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Corrosion


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Hi,

Some time ago I found an article that is very important to boat owners, especially if you go to sea. I did not post it at the time as there had just been a boat sunk on the Broads and all sorts of comments and speculation about hull fittings.

With the credit crunch and cheap components being imported into the UK it is worth knowing what you are paying for and what can happen to your boat.

Have a read of this,

http://www.michel-christen.com/2T-H.pdf

Ian :Sailing

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Interesting that somebody had used a gate vave of any quality attached to a skin fitting Ian. ;)

Whilst not ideal, I think I'd rather have a bronze (or stainless, titanium, or even plastic) gate valve than a brass ball valve.

This is quite a worrying report, as I'm not sure how I'd detect the presence of brass skin fittings during a survey (it was noted that the fitting on the yacht appeared perfectly normal when given a scratch test to the outer mushroom, which would be my normal method). I have also heard of cases where moulds for bronze skin fittings have been sold on and used by other manufacturers to cast skin fittings in brass, which say on them that they're bronze - even less chance of detecting that something wrong before they fail.

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Whilst not ideal, I think I'd rather have a bronze (or stainless, titanium, or even plastic) gate valve than a brass ball valve.

Me too, but that just confuses the issue I'd rather have a proper spec ball, period, it's also quite interesting to see the number of underwater skin fittings that are not electricaly bonded to sacrificial anodes of the correct material for the electrolite they're sat in.

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Me too, but that just confuses the issue I'd rather have a proper spec ball, period, it's also quite interesting to see the number of underwater skin fittings that are not electricaly bonded to sacrificial anodes of the correct material for the electrolite they're sat in.

AFAIK, opinions on whether or not to bond skin fittings are divided. As a rule, American boatbuilders do bond all skin fittings, European boatbuilders don't.

Having seem rampant dezincification (of both skin fittings and manganese 'bronze' propellers) in a boat that was less than 2 years old (it was possible to remove slices of copper from the skin fittings using minimal force on a scraper), I would tend to agree with you that bonding is a very good idea.

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With the rapid erosion of my internal engine anodes as a first sign it’s always been a worry to me TD, what are your thoughts on trying to get anodes in “line of sight†to the stuff they are protecting rather than just bonded? It is something I have discussed with a number of people but never had a definitive answer, I raised the question because the big bar anode which is bonded but on the transom so not “seen†has hardly eroded since our boat was brand new so that’s five years now, the other ten or so are well gone every year but I have fitted a Galvanic isolator to see if maybe their pretty rapid erosion is shore power connected and I worry about all the other boats around me using my anodes.

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Here is a thought. Gate valves fitted as standard equipment by boat builder. gate valves turn and appear to shut off/open.

Boat sinks on its mooring due to a burst hose and although the valve is in it's off position it is obvious that it was not working and hasn't been for some time (gate valves can feel as if they are working when they are not).

Would the insurance pay out?

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Here is a thought. Gate valves fitted as standard equipment by boat builder. gate valves turn and appear to shut off/open.

Boat sinks on its mooring due to a burst hose and although the valve is in it's off position it is obvious that it was not working and hasn't been for some time (gate valves can feel as if they are working when they are not).

Would the insurance pay out?

I am not a lawyer, and this is not legal advice.

With that in mind, this would seem a pretty clear case of a latent defect - the faulty gate valve. In such a situation, the insurer would probably pay out, minus the cost of the gate valve or it's ball valve replacement. There is the possibility that they would then take legal action against the boat builder (or possibly a surveyor if the boat had been surveyed) for negligence in fitting the gate valve in the first place.

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With the rapid erosion of my internal engine anodes as a first sign it’s always been a worry to me TD, what are your thoughts on trying to get anodes in “line of sight†to the stuff they are protecting rather than just bonded? It is something I have discussed with a number of people but never had a definitive answer, I raised the question because the big bar anode which is bonded but on the transom so not “seen†has hardly eroded since our boat was brand new so that’s five years now, the other ten or so are well gone every year but I have fitted a Galvanic isolator to see if maybe their pretty rapid erosion is shore power connected and I worry about all the other boats around me using my anodes.

You haven't had a definitive answer, because their isn't one. Cathodic protection is more of a black art than a science, and most of the serious research that's been done has been on large commercial vessels built from steel or alloy, which doesn't translate very well to small leisure craft built from GRP or wood.

Examining things from first principles, if the fittings are bonded by a wire on the inside of the hull, and both the fittings and the anode are immersed in the same body of seawater, then as far as I can see, the fittings should not need to be within line of sight of the anode in order to be protected, as nothing is actually flowing from the anode to the fitting or vice versa, apart from the electrons travelling through the copper bonding wire (zinc, aluminium or magnesium ions are lost from the anode, and hydrogen is generated at the fitting, but the two do not need to mix or transfer across in order for the anode to do it's job of protecting the fitting).

I would suggest that the next time your boat is out of the water, you (or your surveyor) should use a multimeter to check for electrical continuity between the anode in question and all of the fittings that are (or should be) bonded to it. It takes very little corrosion in the wrong place to cause significant resistance in the circuit, and it's a lot easier and more reliable than following all of the bonding wires and manually checking that all of the connections are secure. Also make sure that the anode is not coated with anything that might insulate it from seawater (you're probably well aware that zinc anodes in fresh water can develop a chalky deposit on them which significantly inhibits their effectiveness, even if they later return to salt water, and that painting anodes is a really, really, bad idea).

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All conections are tight and bright with good continuity which i check every year, I even go to the trouble of brightening the bar anode with production paper every year, go figure, I may move it to the skeg on the next lift just as an experiment.

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All conections are tight and bright with good continuity which i check every year, I even go to the trouble of brightening the bar anode with production paper every year, go figure, I may move it to the skeg on the next lift just as an experiment.

Ok, it sounds like whatever's going on, it's not something easy or obvious. I do wonder if it's something to do with the composition of the anode (and/or all of the other anodes). Not particularly easy to test unfortunately. Since it doesn't seem to be working very well currently, I can't see how changing it's position could make things significantly worse, so it's probably worth a try.

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|Impossible to tell Ian, as you probably know I always used ali anodes on the broads as the boat was in brackish apart from short forays to Beccles, the Horning triangle and a few weeks out to sea so it's not really comparing apples with apples. I do suspect that the zinc anodes I'm using now are being eroded faster than I would expect and that's why I've fitted the galvanic isolator to see if that improves it. The one comfort I take is that at least whilst they are being eaten away the rest of the stern gear is being protected, but at around £250 for a set I would like them to last a bit longer than 8 or 10 months :o

There is an unsuspecting (shh don't tell Perry) control to the experiment as we haven't fitted his isolator yet so we will have something to compare as his were behaving the same and we both replaced with MG Duff at the beginning of the year. :naughty:

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There is an unsuspecting (shh don't tell Perry) control to the experiment as we haven't fitted his isolator yet so we will have something to compare as his were behaving the same and we both replaced with MG Duff at the beginning of the year. :naughty:

They certainly fizz in Shotley.

I have been called a lot of things but never a control Guinea Pig :cry

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|Impossible to tell Ian, as you probably know I always used ali anodes on the broads as the boat was in brackish apart from short forays to Beccles, the Horning triangle and a few weeks out to sea so it's not really comparing apples with apples. I do suspect that the zinc anodes I'm using now are being eroded faster than I would expect and that's why I've fitted the galvanic isolator to see if that improves it. The one comfort I take is that at least whilst they are being eaten away the rest of the stern gear is being protected, but at around £250 for a set I would like them to last a bit longer than 8 or 10 months :o

There is an unsuspecting (shh don't tell Perry) control to the experiment as we haven't fitted his isolator yet so we will have something to compare as his were behaving the same and we both replaced with MG Duff at the beginning of the year. :naughty:

Well that reduces the possibility of my only current idea - that the bar anode is slightly more noble than the stern gear anodes and is therefore being protected by them. MG Duff have about as good a reputation for quality control as there is, so I'd probably look elsewhere before getting into testing all of the anodes against a silver/silver chloride half cell.

How similar is the control in terms of proximity to other boats, shore power, etc?

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How similar is the control in terms of proximity to other boats, shore power, etc?

It’s an accidental control rather than deliberate I must make that perfectly clear, we simply have not gotten around to fitting Perry’s GA yet. :oops:

The two boats have less than a foot gap between them and are plugged in the same shore power post (separate sockets) and we are surrounded by other boats most of which are also on shore power.

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It’s an accidental control rather than deliberate I must make that perfectly clear, we simply have not gotten around to fitting Perry’s GA yet. :oops:

The two boats have less than a foot gap between them and are plugged in the same shore power post (separate sockets) and we are surrounded by other boats most of which are also on shore power.

Accidental or not, that's probably about as good a control as you can get without a very large laboratory. I'd be very interested in the results, whatever they show.

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