Galvanic corrosion on a swinging mooring?

[skyflyer]

Is this possible? What other reasons for rapid anode corrosion.
Technically with a mooring chin around the anchor windlass, there is an electrical connection between the boat earth and the water - but then there sia anyway via the prop, surely?

 

[TheOrs]

What has changed? Have you previously used this mooring with this boat without anode problems?

 

[skyflyer]

What has changed? Have you previously used this mooring with this boat without anode problems?

Not my boat so don’t know. But I’m on a nearby mooring and have never had a problem. The owner has been told that the problem is the use of chain mooring making a circuit to earth!
I’m saying that sounds “unscientific” and just wanted to check if anyone has a different take on it.

 

[vyv_cox]

There is no truth in this theory. Chain is a poor conductor of electricity anyway, so would be unlikely to conduct. Millions of boats berth on chain moorings. Any underwater metallic part of the boat is at earth potential. If there has been a sudden change in anode life the source is in the boat. Careful analysis with a meter should find it.

 

[skyflyer]

Exactly what I thought Vyv. But nice to have it confirmed by a qualified person. Thanks

 

[superheat6k]

This sounds more like a wider electrolysis problem rather than a purely galvanic one. Galvanic current flow is that caused by the inherent difference in nobility of one metal to another when electrically connected and then immersed in an electrolyte, e.g. sea water. However, the boat likely has a 12v dc circuit and should this part of this leak into the seawater somewhere then the rate of anode disintegration can increase inordinately. An electrolytic leakage of DC current can be much more damaging to anodes and other underwater fittings than pure galvanic currents. AC leaks are even worse, but would be very unlikely to be at play here unless the boat has a generator running.

There is a simple test however. Turn everything off that is normally off when the boat is left. Disconnect the +VE from the battery then measure the voltage between the battery post and the just removed terminal lug - this should be 0 V. If any voltage is measured then there is a current flow going somewhere, and likely via the anodes.

 

[Seajet]

A sudden change must mean something on the boat is different - though there's always the chance someone has dumped a vehicle or something by the moorings especially if half tide !

When my original mild steel lift keel plate corroded badly on a half tide mooring in Chichester Harbour I did some researching and discovered from there to Portsmouth is ' an area of accelerated steel corrosion ' being investigated by the Marine Metallurgy Unit of Portsmouth University.

Their pet theory of the time was microbes which excrete stuff that eats steel, same as are destroying the wreck of the Titanic - but they agreed that was not the case with my old keel when they came to see it.

My new keel plate ( like the old one, galvanised from new ) began to show signs of corrosion quite quickly - when I fitted an anode it stopped instantly - there is nothing electrical touching or near the keel structure inside the boat - so I gave up on the theory, just glad the keel remains untouched.

There are several local factors which may be relevant; it's close to the main power lines to Hayling Island, and a marina close by with shore power - and the whole area was a decoy in WWII to lure the Luftwaffe away from Portsmouth docks, so the mud is full of ordnance inc Magnesium and Phosphour.

 

[eddystone]

This sounds more like a wider electrolysis problem rather than a purely galvanic one. Galvanic current flow is that caused by the inherent difference in nobility of one metal to another when electrically connected and then immersed in an electrolyte, e.g. sea water. However, the boat likely has a 12v dc circuit and should this part of this leak into the seawater somewhere then the rate of anode disintegration can increase inordinately. An electrolytic leakage of DC current can be much more damaging to anodes and other underwater fittings than pure galvanic currents. AC leaks are even worse, but would be very unlikely to be at play here unless the boat has a generator running.

There is a simple test however. Turn everything off that is normally off when the boat is left. Disconnect the +VE from the battery then measure the voltage between the battery post and the just removed terminal lug - this should be 0 V. If any voltage is measured then there is a current flow going somewhere, and likely via the anodes.

Does this test work when the boat is out of the water?

 

[VicS]

Does this test work when the boat is out of the water?

Obviously you won't detect a current flow via the anodes if out of the water but it may still be worth doing the check. If you find a current flow there is a problem but I would confirm a current flow with an ammeter / milliammeter because high impedance digital voltmeters will give a reading when an analog meter won't.... Guaranteed to make a fool of you sooner or later

 

[Graham376]

There is no truth in this theory. Chain is a poor conductor of electricity anyway, so would be unlikely to conduct. Millions of boats berth on chain moorings. Any underwater metallic part of the boat is at earth potential. If there has been a sudden change in anode life the source is in the boat. Careful analysis with a meter should find it.

Would the fact that for some reason the mooring chain is wrapped around the windlass, which may not be isolated from pos and neg boat supply, have any bearing?

 

[vyv_cox]

Would the fact that for some reason the mooring chain is wrapped around the windlass, which may not be isolated from pos and neg boat supply, have any bearing?

Only if there is a fault in the windlass insulation I suspect. Although it does not alter the fact that chain is a poor conductor. I think the fault lies on board, along the lines that superheat has suggested. There are several examples of this type of fault on my website. Electrolytic corrosion

 

[skyflyer]

Surely the windlass, like most equipment on boats and cars, use the body of the device as a negative ground. So in effect the windlass is bonded to earth and hence (assuming the boat is properly bonded) there is already an electrical connection from windlass to battery negative to prop shaft to seawater to earth?
I do not know whether this is a new phenomenon for the boat owner or not. Simply that he has asked (me) for a rope mooring strop because his anode corroded really quickly when he first took the mooring on a chain and he was advised by a "marine engineer: that this was probably due to galvanic corrosion via the mooring chain. I'm afraid I dont know where he kept his boat previously.
It seemed to me that this "marine engineer" was calling himself such simply because he can identify a hammer and lives near the sea. Hence I thought I'd float the idea on this forum for some more professional advice.
What I dont quite understand from the above replies is how DC current "leaks". is it literally a case of a stray +12v wire that is (eg) immersed in seawater? In other words if the above test is performed and there is found to be a residual voltage then what do you look for next?

 

[vyv_cox]

Surely the windlass, like most equipment on boats and cars, use the body of the device as a negative ground. So in effect the windlass is bonded to earth and hence (assuming the boat is properly bonded) there is already an electrical connection from windlass to battery negative to prop shaft to seawater to earth?
I do not know whether this is a new phenomenon for the boat owner or not. Simply that he has asked (me) for a rope mooring strop because his anode corroded really quickly when he first took the mooring on a chain and he was advised by a "marine engineer: that this was probably due to galvanic corrosion via the mooring chain. I'm afraid I dont know where he kept his boat previously.
It seemed to me that this "marine engineer" was calling himself such simply because he can identify a hammer and lives near the sea. Hence I thought I'd float the idea on this forum for some more professional advice.
What I dont quite understand from the above replies is how DC current "leaks". is it literally a case of a stray +12v wire that is (eg) immersed in seawater? In other words if the above test is performed and there is found to be a residual voltage then what do you look for next?

If you look at the case histories in my link above you will see that in each case the problem was caused by connection of the positive supply to the negative via seawater. Two of these were fairly extreme, the other less so. If tests with a meter show positive voltage at the anode(s) it is a matter of painstakingly following each circuit until the short is found.