What's so special about anode fixing studs?

[prv]

I need to change KS's anode, together with the studs that hold it on (studs are well corroded outside). I can get the anode for what seems a reasonable price from ASAP, but they don't sell the studs. Force 4 do, but £16.50 seems a bit steep for a couple of nuts and bolts: http://www.force4.co.uk/1510/M-G-Duff-One-Set-of-Fixing-Studs-for-Anodes.html . Is there something special about these ones, or should I just go and buy some stainless (assuming that's what it is?) M10 threaded bar and appropriate nuts and washers from a non-marine supplier?

If these are made of a special material or whatever I'm happy to pay for them, but not if they're just ordinary bolts with a "marine" price tag.

Cheers,

Pete

 

[VicS]

Need not be stainless. Zinc plated are adequate as the anode protects them but the choice is yours to make as both are available

Special studs though with a square "head" on the outside of the hull so that the through hull bit remains undisturbed when changing anodes.

Picture not meant to imply that two connections are required and ceratinly not positive and negative as a chandler recently tried to tell one forumite!

BTW 4mm² PVC covered multistranded cable recommended

 

[guernseyman]

My usage differs in two respects.

1. I disassemble the bolts completely in the autummn to clean them and review their condition. Along with the anode, of course.

2. On reassembly, I allow the square 'heads' to contact the anode connecting bars (I use the pear shaped anode) for the sake of electrical conductivity.

Not saying I'm right but, in practice, it seems that the bolts get disturbed during dissassembly/assembly and I prefer to make a fresh seal in the spring.

 

[prv]

Need not be stainless. Zinc plated are adequate as the anode protects them but the choice is yours to make as both are available

OK - so I might as well just go and buy something from B&Q then.

Special studs though with a square "head" on the outside of the hull so that the through hull bit remains undisturbed when changing anodes.

Yep - that's the only reason the marine-pricetag ones appeal. Pity my welding (well, my welder) is a bit too agricultural to do a neat little job of tacking a square washer onto a bar. A friend of a friend does TIG welding; perhaps I'll mention it next time I'm around.

Cheers,

Pete

 

[VicS]

OK - so I might as well just go and buy something from B&Q then

well the £16.50 ones from Force 4 are not the stainless ones they are just the zinc plated ones!

 

[VO5]

I have been told that the studs are not stainless for a reason, being that the conductivity of stainless is much superior and thus the anode would get nibbled very quickly.

 

[VicS]

I have been told that the studs are not stainless for a reason, being that the conductivity of stainless is much superior and thus the anode would get nibbled very quickly

Other way round I think you will find. Not that I suspect that it would make a significant difference any way.
(resistivity of carbon steel 17.0, 18/8 stainless steel 66.3 according to Kaye and Laby)

The reason for not using stainless steel is the relative positions of stainless steel, mild steel and zinc in the galvanic series .

zinc -0.8v, mild steel approx -0.4v, 316 stainless steel +0.2v

 

[Allan]

I thought the reason for not using stainless was that it corodes under water, ie when the oxygen is removed. Am I wrong or does it depend on the type of stainless?
Allan

 

[vyv_cox]

I thought the reason for not using stainless was that it corodes under water, ie when the oxygen is removed. Am I wrong or does it depend on the type of stainless?
Allan

I think you are talking about the difference between the active and passive states of stainless steel. Active state is the metal itself, whereas passive is the normal condition with an oxide film. This forms readily and virtually instantaneously in air, and to all intents and purposes in seawater, which contains plenty of dissolved oxygen.

The difference between the active and passive states is a problem where oxygen is partly excluded, in crevices, pits, scratches, etc. Inside such locations the metal tends to remain in the active condition. There is a galvanic potential between the active and passive states, so corrosion occurs at the crevice.

The main reason for preferring 316 SS to 304 for marine use is that the former has better resistance to pitting and crevice corrosion by virtue of its additional molybdenum.

 

[davidej]

Arn't we forgetting one important point - the square washer should be welded to the stud.

This is because if the outer (ie underwater) stud end or nut corrodes and falls off (and believe me, this can happen) the inner portion of the stud stud will not come out leaving a neat round hole in the hull.

They do seem expensive but, IMHO, there is no alternative to fitting the proper article.

 

[VicS]

Arn't we forgetting one important point - the square washer should be welded to the stud.

No. thats why i called it a "head" rather than a washer and why prv was discussing welding a square washer on.

If the external part of the stud and the nut corrodes and falls off though it doesn't say much for the effectiveness of the anode.

 

[longjohnsilver]

I thought the reason for not using stainless was that it corodes under water, ie when the oxygen is removed. Am I wrong or does it depend on the type of stainless?
Allan

I used stainless one year in error. By the end of the season one of the studs had effectively disappeared. Never again.

 

[prv]

I used stainless one year in error. By the end of the season one of the studs had effectively disappeared. Never again.

That's interesting. I've seen stainless anode studs offered somewhere online.

Sounds like the norm is zinc-plated steel though.

I might have a bash at welding some parts from B&Q, just to see what sort of quality I can produce with my big clumsy welder. If it turns out better than I expect it will, I'll put them on.

Pete

 

[vyv_cox]

I used stainless one year in error. By the end of the season one of the studs had effectively disappeared. Never again.

That's the problem with exposed threads underwater. They are just one continuous crevice!

 

[gandy]

If the external part of the stud and the nut corrodes and falls off though it doesn't say much for the effectiveness of the anode.

Our anode nuts and studs get very corroded. I had to replace one of the studs a couple of years ago, and this year when replacing the anode they're so bad I think I'm going to replace both. I can't understand how the anode is supposed to protect zinc-plated bolts. Surely the zinc will come off the bolts just as fast as the anode erodes.

 

[Bosun Higgs]

I have in the past bused 316 studding with a nut and washer on either side of the hull plus a further nut to hold the anode on. Worked without problems

 

[VicS]

Our anode nuts and studs get very corroded. I had to replace one of the studs a couple of years ago, and this year when replacing the anode they're so bad I think I'm going to replace both. I can't understand how the anode is supposed to protect zinc-plated bolts. Surely the zinc will come off the bolts just as fast as the anode erodes.

I think the zinc plating is only to protect the bolts while they are out of the water or maybe just to make them look nice
Once in the water the zinc plating will go at the same rate as the anode, so a few μ won't last long, but from then on in the anode should protect the bolts from serious corrosion. If it doesn't then they cannot be in good electrical contact and if they are not then the bonding wire on the inside will not be effectively connected to the anode either.

 

[Bosun Higgs]

Are you sure about that Vic? Electrolytic corrosion via the formation of a cell is one thing and exactly what an anode is used to counteract but I cant see why an anode should stop straight forward rusting of the then bare steel stud in the presence of oxygenated water

 

[VicS]

Even rusting in oxygenated water proceeds by an electrochemical process, corrosion of the steel occurring at anodic areas.

 

[rob2]

Surely the biggest obstacle to manufacturing your own studs is that bolts are normally bought already zinc plated and that will prevent welding the "washer" on. You could more easily make them from mild rod and thread them after welding. As already mentioned, any zinc plating is eaten away very quickly once submerged, so not much lost there! What a shame that bronze studs would only load the anode more.

With chemists and metallurgists inputting such detail, is there any material which is strong, conductive and inert in the galvanic series? I guess mild steel it is then!

The only other alternative would be a fairly complex design for the anode which protects the stud sealing against water ingress to it. That would be possible with a disc shaped anode, but probably not for the more common pear shape. I believe there was another option offered which was a through hull fitting in which a cylindrical anode is inserted flush with the hull whilst sailing or push further through to expose it when moored.

Rob.