In situ replacement of hull anode??

omega2 said:
You can give in if you like Rob, but WHAT MADE OUR BOLTS CRUMBLE?? and others I have come across "My anodes are untouched and they have been on for over a year" the reason was found to be that the bolts holding them were in two pieces and the only thing holding them in place was the sikaflex sealant.
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Maybe a phenomenon they call "crevice corrosion"?

PS Not giving in as such as I prior to the lesson from maby I was certain that anodes did'nt protect from rust! But can't argue with the experts he's quoting.
 
RobWales said:
Maybe a phenomenon they call "crevice corrosion"?
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No crevice corrosion is a different thing altogether and does not occur in mild steel, but in stainless where the material is damp and starved of oxygen. Typically in threads on bolts - which is why stainless is not used for anode attachment as mild steel is suitable, and protected from corrosion by the anode, if as maby says it is in good electrical cobtact with the zinc of the anode.
 
Tranona said:
No crevice corrosion is a different thing altogether and does not occur in mild steel, but in stainless where the material is damp and starved of oxygen. Typically in threads on bolts - which is why stainless is not used for anode attachment as mild steel is suitable, and protected from corrosion by the anode, if as maby says it is in good electrical cobtact with the zinc of the anode.
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FFS! Wrong again! lol....Cheers :)
 
Thanks everybody for the advice and as usual an interesting debate. Taking on board the suggestions on here, here's my plan- I will review the inside fixing to see if there is any corrosion and look to see how its assembled i.e is it worth trying, if yes, I will make a feeble attempt at turning the nut with someone checking the inside for any movement - if the nut turns relatively easy then I will continue on - if not, then its the lift out and all anodes replaced and antifouled.

Once again thanks to all- this forum is a fantastic resource and I am grateful for all the responses. I will post back the outcome but it could be a couple of weeks when i am down there again.

Cheers
Mike
 
Tranona said:
No crevice corrosion is a different thing altogether and does not occur in mild steel, but in stainless where the material is damp and starved of oxygen. Typically in threads on bolts - which is why stainless is not used for anode attachment as mild steel is suitable, and protected from corrosion by the anode, if as maby says it is in good electrical cobtact with the zinc of the anode.
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Tell "Shell" that.
All of the subsea assets with anode skids are connected by stainless wire, and nuts and bolts... All are 316L. And wont see any light of day for at least the next ten years.. We put hundreds of tonnes of anode skids on the sea floor every year.
Corrosion cell can be in any material, not just a stainless, to form crevice corrosion.
Most of the valves and controls on the wells are stainless steel, and will be down here for a long time.
Cameron valves and vetco gray have no issue using it, (well head/tree manufacturers) as do all the oil companies...
If i could remember back to when i studied metallurgy... Or even my cswip subsea inspection course ;)
 
Robg71 said:
Tell "Shell" that.
All of the subsea assets with anode skids are connected by stainless wire, and nuts and bolts... All are 316L. And wont see any light of day for at least the next ten years.. We put hundreds of tonnes of anode skids on the sea floor every year.
Corrosion cell can be in any material, not just a stainless, to form crevice corrosion.
Most of the valves and controls on the wells are stainless steel, and will be down here for a long time.
Cameron valves and vetco gray have no issue using it, (well head/tree manufacturers) as do all the oil companies...
If i could remember back to when i studied metallurgy... Or even my cswip subsea inspection course ;)
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But crevice corrosion is not the same as galvanic corrosion and zinc anodes will not prevent crevice corrosion. It will also occur when there are no other metals in contact with the SS - most commonly in the threads where oxygen is excluded and the surfaces are wet. Another common place on boats is on prop shafts where they go through water lubricated bearings and are static for long periods so starved of oxygen from moving water. Again anodes will not help as there is no galvanic cell to tap into.

Plenty of references explaining it if you Google the term.
 
Update just got back from the boat and the anode change went well- 17mm socket on the ratchet reached down and applied some mild force - some movement as anode and nut began a little turn simultaneously, so I checked below to look at the bolt from the inside which was solid and no leaks!!

Continued with turns and off she came with the nut two washers in my hand. Replaced with new 1.25kg disc anode from Zincsmart ,a new nut and washer and tightened back up. checked the continuity between anode and thread of bolt - all fine then continuity from the inside nut to the boding terminal all fine too. Result.- thanks for advice again.
 
very good to see a thread develop informatively, and the OP come back with a report and thanks. That's what this is all about.
 
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