Shore side electrical problem

[ean_p]

just berthed a steel boat along side in a marina berth. fendered off with polyprop mooring lines and no electrical connection nor any batteries etc on board. no metalic connection to shore nor power generation on board.....220mV between boat and pontoon with 20-25mA current, and still there when boat and pontoon connected with tempoary cable ......?, is this a cause for concern?

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[MainlySteam]

Sorry if I state what you already know, but have assumed you would not have asked the question if you knew the following.

Any two dissimilar metals placed in an electrolyte (such as sea water) will form an electrolytic couple and a voltage will exist between them. If they are connected together by a conductor a current will flow through the wire or resistance because of that voltage difference.

The shore power ac earth is a metal(s) in the ground/water and your metal boat is in the seawater so there will always be a measurable voltage between them. If a piece of wire is used to connect the two then a current will flow through the wire and one metal(s) and the most "anodic" one(s) will erode. This is galvanic corrosion and is the same reaction as occurs in a battery. It seems that this is the observation that you have made - with respect to the current measured your multimeter in the current setting is effectively a short circuit from the boat to the shore earth so a current will flow through it due to the voltage between them.

Furthermore, other boats nearby will be connected to the shore ac earth by means of their shore cables and so if you are connected to the shore ac earth you are connected to them too - that is you may be connected to the metals on their hull which their own shorepower cable earth is connected to and one or the other, or both of you will suffer galvanic corrosion(assuming they do not use an isolating transformer or galvanic isolator).

So, as long as your boat's hull is not connected to the shore ac earth or in any other way to other metals by a conductor then you have nothing to worry about. If your boat is connected to the shore ac earth, usually through the shore ac cable earth conductor, then you must use a galvanic isolator in that conductor on the boat or else a proper marine isolating transformer. Then no current can flow and no galvanic corrosion can occur - that is what you must aim for. Do not do as some propose, disconnect your ac ground on the boat from the hull to break this current path as it is unsafe.

All the above can be easily shown in the kitchen by mixing up some salty water and putting a piece of aluminium (eg a pie dish) in it together another different metal and measuring the voltage between them (usually the other piece of metal can just be the multimeter probe itself). Similarly a current will be measured if the multimeter in ammeter mode is put between them. Take away the ammeter though and it is obvious that no current can flow (there is no conductor to complete the circuit).

So, you do not have a problem if your boat is not connected by a conductor to other metals (eg the shore side ac ground) in contact with the water even though you may be able to measure a voltage between them. If it is connected to other metals by a conductor (usually the shorepower earth) then you may have a problem and regardless it should be corrected.

John

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[BrendanS]

Is that a yes he has a problem, or he doesn't then? /forums/images/icons/laugh.gif

Woops! sorry! <G>

<hr width=100% size=1>Me transmitte sursum, caledoni

 

[VicS]

Firstly some comments/resevations about your observations, but not in any particular order of relevance.

You do not give the polarity of your readings. If the boat is anodic, ie +ve then there is potential for corrosion but if the boat is cathodic (-ve) corrosion is not likely to occur.

When taking current readings with such low volages involved it is very important that very good connections are made with the multimeter, this means surfaces cleaned to bright metal.

An important chracteristic of an ammeter is that it should have a very low resistance. This is unfortunately not normally the case with cheap (and not so cheap) multimeters. You are likely to find they have resistances of from several ohms upwards on the mA ranges. The way round this is to use the highest current range that will give a meaningful reading. It is better to sacrifice a little pecision in order to get better accuracy. I would not be surprised if with 220mV difference it were possible to get a much greater current flow through a truly low resistance connection

Taking your observations at face value its is possible to calculate that a current of 25mA will in the course of a year remove approx 0.23Kg iron from the hull, but ony if it is anodic.
Spread evenly over several tens of square metres it isn't significant but paint coatings will reduce the effective area and could lead to much greater rates of corrosion locally. It would also be interesting to compare this with literature values for the rate of corrosion of unprotected steel in sea water.

It is worth considering possible reasons for the potential difference between the boat hull and the marina pontoon. There are two possible lines of thought here. Either there is a dissimilar metals situation or there is a current flow between the pontoon and the water.

The dissimilar metals situation would arise if the pontoons are aluminum or galvanised or protected by sacrifical anodes. Each of these would lead to a situation in whch the hull is cathodic and therefore protected to some degree. Even the sacrificial anodes on nearby boats could be the source if they are connected to the pontoons via their shorepower earths.

The current flow could arise if the pontoons were protected by an impressed current cathodic protection system. This again would afford some measure of protection to the hull and were it the case there would be benefits to be gained from bonding the boat to the pontoon. The current flow could also arise from installations on nearby boats and depending on just what is the cause could be in either direction. It is therfore important to establish whether the hull is anodic or cathodic.

Potential problems can probably be avoided by not making any electrical connection with the pontoon or shore and by the use of some sacrificial anodes. If you don't want to fit anodes then you could consider suspending some in the water as owners of aluminium boats often do.

Anyway the first thing is to establish the polarity of your 220mV then you know if there is a potential problem or not.

Just some thoughts which I hope haven't muddied things too much.



<hr width=100% size=1><font color=purple>Ne te confundant illegitimi.</font color=purple>

 

[MainlySteam]

Read the last paragraph of my post Brendan /forums/images/icons/smile.gif - he has a problem if connected with a conductor and does not if he isn't. The poster's question does not state his situation in that respect so an exact yes/no answer cannot be given.

John

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[ean_p]

its at this point my head starts to spin , what with 'conventional' flow etc.......anyway the instrument used was a good quality Fluke and the polarity with the black / common probe to the pontoon and the red probe to the boat produced a negative value !!

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[VicS]

So the boat is -ve. So corrosion will not be caused by the 200 or so mV that you have observed, in fact what ever it is that's giving this effect will be giving you a small degree of protection.

<hr width=100% size=1><font color=purple>Ne te confundant illegitimi.</font color=purple>

 

[MainlySteam]

But the voltage is of no relevance whatsoever if the boat's hull is not connected to the shore by a conductor eg a shore power cable, which is what I think the original post says. Such voltages always exist (refer my kitchen experiment) and are of no consequence whatsoever to the boat.

John

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