Galvanic corrosion on Pearly Miss

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Rapier
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Galvanic corrosion on Pearly Miss

Post by Rapier »

Having complained about various marinas turning my boat into an anode am not sure that my electrical wiring isn't the cause. The ali transom plate is pitting, there are brownish 'burn' marks on the spray rail / hull that is immersed at mooring, and dusting on the prop cone, skeg of the motor and an unpainted area above the underwater exhaust ...

I have a bilge pump wired into a 3 position Jarvis Walker marine switch on the dash and the regular Mercury cobrahead-style loom plugged inot the side of the motor, but the key / old choke and neutral switch removed from a MerControl remote box, and mounted on the dash. There are two batteries, one is for the e-start Merc, the other, a lightweight Odyssey, powers the bilge pump. At Neyland I was removing the positive cable from the battery box at night (although is a 'smart' box with the + & - cables connected to the lid inside).

Am wondering where there is stray voltage and how I test for it? As other members can attest, when it comes to wiring I belong to the luddites...
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haventaclue
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Re: Galvanic corrosion on Pearly Miss

Post by haventaclue »

As it's an alloy boat, did you earth the boat itself?
Do you many stainless steel fittings/bolts? Aluminum + S/S + saltwater = electrolysis
no matter how bad it is,it can always get worse,I'm an optimist

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Rapier
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Re: Galvanic corrosion on Pearly Miss

Post by Rapier »

There's not much more stainless fittings on the boat than the last one, and that had the same electrical setup (only one battery though) without any issue. I've never had to fit, or dangle anodes overboard either. Can only think that I need to connect it all up again and check for electrical grounding, perhaps the bilge switch is faulty too..it looks like it's taken some flak over the month it's been installed.
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haventaclue
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Re: Galvanic corrosion on Pearly Miss

Post by haventaclue »

Only installed a month, even in salt water environment, I wouldn't expect it to corrode to that extent unless water dripping directly on to it
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Diomedea
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Re: Galvanic corrosion on Pearly Miss

Post by Diomedea »

Galvanic corrosion needs contact between two metals which are apart on the periodic table and moisture. The same materials can cope if it’s wet, different materials if it’s dry. Two or more different materials and moisture equals problem. I think there are two issues here, one at the switch body which you could fix by using Duralac as a barrier so there is no contact.
Regarding the hull I guess you have a different circuit for starting and charging and so parts of the engine system will be in the water somewhere. The normal fix would be to use a sacrificial anode, for fresh water I believe that magnesium is preferred. The anode is connected to the negative of the battery by a bolt through the hull. Any stray currents then exit through the magnesium anode which slowly corrodes away as it is further away on the table.
I suggest that it would be a good idea to chat to an anode supplier, they will advise the correct material and how it should be connected.
Btw, if there is no anode, the hull itself becomes the anode and then you get the corrosion.
Hope this helps.
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Rapier
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Re: Galvanic corrosion on Pearly Miss

Post by Rapier »

Thanks for this. I'd had some time to mull it over, and read comments from the vintage ali boat crowd.

The true cause of the problem was revealed when I took the '60 Mercury 300 off the boat (to be added to the motors that need bearing / seal work). That loom now shows the normal Mercury wiring issues, where the insulation perishes over time and leaves exposed wires. The bilge switch, mounted below the ignition switch, was in contact with that wiring, or the dash.

Later, in the quiet of the night, I'd realised a. that none of the 4 Pearly Miss' and the Lone Star had been used in saltwater for any length of time, b. if they had been used in salt, they'd not been moored up where there was shore power connected, c. that, bar one, all had sacrificial trim anodes on the motors used, d. none had charging systems fitted, or if they had, I'd disabled them - kill switches being pulled underway (in early Mercurys) do fry the rectifier.

Some owners hang a line clamped to the gunwhale when mooring, with the required anode, but I need to talk to a retired expert as to the best method though.
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Diomedea
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Re: Galvanic corrosion on Pearly Miss

Post by Diomedea »

I’ve done some checking and it seems that “Galvanic Isolators” place a block in the Earth lead which, from what I can tell is probably four diodes in bridge format, ie two pairs in series but back to back. This gives a 1.2 volt barrier so there is no galvanic current path. For all other purposes the diodes would turn on and the 1.2 volts is not an issue.
In other words, there is no path for the tiny voltages to cause a current so no corrosion. Seems to make sense.
Hope that helps.
Wherever I have two metals I always use Duralac as a barrier.
Just wish I could get more water time at the moment to see it it makes any difference.

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