limited use of shore power and the risk of galvanic corrosion. But at the moment the OP is effectively bringing an extension lead onto his boat, so until there is a proper mains installation with a hull-earth bond any use of shore power is irrelevant when it comes to galvanic corrosion. Safety issues aside, it's only once the hull-earth bond is made that the shore power connection and installation of a GI or IT becomes an issue.

Edited by blackrose
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That is exactly it, blackrose. Safety dictates I bond the earth so I will but that then opens, if only slightly in my case, the risk of GC! I could just rip it all out and use a normal extension lead but the current arrangement is safer than that option in that there are no wires draped around the boat and the two sockets driven by the lead are securely fixed. So a quick bond of earth upon entry to the boat and then buy an inline GI seems to be the only way.

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20 minutes ago, Traveller said:

That is exactly it, blackrose. Safety dictates I bond the earth so I will but that then opens, if only slightly in my case, the risk of GC! I could just rip it all out and use a normal extension lead but the current arrangement is safer than that option in that there are no wires draped around the boat and the two sockets driven by the lead are securely fixed. So a quick bond of earth upon entry to the boat and then buy an inline GI seems to be the only way.

Yes, ok. It's not a major investment and in my opinion worth having some form of isolation even if you think you're not going to be on shore power very much.

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  • 3 weeks later...

Some great advice and info, as usual, many thanks. On the back of this I checked through the wiring and found there is an earth, it runs from the back of the last plug socket in the chain to a stud in the engine room. The earth on the plug tracks back to the bus bar in the consumer unit so would I be correct in concluding that this is sufficient?

Edited by Traveller
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29 minutes ago, Traveller said:

Some great advice and info, as usual, many thanks. On the back of this I checked through the wiring and found there is an earth, it runs from the back of the last plug socket in the chain to a stud in the engine room. The earth on the plug tracks back to the bus bar in the consumer unit so would I be correct in concluding that this is sufficient?

Electrically yes, the wire from the back of a plug socket provides the earth bond. From a 'best practice' or even 'usual practice' perspective it's poor. Imagine a future owner who is making some alterations to the fitout and who decides to remove that socket. He might simply put the four wires into a 4-way terminal block, wind some tape around it and hide it behind a panel. Everything would still work but now he's lost his Earth bond and he wouldn't realise that he'd gone anywhere near it.

If the Earth bond comes from the CU's Earth busbar then it's logical and tidy. 

Also, if the Earth connection between any two sockets became disconnected you'd lose the bond. 

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