Why is my shaft pitted here (see pic)?

demonboy

demonboy

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Removed the shaft the other day and upon inspection it has some pitting in one area. It's the part that fits through the stuffing box (now to be replaced with a dripless seal) and the pits are quite severe. I'm guessing it is electrolysis but wondered why only this part. The part outside the boat in the water, and the part at the flexible coupling, are fine. The shaft is 10 years old, has done many hundreds of engine hours, but the stuffing box was diligently greased regularly. Needless to say we're replacing the shaft but I wondered if anyone has any clues as to why it's happened only in the stuffing box.

P9060112.jpg
 
That is anaerobic pitting. For stainless to be stainless it has to have a constant supply of oxygen. When the shaft sits idle, the tiny sliver of water between the cutlass bearing material and the shaft quickly become denuded of oxygen and the shaft corrodes.

Interestingly this is almost only seen in tropical (very warm) waters. I have seen a hole appear in the tubing of a boarding ladder in a matter of weeks where a barnacle has become attached. It drilled a clean hole straight through.

With you in Thailand it's also a severe problem with the stainless steel expansion bolts that have been placed on the sea cliffs for belays for all the rock climbers. The anaerobic atmosphere in the 'drilled hole' quickly reduces the stainless steel threaded bolt and expansion cone to nothing.

It's much less common in temperate waters but I have seen an extreme case on the backing plate in the structural box for the upper chainplates in a Vancouver 34, but she had spent her life cruising the Med.

The solution with the propshaft is to make sure the engine/propshaft is run regularly.
 
local corrosion and pitting probably initiated by particles in the stuffing box area. The corrosion may extend more deeply into the metal than the surface pits seen, weakening the shaft and making it vulnerable to shear failure. Some kind of insidious ion/anion interaction.

What is the material of the stuffing box and liner ? Is there a linear weld in it ? (pits seem to be in a line)

Solution - VicS and Vyv Cox, I suggest.

And Tim Bennet :)
 
TimBennet said:
That is anaerobic pitting. For stainless to be stainless it has to have a constant supply of oxygen. When the shaft sits idle, the tiny sliver of water between the cutlass bearing material and the shaft quickly become denuded of oxygen and the shaft corrodes.

Interestingly this is almost only seen in tropical (very warm) waters. I have seen a hole appear in the tubing of a boarding ladder in a matter of weeks where a barnacle has become attached. It drilled a clean hole straight through.

With you in Thailand it's also a severe problem with the stainless steel expansion bolts that have been placed on the sea cliffs for belays for all the rock climbers. The anaerobic atmosphere in the 'drilled hole' quickly reduces the stainless steel threaded bolt and expansion cone to nothing.

It's much less common in temperate waters but I have seen an extreme case on the backing plate in the structural box for the upper chainplates in a Vancouver 34, but she had spent her life cruising the Med.

The solution with the propshaft is to make sure the engine/propshaft is run regularly.
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Great answer, thanks Tim. We sat idle for three years in India, only turning the engine occasionally and rarely putting it in gear (maybe once every three months for thirty seconds). Lesson learnt.
 
Not quite. Stainless steels most definitely do not need a constant supply of oxygen, the alloys are used in all sorts of environments that contain no oxygen whatsoever, eg natural gas, a whole range of chemical manufacturing, etc. Pitting and crevice corrosion occur by the same mechanism, explained in detail on my website under 'metallurgy'.

Briefly, stainless steels depend for corrosion resistance on a film of chromium oxide that forms very rapidly in air and is extremely stable. Thus the extraction of chromium from its naturally occurring oxide requires a huge input of energy. This film can be ruptured quite easily in the form of micro cracks and grain boundaries, which then allows corrosive media to attack the metal below. The galvanic potential of the 'active' metal below and the 'passive' film above differs, with the result that a corrosion cell develops, attacking the active metal. Thus pits just keep going once they have started. In the case of the shaft beneath propellers, cutless bearings, rope cutters, fouling, the cause is crevice corrosion but ultimately it is all pitting. Lots of examples on the website, with good photos. http://coxengineering.sharepoint.com/Pages/Crevice.aspx
 
vyv_cox said:
Not quite. Stainless steels most definitely do not need a constant supply of oxygen, the alloys are used in all sorts of environments that contain no oxygen whatsoever, eg natural gas, a whole range of chemical manufacturing, etc. Pitting and crevice corrosion occur by the same mechanism, explained in detail on my website under 'metallurgy'

Briefly, stainless steels depend for corrosion resistance on a film of chromium oxide that forms very rapidly in air and is extremely stable. Thus the extraction of chromium from its naturally occurring oxide requires a huge input of energy. This film can be ruptured quite easily in the form of micro cracks and grain boundaries, which then allows corrosive media to attack the metal below. The galvanic potential of the 'active' metal below and the 'passive' film above differs, with the result that a corrosion cell develops, attacking the active metal. Thus pits just keep going once they have started. In the case of the shaft beneath propellers, cutless bearings, rope cutters, fouling, the cause is crevice corrosion but ultimately it is all pitting. Lots of examples on the website, with good photos. http://coxengineering.sharepoint.com/Pages/Crevice.aspx
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Bit confused by this; referring to one of the photos on your website where possible cause of corrosion is "spash of antifouling"; wouldn't that be lack of oxygen? Spent a very unpleasant hour yesterday snorkelling in the River Itchen trying to scrape barnacle encrustation of the prop; unable to make much impact on the shaft which was also covered with them. If in same location next year will need to antifoul both prop and shaft - will that likely cause corrosion?
 
The example on the website is a fairly unusual one in that the shaft is on a Jeanneau, who choose to use a 400 series stainless steel for their shafts. This lends it more susceptible to crevice corrosion. My 300 series shaft has had its fair share of barnacle encrustation but no sign of crevice corrosion.

I recommend Velox antifouling for your shaft. This stuff is brilliant, not cheap but it really does do the job. We have lots of tube worm growth in the Med but since using Velox on my prop, P-bracket and parts of the shaft as a trial, none at all.
 
vyv_cox said:
The example on the website is a fairly unusual one in that the shaft is on a Jeanneau, who choose to use a 400 series stainless steel for their shafts. This lends it more susceptible to crevice corrosion. My 300 series shaft has had its fair share of barnacle encrustation but no sign of crevice corrosion.

I recommend Velox antifouling for your shaft. This stuff is brilliant, not cheap but it really does do the job. We have lots of tube worm growth in the Med but since using Velox on my prop, P-bracket and parts of the shaft as a trial, none at all.
Click to expand...
Thanks for that I'll try that next time its out of the water.
 
vyv_cox said:
Not quite. Stainless steels most definitely do not need a constant supply of oxygen, the alloys are used in all sorts of environments that contain no oxygen whatsoever, eg natural gas, a whole range of chemical manufacturing, etc. Pitting and crevice corrosion occur by the same mechanism, explained in detail on my website under 'metallurgy'.

Briefly, stainless steels depend for corrosion resistance on a film of chromium oxide that forms very rapidly in air and is extremely stable. Thus the extraction of chromium from its naturally occurring oxide requires a huge input of energy. This film can be ruptured quite easily in the form of micro cracks and grain boundaries, which then allows corrosive media to attack the metal below. The galvanic potential of the 'active' metal below and the 'passive' film above differs, with the result that a corrosion cell develops, attacking the active metal. Thus pits just keep going once they have started. In the case of the shaft beneath propellers, cutless bearings, rope cutters, fouling, the cause is crevice corrosion but ultimately it is all pitting. Lots of examples on the website, with good photos. http://coxengineering.sharepoint.com/Pages/Crevice.aspx
Click to expand...

May I ask a qn? When using SS screws to secure items to the hull, should they be bedded in a mastic (or similar) of sorts to prevent the 'rust runs'?
 
Piers said:
May I ask a qn? When using SS screws to secure items to the hull, should they be bedded in a mastic (or similar) of sorts to prevent the 'rust runs'?
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Yes. My Blakes seacocks and P-bracket are all bolted to the hull using stainless steel countersunk bolts. They are bedded in mastic and have suffered no corrosion in 30 years. When I install stainless steel items, bolts, U-bolts, etc I bed them in Sikaflex 291, which is usually pretty effective. No water = no corrosion ( in most cases)

Anyone know how do I do multiple answers in one post, please?
 
vyv_cox said:
Yes. My Blakes seacocks and P-bracket are all bolted to the hull using stainless steel countersunk bolts. They are bedded in mastic and have suffered no corrosion in 30 years. When I install stainless steel items, bolts, U-bolts, etc I bed them in Sikaflex 291, which is usually pretty effective. No water = no corrosion ( in most cases)

Anyone know how do I do multiple answers in one post, please?
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That's really helpful. Thank you.

Piers
 
Hydrozoan said:
"Multi-Quote This Message" - symbol like this "+ at bottom extreme right of each post, alongside other reply options. If that's what I think you mean.
Click to expand...

vyv_cox said:
Yes. My Blakes seacocks and P-bracket are all bolted to the hull using stainless steel countersunk bolts. They are bedded in mastic and have suffered no corrosion in 30 years. When I install stainless steel items, bolts, U-bolts, etc I bed them in Sikaflex 291, which is usually pretty effective. No water = no corrosion ( in most cases)

Anyone know how do I do multiple answers in one post, please?
Click to expand...

Ah, right, thanks. I didn't realise I had to pre-select each one and then 'reply to post' for one of them.
 
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