Crevice corrosion

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andybussell

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I have just removed a spurs rope cutter that I fitted a year ago and found signs of corrosion where it had been clamped to the shaft. Shaft seems OK. Earlier discussions suggest bedding in sealant to exclude water, any comments please?
 
Well I've had my Spurs for around 12 years or more now and remove it and the prop every winter. There is no sign of any corrosion on the shaft or the Spurs.

The Spurs 'fixed' cuter used to be connected to the bolted on 'V' section via a s/s wire, but this broke after several years (now they supply at much cost little anodes instead) so I now make my own anodes and bolt these on.

I do not understand the recent suggestions on here about using mastic between the cutter and the shaft. On mine at least, it would be squeezed out completely when the unit was tightened as required.

IIRC the Spurs cutter is made from hardened 316 ss, so should be compatible with my 316 shaft, and the lack of corrosion in either suggests that is the case. I assume the Stripper type is similar.
 
That was a suggestion that I and others made a couple of weeks ago, although it was by no means clear whether it was in fact crevice or galvanic corrosion. Not knowing the shaft material it was impossible to be certain, but its appearance suggested to me that it was galvanic. Whichever it was, excluding water would largely overcome the problem.

I agree with the second poster that most of the sealant would be extruded out, but the point is that some would be left, which should keep any water out. If the problem was galvanic, rather than crevice, then you might finish up with two narrow rings of corrosion, one each side of the cutter. Let us know in a year which it was!
 
Thanks Jerryat & vyv. I suppose tightening the cutter after the sikaflex has gone off, with just a smear immediately before doing so, would minimise extrusion and maximise water exclusion. I'm pretty sure it's crevice corrosion as it is in the part of the fitting in contact with the shaft, in a narrow band just inside the forward rim. The rim is completely intact.
 
Thanks Tranona. I wondered about that, but one poster here has had no probs for 12 years. Also the anode only protects the fixed blade which is isolated from the rest of the fitting by non-metallic bearing material. I guess I'd better review the galvanic protection of my shaft just in case......
 
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Whichever it was, excluding water would largely overcome the problem.
...
Let us know in a year which it was!

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Why are you only considering the two? there are other common forms and causes of corrosion. ie fretting as above
 
Yes, I think you are correct. I just looked up their website, where I found:

'The rotating and stationary cutter blades themselves are made from specially hardened stainless steel, using three heat processes to develop a hardness of approximately 45 Rockwell C. This is very hard, just under brittle, and the cutters remain stable under saltwater with anode protection. The hardness is extremely essential in severe cutting conditions, as from time to time commercial fishermen will encounter hooks and toggles. '

316 is not hardenable by heat treatment, so the cutter blades are clearly something else, I would hazard a guess that they might be a 400 series stainless. In which case an anode is clearly needed to protect the cutter itself, which is more noble than the shaft and the block, made of 316.

Further down, the site says:
Zinc anode protection in saltwater environment. A specially fitted zinc anode is mounted to the hardened stainless steel stationary cutter blade, and is provided with every cutter system from the factory.'

So that seems to be the answer. It's galvanic corrosion and you need an anode. Sealant may help a little but the cutter would need to be electrically isolated from the shaft to be 100% effective.
 
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... I'm pretty sure it's crevice corrosion as it is in the part of the fitting in contact with the shaft, in a narrow band just inside the forward rim. The rim is completely intact.

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Well yes.. exactly where you would expect the two surfaces to fret together.
 
Fretting is very different from either of these, and I'm not even sure whether it would occur underwater. It occurs when asperities on the two machined surfaces weld together and then fracture. The resulting alpha-iron then oxidises to alpha - Fe2O3, which is the red coloured product mentioned. It is generally reckoned to occur at vibration amplitudes between 6 and 24 microns. It isn't really a corrosion mechanism at all, actually it's a high-temperature process but on a micro scale (the asperity welding).
 
I suspect the primary purpose of the anode is to protect the hard stainless while it is immersed in a low oxygen, high chloride environment and suceptible also to SSC.
 
Yes, you are right it is galvanic. On some boats it seems not to matter, but on others I have seen severe pitting on the bearing faces and around the clamp bolt holes.
 
Yes I know, but I've seen it on oil bathed shafts at no more than 50 Deg C so I don't see why water should be any different. And also on dry shafts - between couplings and their tapers.
 
[ QUOTE ]
....Further down, the site says:
Zinc anode protection in saltwater environment. A specially fitted zinc anode is mounted to the hardened stainless steel stationary cutter blade, and is provided with every cutter system from the factory.'

So that seems to be the answer. It's galvanic corrosion and you need an anode. Sealant may help a little but the cutter would need to be electrically isolated from the shaft to be 100% effective.

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So if he has this type of cutter has he not GOT the anode? Or has it corroded away?
And if he HAS the anode referred to - is it likely to be galvanic corrosion he is seeing?
 
<<< I suspect the primary purpose of the anode is to protect the hard stainless while it is immersed in a low oxygen, high chloride environment and suceptible also to SSC. >>>

Eh?? Seawater around a propeller, and almost anywhere within a couple of metres of the surface, is highly oxygenated.

Seawater would not be recognised as a high chloride environment.

SSC is sulphide stress corrosion. Where has that suddenly come from? I suspect you might mean chloride stress corrosion. Anodes can not protect against it. It occurs in components hardened to greater than 22 Rockwell C at a temperature normally recognised as above 65 degrees C.

And believe me, this problem has nothing to do with fretting.
 
>> I wondered about that, but one poster here has had no probs for 12 years. Also the anode only protects the fixed blade which is isolated from the rest of the fitting by non-metallic bearing material. I guess I'd better review the galvanic protection of my shaft just in case...... <<

Hi Andy,

Yes, I should have added to my first post that my propshaft and prop are wired back to the main anode on the hull, so the little anode bolted to the 'floating' part of the cutter system protects ONLY that and not the section clamped to the shaft. This of course, is protected via the shaft connection to the main anode.
 
You are correct, I should have said chloride, however, not high chloride? you might refer to Norsok M001 para 4.2.3 re the issues of stainless in marine environments, particularly under a clamp or similar.

Also question of whether he has anode as described as being fitted on his kit has not been answered.

edit - Just spotted answer.
All a bit off-topic now!
 
Re: Crevice corrosion, thanks to all

Thanks to vyv, Mike, Tranona and Jerryat. I understand a bit more clearly now and I will review my shaft protection (which I think is suspect) AND slosh on some sealant when I remount the stripper. And if I remember I will report back in a years time ;-)
 
A small correction 316l is hardenable, we've developed a process that is protected. hence early quicKutters were hardened 316l.

We moved on from this a year or so ago and have developed our own stainless alloy which is significantly harder than 316 and is also resistant to crevice corrossion.

We advise that all bolts are bedded on epoxy as this removes opporunties for vinration loosening, it also spreads the loads and prevents stagnant salt water in the threads which could lead to crevice corossion in th ebolts which are 316l.
 
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