Hull Anode Bolts - Stainless ??

Halo

Halo

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Dear All
One of my through hull anode bolts has sheared off. There are stainless and non stainless replacements available. Whilst I usually go for stainless bolts I wonder about the sense in using these on a hull anode - after all the anode should protect it. Also I am concenered that stainless will not conduct as well and also that it may stop the galvanic protection to my engine and fittings if it is immersed in the sea water at the same time as the anode. Any expert views out there ? Will the galvanic action work through stainless bolts and copper wire to protect aluminium ?
 
I haven't used stainless ones and the mild steel ones do rust. I always worry that the rust will hardly improve the contact - yet they seem to work when reused. I doubt it makes a significant difference to the circuitry or the depletion rate of the anode, but one would hope that the stainless option would last a little longer as you are at less risk of shearing them when fighting to undo that rusty thread.

Rob.
 
Galvo
it is protected by the anode

Halo said:
Dear All
One of my through hull anode bolts has sheared off. There are stainless and non stainless replacements available. Whilst I usually go for stainless bolts I wonder about the sense in using these on a hull anode - after all the anode should protect it. Also I am concenered that stainless will not conduct as well and also that it may stop the galvanic protection to my engine and fittings if it is immersed in the sea water at the same time as the anode. Any expert views out there ? Will the galvanic action work through stainless bolts and copper wire to protect aluminium ?
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The steel ones are protected by BZP from surface rusting when out of the water ... for as long as the BZP lasts. When immersed the anode protects them.

The resistivity of carbon steel is 17 x 10-8 Ω m and that of 18/8 stainless steel 66 x 10-8 Ω m

if you work out the resistance of the bolts I think you will find the result is in micro ohms . So low I'dt think not to be significant.

Stainless is , I suspect, aimed at the shiny owners of shiny mobos who dont want rusty looking bolts visible when the boat is out of the water


By the way a hull anode does not in anyway protect your engine

Copper wire used for bonding the anodes to the items they are to protect should all be inside the hull, not in the water.

MGDuff recommend a cable size on their website
 
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I fitted stainless steel (316) studs to hold my anodes on and after 4 years are still in perfect condition but my boat is steel and the anode studs are welded to the hull.

I think in your case that the stud failed due to crevice corrosion where the stud passed through the hull and water peculated between the hull and stud.
 
macd said:
Galvanised is generally preferred to stainless due to the risk of crevice corrosion in the latter in the portion bonded into the hull and thus deprived of oxygen, although it seems to some degree to depend who you ask.
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Good point.
If I go stainless I will give them a really good passive layer using citric acid before I install them.
Also thanks to others for the very interesting points
 
Id think the citric acid is to clean the stainless before allowing the oxide film to form.


However ITYWF that crevice corrosion occurs because when deprived of oxygen the protecting oxide layer is eventually lost
 
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vyv_cox said:
Can you explain what this is about? The passive layer is chromium oxide. How does citric acid help?
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Vyv
You are right that less steel self passivates with a layer of oxide. However the layer formed in natural passivation may not be optimal and there may be traces of iron on the surface which can initiate serious problems. Passivation using Nitric or Citric acids is a well established process and is detailed in ASME standards. The technique serves to dissolve any surface iron and ensures an optimal oxide layer.
I make surgical instruments and the corrosion resistance of passivated stainless is far, far better than would otherwise be the case.
The process involves cleaning, immersion in hot acid , agitation and rinsing off.
Hope this helps
Martin
 
Halo said:
Vyv
You are right that less steel self passivates with a layer of oxide. However the layer formed in natural passivation may not be optimal and there may be traces of iron on the surface which can initiate serious problems. Passivation using Nitric or Citric acids is a well established process and is detailed in ASME standards. The technique serves to dissolve any surface iron and ensures an optimal oxide layer.
I make surgical instruments and the corrosion resistance of passivated stainless is far, far better than would otherwise be the case.
The process involves cleaning, immersion in hot acid , agitation and rinsing off.
Hope this helps
Martin
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That's interesting. Just has a look at the liquid I use to clean the TIG welds and the stainless items I make for my boat and it turns out to be Nitric acid but it does tend to leave the surface dull so I mechanically polish after to get a shiny finish.
 
Rogershaw said:
That's interesting. Just has a look at the liquid I use to clean the TIG welds and the stainless items I make for my boat and it turns out to be Nitric acid but it does tend to leave the surface dull so I mechanically polish after to get a shiny finish.
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Roger
Polishing should enhance the passivation as long as there are no steel particles arising or use of abrasives/tools which may be contaminated with steel or (worst of all) rust. Once rust starts acting on the surface of the stainless it acts like a wick drawing more iron to the surface. The only answer then is passivation or serious polishing. What is the name of the liquid you use?
 
I try to keep Mild steel away from my polishing tools but contamination can happen.

The liquid is some thing I got from my local welding supplier, locally made, and the only name on the bottle is "Thermadynes" if that means any thing to you.

The other problem is where I keep my boat is very industrial and we do get various ore dust's in the air from bulk ship loading. specially Richards Bay and Port Elizabeth.
 
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rob2 said:
...one would hope that the stainless option would last a little longer as you are at less risk of shearing them when fighting to undo that rusty thread.
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When fitting hull anodes, always put 2 nuts on each stud to secure the anode. When the time comes to change the anode, use one spanner to hold the nut next to the anode, and use a second spanner to unscrew the outer nut and clean the thread.
 
For many years I have used the steel type and used A4 stainless nut, never corrode and always come off ok.

Proir to that it was always a pain to renew every year.
 
If your boat is craned in/out or even launched and recovered with a travelling boat lift be aware of the location of hull anodes and the damage the fixings can do to slings.

The one below did terminal damage to a sling.

DSCF0342.jpg
 
I too had a mild steel stud shear off after 2 years , the stud went thro the hull and a piece of hardwood fixed to the inside of the hull with silicon . annode was a dome style . While trying to remove the broken stud from the hull / hardwood block , the block came off with not alot of force . it could have sunk the boat (not my solution it came with the boat ).
a note to all, make sure annodes and what they are fixed to are secure.

solution sikaflex new block to the inside of the hull
also put a die nut down some bronze bar and used as screwed bar
do you think i will be ok ?
should i have fixed the wood block with epoxy ?


Michael
 
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