Rudder bearing housing and anodes

[mowa]

Hi everyone. I saw in this video (http://youtu.be/9upUBC2tu1c) the use of sacrificial anodes to reduce the corrosion in the metallic ruder bearing housing. The past June, I installed anodes (see picture) in the iron keel of my B39Cr (2005) to try to reduce its corrosion. For the moment it seems that it is working. Respecting the metallic ruder bearing housing, I found some small holes caused by corrosion in the side in contact with the water. After I saw the video, I am thinking to connect electrically with a cable the bearing housing with the keel to protect this part by consuming the anodes in the keel. Someone thinks this could be not a good idea?

[Symphony]

I am afraid your anode on the keel is a complete waste of time. The keel is cast iron or lead, neither of which suffer from dezincification and is not connected electrically to any other part of the boat. So that anode cannot protect anything.

Normally there is no need for an anode on the rudder bearing housing either as it is aluminium encased in GRP and the bearing inside is non metallic so again there is no possibility of galvanic action. Any corrosion there is the aluminium corroding, not electrolysis and an anode will not prevent it. There is an argument in favour of having an anode attached to the rudder shaft because some fittings on the steering may be stainless steel. However they are not in seawater which would be necessary for electrolysis to occur. The corrosion on rudder shafts is usually caused by copper based antifouling getting on the shaft in the gap between the rudder and the hull. This can be reduced by using a copper free anti fouling such as Trilux in that area.

You can find more information on galvanic corrosion and anodes on the MG Duff website and specific advice on corrosion of rudders and steering on the Jefa website.

[MarkTheBike]

Hi Symphony

I was really interested to read what you said about the keel and would have agreed with you, except... (and I have no idea of the cause!), when we hauled the boat out last winter for the first time since we bought, I discovered this what appears to be electrolysis pitting. I have also noticed that the keel bolts are joined by earth leads. I'd assumed everything was normal about the setup (we've not owned the boat a year yet and there was no questions raised by the surveyor) but having read your post, now I'm not so sure. Perhaps, in our case, a keel anode might be justified if the pitting is electrolytic. Any views are welcomed...

Regards

[mowa]

Respecting the keel bolts they are also joined by earth leads in my boat.

[MarkTheBike]

mowa - interesting, have you noticed any pitting in your keel?

[mowa]

Yes, not as extended as in your case but this was the reason to ask and the person in charge of the boatyard recommends me this solution.  After three months in the water the aspect of the anodes in the keel are similar to the aspect of the anodes in the saildrive, that were changed at the same time, showing similar corrosion. In both case the corrsion is apparent.

[Symphony]

Think you will find the wire to the keel is to ground the mast against lightning. Don't think it is connected to the electrical system of the boat. The corrosion on keels is straightforward rust.  The only thing that needs an anode is the saildrive to protect the aluminium housing, and possibly the propeller if it is a folding or feathering type which has a mixture of metals in its construction.

[mowa]

Yes that is right but this does not matter. Here we are dealing with electrolytic corrosion of a metal in salt water. To minimize this corrosion you only need a sacrificial anode connected to the metal you want to protect (you can check the book "The 12v doctor´s practical handbook"). In the case of the aluminium bearing housing connected to the keel attached to the sacrificial anode, the problem is that if the zinc anode is consumed, the aluminium goes after to protect the iron keel... and this is no good.

[MarkTheBike]

Think you will find the wire to the keel is to ground the mast against lightning. Don't think it is connected to the electrical system of the boat. The corrosion on keels is straightforward rust.

Thanks, Symphony. Actually, that does make a lot of sense so I guess you're right. When we lifted her for the first time, there was a lot of the keel where the anitfoul had completely worn away, leaving bare metal. I'll just make sure the keel is well painted.

[Symphony]

Yes that is right but this does not matter. Here we are dealing with electrolytic corrosion of a metal in salt water. To minimize this corrosion you only need a sacrificial anode connected to the metal you want to protect (you can check the book "The 12v doctor´s practical handbook"). In the case of the aluminium bearing housing connected to the keel attached to the sacrificial anode, the problem is that if the zinc anode is consumed, the aluminium goes after to protect the iron keel... and this is no good.

How is the keel connected to the bearing housing? Have you added a bonding wire? If the purpose of that anode is to protect the aluminium housing it must be connected directly to it and mounted close by, so a hull mounted anode under the stern is more appropriate. However, electrolysis occurs between dissimilar metals touching each other in sea water, and as I pointed out earlier, this does not apply to the rudder bearing housing which is mounted in GRP and islolated from the shaft (which is also aluminium) with a non metallic bearing.

So, don't think your anode will do anything about possible corrosion of the rudder bearing housing, which is why the factory do not fit one.

[Baltic]

.... Don't think it is connected to the electrical system of the boat. The corrosion on keels is straightforward rust.  The only thing that needs an anode is the saildrive to protect the aluminium housing, and possibly the propeller if it is a folding or feathering type which has a mixture of metals in its construction.

Hi

I think that is connected..
First what come to my mind.
Antennas. Coaxial cables are probably connected to the earth.

Last spring I added same kind keel anode like there mowa´s picture.
Short dive during the summer.
Look like is working...

[Symphony]

.... Don't think it is connected to the electrical system of the boat. The corrosion on keels is straightforward rust.  The only thing that needs an anode is the saildrive to protect the aluminium housing, and possibly the propeller if it is a folding or feathering type which has a mixture of metals in its construction.

Hi

I think that is connected..
First what come to my mind.
Antennas. Coaxial cables are probably connected to the earth.

Last spring I added same kind keel anode like there mowa´s picture.
Short dive during the summer.
Look like is working...

What "earth" is the radio antenna connected to?  The whole point is that the anode needs to be connected to the item it is protecting, in this case the rudder bearing housing and it is not unless there is a cable specifically running from the anode to the housing. The anode also has to be in line of sight to the item it is supposed to protect, and it is not.

Your anode may be corroding away, but that does not mean it is doing that in preference to the rudder bearing housing, nor anything else on the boat except possibly the mast, but even then there needs to be seawater connection for it to work. Masts do not suffer from electrolysis except where there are stainless fittings and this is avoided by isolating the two materials with a sealant or a paste such as Duralec.

[mowa]

Just to clarify. The zinc anodes in the keel, as I installed in my boat, protect the iron cast keel for electrolysis because both metals are in salt water and electrically conected due to that they are in contact.

Of course, to protect the aluminium bearing hausing that is in water to the same zinc anode, I would need to contact both metals with a wire inside the hull to the keel bolts. Of course a better solution is, instead to use the zinc anode in the keel, to install an aditional anode, as in the video, but this involves to make holes throhggh the hull, as in the video.

[Symphony]

Just to clarify. The zinc anodes in the keel, as I installed in my boat, protect the iron cast keel for electrolysis because both metals are in salt water and electrically conected due to that they are in contact.

Of course, to protect the aluminium bearing hausing that is in water to the same zinc anode, I would need to contact both metals with a wire inside the hull to the keel bolts. Of course a better solution is, instead to use the zinc anode in the keel, to install an aditional anode, as in the video, but this involves to make holes throhggh the hull, as in the video.

Sorry, as I explained earlier the anode on the keel does nothing. The keel is cast iron as you say and by definition does not corrode through electrolysis. Electrolysis occurs when two dissimilar metal of different potential are in contact in an eloctrolyte (seawater). This does not exist as the iron keel is not connected to anything metal under water. The keel bolts are stainless steel, but they are not in the water.

If an iron keel corrodes it is the action of water on the iron which forms ferrous oxide - rust. Nothing to do with electrolysis. Rust can be particularly severe in castings where there are imperfections or pinholes in the casting, so corrosion can be avoided by keeping the casting coated - if possible shot blasted clean and then epoxy coated. Attaching an anode is more likely to start corrosion unless you seal the attachment point to exclude water.

[MarkTheBike]

... The keel bolts are stainless steel, but they are not in the water...

Unless the seal between the hull and keel isn't quite perfect. Now there's a scary thought.

[Symphony]

... The keel bolts are stainless steel, but they are not in the water...

Unless the seal between the hull and keel isn't quite perfect. Now there's a scary thought.

Any corrosion then will still not be electrolysis, but likely to be crevice corrosion of the stainless steel bolts. Not unknown where the hull/keel joint fails.

[MarkTheBike]

Hi Symphony

OK, I don't understand why that wouldn't cause pitting. Is there no electrolytic difference between cast iron and S/S, or is it that they are so close in nobility that the effect is negligible? Or does it only affect alloys of dissimilar metals (e.g. copper/tin)? I probably have less than a layman's level of knowledge on this stuff but do find it interesting. Hmm, never heard of 'crevice corrosion' before and now, having read Wiki's entry, I'm not sure I want to...   

[Symphony]

Hi Symphony

OK, I don't understand why that wouldn't cause pitting. Is there no electrolytic difference between cast iron and S/S, or is it that they are so close in nobility that the effect is negligible? Or does it only affect alloys of dissimilar metals (e.g. copper/tin)? I probably have less than a layman's level of knowledge on this stuff but do find it interesting. Hmm, never heard of 'crevice corrosion' before and now, having read Wiki's entry, I'm not sure I want to...

 Zinc is less noble than most metals so if it is connected to other metals where there is a differential it erodes in preference to the other metals. The difference between stainless and cast iron is small and the bulk of iron in a keel is so huge that there is little significant effect.

Stainless gets its stainless, not corroding properties from surface oxidisation. If it is a location where it is damp and starved of oxygen the surface will pit and the underlying metal corrode. Common in threads and places where the metal passes through wet wood or close fitting water lubricated bearings. So you see it on fastenings into wood, leaking keel bolts and shafts passing through close fitting cutless or composite type bearings, particularly if the boat is left unused in seawater for any length of time. Can be avoided in fastenings and keel bolts by using plenty of sealant to keep the embedded surface away from seawater. 

[Ripster]

Just for information, lots of information on Vyv Cox's site about metallurgy etc.  http://coxengineering.sharepoint.com/Pages/default.aspx 

[MarkTheBike]

Cheers, Symphony. Thanks for the explanation... (and for the website, Ripster).