Electrolysis/bonding tutorial

[Sinbad1]

To celebrate finally getting access to this forum after 12 months of technical frustration (resolved by Kim...thanks) I no longer have to lurk in the wings.
The posts on anodes and electrolysis have been many, and interesting tho I feel they need clarifying.

Having the underwater metal parts of your boat eaten away can be caused by one of two events occurring. Namely 'stray current corosion' or 'electrolysis'.

This is about electrolysis. Electrolysis arises when you put disimilar metals in a common 'bath' of water. For us guys this is simply your bronze prop and your stainless steel shaft and will also include other metals in the near vicinity such as thru hulls.

There are two solutions to avoid electrolysis. Unbond and isolate each piece of metal. This is as simple as having a through hull connected to a rubber or plastic hose inside the boat. It is effectively isolated and not being connected to any other metal component, it is unbonded.

or

bond the metal underwater components to a sacrificial zinc anode which allows the anode to 'corrode' instead of more important pieces like the prop!

Ok thats the basics. To check the effectiveness of your boats protection is surprisingly simple. Despite boatyard gossip this is NOT a black art. Whilst the boat is out of the water, buy yourself a decent digital multimeter, one that has clips on the back to hold the probes. Switch it to ohms and set on the most sensitive setting. The readout will say 1. Now touch the two probes together. The readout will state 0.000 You have the perfect connection.

This is the position that you want to achieve throughout your bonded circuit. Now test the zinc. Put probes both ends of zinc. Readout should be 0.000 (this is to check that your zinc hasn't become 'passivated') Now put one probe on the zinc and one on the anode studs. Check each stud separately to ensure that the zinc is properly connected to the studs.

A word of caution here. Many yards use galvanised steel studs for holding the anode on because the anode is more electrically volotile and will corrode before studs do. Well lets hope that your anode doesn't drop off cos then your studs will become the anode and as they go through your hull the end result will be dampness, lots of it. I use bronze bolts.

Still standing outside with your multimeter and anode, now place a probe onto the zinc and the other probe onto your cutlass bearing housing, then the shaft, then the prop. All will read 0.000 if your circuit is correctly wired inside the boat.

If you do not get this reading on each metal component that you want protected by the anode then I'm afraid its inside the boat you go and work from the anode studs to check each connection for electrical continuity, ie a multimeter readout of 0.000

This winter I have reconnected and replaced my whole zinc bond circuit. From inside the boat one wire goes from the anode stud to the engine block. Another goes to the shaft via a brush,(this pretects the shaft and prop) another to the stern tube (which then also protects the external cutlass bearing housing)

Don't leave your little conical prop anode doing all the work of protecting the stern gear. Its not difficult to correctly wire it all into your main zinc and it will be another step up on your learning curve. It was for me.

 

[castaway]

Very interesting post. I asked the question last week whether anyone knows an easy way to get a good electrical contact on to all the various bronze bits and pieces. I had been checking my "bonds" and had to renew some of the tags but found it realy dificult to get a decent contact on to even the easy to get to bits.. ie stern gland.. so I wonder how Im going get a contact on to some bits that are tucked away....... Any ideas.... I did get the suggestion ok Oxalic acid but havnt tried it get (seems to cure most things!) Nick.

 

[Sinbad1]

Nic, once you have established that the anode and the studs are giving you 100% contact, then you merely have to run your internal bonding wire from one of the studs to the part to be protected. The essential ingredient is good contact.

I achieved this by using a electric drill with an extending flexible piece B&Q, into which a put a brass wire brush. You clean the anode studs, screw on a stainless or brass nut, then you attach all you wires to a piece of stainless or brass plate say 2"xhalf" which is drilled to take the anode stud one end and a bolt the other. Use thin metal which you can bend by hand if required to get the correct angle, and then attach it to the stud with a suitable nut and bolt it up tight to the nut you have already put on.

At all stages/ joins check the electrical continuity. Run one wire to a bearing housing bolt head (not the nut as it may have a plastic internal lock which can insulate it) and that should adequately protect the bearing housing and stern tube.

The only safe way of protecting the bronze prop is to run a wire from the anode direct to the shaft via a brush. MG Duff make a system for this and most chandlers stock it.

 

[castaway]

Thanks for that Sinbad ...all makes sense when you think about it I think I will just start again and replace it all rather than messing around fixing bits and pieces that way i can measure it up and make the leads at home and fit the lot in one shot.. Thanks Nick