Advice on prop shaft (metallurgical)

Neil_Y said:
the metalurgists reply is that it may be a lower grade of Stainless or that it wasn't electrically connected to the anode, or both.

Just bad corrosion with signs of it all along the shaft.
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Bad corrosion? What does that mean? There are nine recognised varieties of corrosion, but 'bad' is not one of them. Just a few:

General - clearly not as the damage is localised
Pitting - again too localised and completely wrong shape. Pits are deep and narrow in generating aerobic/anaerobic cells. These are not.
Crevice - possible if there has been fouling long term
Galvanic - doesn't seem likely assuming the area near the prop is ok.

Grinding the pits back may give some clues. Slag inclusions in hot rolled bar come out as stringers running along the length, so the damage seen does not conform. Porosity is a possibility but I need to see the macros before deciding.
 
It looks very similar to the crevice corrosion I have had on Volvo Penta stainless duoprops; the only outward signs were very small holes with a slight rusty stain around them. Poking at those holes revealed there were very thin skins of intact stainless covering much bigger holes and pits; full initially of a black sludge. Following thorough washing out the metal exposed in those pits was black.

You may have inadvertently removed such a thin layer of stainless and exposed the damage underneath when the fouling was removed.
 
Troubadour said:
Are you referring specifically to one of the cases in this thread please? It's grown from my original one!
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In connection with the original post photos and as Vyv sais there are a few defined types opf corrosion, but difficult to tell without detailed examination but our metalutgist/shaft/bearing expert said...

"shaft looks riddled with corrosion. Shaft appears insufficiently protected. Was a zinc anode clamped on the shaft or an earth strap connecting the shaft to a zinc anode?

What grade of stainless?"

So he's thinking galvanic corrosion, th elarge areas could have been started off by marine growth crustation? or something being clamped on the shaft or shaft surface damaged. It is a large area of pitting but there seesm to be similar but smaller areas all along the shaft, suggesting shaft material or insuficient galvanic protection.

If you want a full answer it needs to be sat on the bench of an expert with the full history.

We're all just doing some informed guesing I think.
 
Neil,

316 Stainless steel does not need protection, it will exist perfectly happily underwater for years provided there is nothing to disturb the chromium oxide film, such as a crevice. Anodes are needed if there is something with a different composition in contact with it, e.g. a propeller.

I agree it could be an inferior grade and seem to recall a case some years ago where this was found on a French boat.
 
Many thanks everyone.

I took the shaft out today and showed it to Mike Bellamy of local company Lancing Marine who I think is quite well known. He blames blobs of antifoul left on the shaft initiating crevice corrosion.

There is also severe corrosion in stripes at the cutlass bearing, some corresponding to the cutlass type bearing built into the Volvo shaft seal and some corresponding to the space between the two lips in the Volvo shaft seal. I knew the boat was underused before I bought it - it must have been static for a while!

There's also some pitting on the taper where the prop (Kiwi) fits. Again I'm pretty sure that wasn't there when I fitted the Kiwi in May. All quite worrying.

Can all this really have occurred in less than a year? Is crevice corrosion fast when it starts? I do suspect very poor metal quality.

I'm certainly going to have to replace the shaft. It's 25mm dia, tapers and keys at both ends and M16 end threads. Total length is just under 4ft. A new one in 316 from Lancing Marine is £280 inc VAT. Quite painful. Can I do much better anywhere else? At least I avoid carriage charges here.

It did have a clamp on shaft anode but that has disappeared - fallen off I guess. The dealer fitted it just before I collected the boat in March. As the Kiwi prop is entirely stainless and plastic, and the shaft is isolated from the engine, based on what Vyv said, do I need an anode on it? The boat does not have any other anodes. The cast iron keel is epoxy encapsulated. The epoxy has gone on the bottom edges but it doesn't seem to be corroding much. The swing blade was galvanised but most of that has gone and I'm epoxying it after blast cleaning.

When fitting the Kiwi prop should the taper be greased or dry or what (the Kiwi agent says grease it)? How do you guard against this corrosion? You can't guarantee a perfect fit all over the taper unless it's machined to micro accuracy!

Vyv, do you want the extra photos or no point?
 
Troubadour said:
Many thanks everyone.

I'm certainly going to have to replace the shaft. It's 25mm dia, tapers and keys at both ends and M16 end threads. Total length is just under 4ft. A new one in 316 from Lancing Marine is £280 inc VAT. Quite painful. Can I do much better anywhere else? At least I avoid carriage charges here.
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I had a new one made up by a local steel fabricator (local to me). He just sourced a blank and copied the old one.
There are thousands of them on industrial estates all over the country. Just search yellow pages then go round waving cash.
 
Yes please if you don't mind.

Suggest you try a magnet on your shaft. 300 series will not be attracted, but inferior 400 series will.

The damage you are describing in way of the cutless bearing and prop sounds like crevice corrosion. For the prop this implies not ideal fit, for the bearing probably just a long time static.

Edit: I agree with other posters about a new shaft. A small engineering shop will be far cheaper, e.g. £60 at one I know in Bangor.
 
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It's quite magnetic :mad:

Will do photos later. Now I have to work on the car, that's playing up too! :mad::mad:

I can get a piece of 25mm 316 for about £85. Duplex seems about double and I can only find 1" not 25mm at the moment. Is it worth it?
If anyone knows a good machine shop in Brighton, tell me who!
 
vyv_cox said:
Yes please if you don't mind.

Suggest you try a magnet on your shaft. 300 series will not be attracted, but inferior 400 series will.
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Vyv, I had a similarly pitted propshaft as Troubadour's , and on advice from a boatyard four years ago, I fitted a new propshaft in 4462 'Remanit- Vetus'. It is also magnetic, but google's infalliable literature says it is the most corrosion resistant SS. I assume 400 series and 4000 series is a different beast ( I hope) ? I took it out again last year and it was still pristine.
Will.
 
Stainless steels with a four digit code are duplex. Your 4462 is ferritic/austenitic and so will have some magnetic attraction. It is also heat treatable, so potentially stronger than a 316.

It sounds likely that Troubadour's is a 400 series, anything from 5 to 13% chromium but no nickel. Nowhere near as corrosion resistant as a 316.
 
Vyv
Is it worth going for the duplex F51/2205 or will 316 be adequate please? The hub of the KiwiProp is 316 maybe I shouldn't mix.
I notice one prop shaft maker advertises that he uses 316L - is that an advantage for this application?

The car now has brakes again so I'll go and do those photos!
 
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I've added some more photos to https://picasaweb.google.com/106544940609481176193/20120221 showing the corrosion at the cutlass bearing, the shaft seal and the propeller taper.

Vyv, let me know if you want any emailed to you in hi-res. Some of these are not completely sharp, taken in the garage with available light hand held. If you want any for your chamber of horrors I'll do again with a tripod.
 
I know it's 9 years old and I wouldn't expect any compensation but I think I would take it to a Jeanneau dealer and let them see it and get their opinion on it.
 
Useful photos. The ones at the cutless bearing, seal and prop taper are definitely crevice corrosion. Your magnet test probably identifies the shaft as being 400 series steel, 410 seems likely. The resistance of this alloy to crevice corrosion is considerably less than would be expected from a 300 series, even 304, which is a lot less than 316. The majority of boats use 316 without problems, so I would not be considering anything else.

Can I use some of your photos for the website please? No need for hi res.
 
The ones with sharp edges look like rolling laps ie surface defects that have then been rolled back in the mill. Seen lots of them in the past but if that is the case, then where are the bits of steel that have been rolled over?

In a way it doesnt matter. That shaft clearly is made from cr*p steel and needs replacing. Whilst I guess that it wont break under load as it looks at the moment, the problem is you dont know what else is lurking unseen within the bar.
 
davidwf said:
I think you may have to think about changing the Volvo Shaft seal as well, just in case the corrosion has damamged the lip seals.
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It was a new one last March :(
It hasn't dripped at all and looking at the marks I think the corrosion is in the dead space between the lips. I presume it won't corrode at the lips.
 
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