Corroded aluminium alloy sheaves

Had some stuck masthead sheaves. Mast is down so this is the time to fix. Turned out they were a mess of corrosion.

I could buy new ones but thought I'd see if I could save these first. The corrosion was quite hard and intractable. After some reading around, I tried boiling the sheaves in citric acid (cheap bulk bag from ebay, had it around anyway). Had to keep adding more acid as the corrosion used it up, and after a while took one sheave at a time out to scrub off the loose and put back in. I was wary of overdoing it so stopped short of perfect, then took them out and finished with a brass brush and a touch of emery cloth. Result! they are perfectly good enough for me to keep using.

Wish I'd taken before-and-after pics but phone camera on the fritz. Just thought I'd share the method.

justanothersailboat said:

Had some stuck masthead sheaves. Mast is down so this is the time to fix. Turned out they were a mess of corrosion.


Wish I'd taken before-and-after pics but phone camera on the fritz. Just thought I'd share the method.

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Not taking photos is a surprisingly common issue (you are not alone) - even though we all have a camera in our phone. I still forget and regret. We still have not got used to the idea, the pictures are free and easily distributed. I wonder how long before it becomes second nature.

But at least you admitted to the omission.

But take some pictures of the completed sheaves - if the mast is still down.

And.... yours is a useful tip (another admission - I would not have thought of boiling sheaves in the acid).

They will continue to be prone to corrosion - if they had an anodised layer it will now have been completely removed - just check when you are up the mast. You can re-anodise - but it is a faff, but not difficult if you have a 12v battery (or battery charger), acid don't recall which, Sulphuric or Hydrochloric and some patience. Lots of UTube vids. Don't bother with a coloured anodised coating - simple anodised.

If the housings are good you can buy 'plastic' sheaves (or have them made).

Jonathan

Good fix to know about. Thanks.

Were the sheaves corroded to stainless steel axles or to the mast head somehow?
As Neeves has mentioned, the anodizing has gone so it would be worth checking the price of new sheaves.
Could be cheaper than you think.


gary

If you do need new sheaves then replacing them with the mast on the ground is the way to go. And - if any of your halyards are wire and rope now is a really good time to replace them with dyneema - which might mean a new sheave (to take the larger diameter rope not the thinner wire).

If you like the idea of using dyneema but do not want to do it now - you could explore where you might get a sheave box that fills the existing slot by taking the measurements now and replacing later.

Jonathan

Aluminium forms a hard, resistant oxide layer almost instantly on exposure to the air; aluminium is highly reactive and would not be stable at normal temperatures and pressures if it didn't form an oxide layer. Anodizing merely thickens the layer. As aluminium oxide is corundum, the standard for 9 on Moh's scale of hardness, it won't be eroded by rope. If you think about it, aluminium boat hulls aren't anodized, and they are just fine.

Incidentally, soaking the sheaves in ammonia or sodium hydroxide solution might have been more effective, but it would require care.

Anodising produces a factorially thicker layer of aluminium oxide than develops naturally, according to a very quick google search - 1,000 times thicker. This may not mean that anodising lasts 1,000 times longer as 'hardness' will depend on density of the aluminium layer, as well as its thickness -

Does anodising have a greater or lesser density than naturally forming aluminium oxide?

Aluminium sheaves, low friction rings are all anodised - to give a longer life (not because anodising comes in pretty colours). If you take a sheet of aluminium and rub it with a mild abrasive - you can soon remove the naturally developed aluminium oxide. It will take a long time to remove the much thicker coating, same chemistry and mineralogy, on a LFR.

The anodised layer might have a high hardness, not far off diamond. But anodising does wear, so much for being almost one of the hardest (most abrasion resistant) materials - evidenced by looking at an aluminium sheave

Aluminium boat hulls do not need to be anodised as they are not subject to abrasion - other than the abrasive characteristics of sea water. Aluminium hulls are subject to corrosion despite the abrasive resistant nature of the thin layer of aluminium oxide - chemical and physical influences are not necessarily related.

Jonathan

Know it well !!



They were cleaned up ... polished and put back.

Neeves said:

Anodising produces a factorially thicker layer of aluminium oxide than develops naturally, according to a very quick google search - 1,000 times thicker. This may not mean that anodising lasts 1,000 times longer as 'hardness' will depend on density of the aluminium layer, as well as its thickness -

Does anodising have a greater or lesser density than naturally forming aluminium oxide?

Aluminium sheaves, low friction rings are all anodised - to give a longer life (not because anodising comes in pretty colours). If you take a sheet of aluminium and rub it with a mild abrasive - you can soon remove the naturally developed aluminium oxide. It will take a long time to remove the much thicker coating, same chemistry and mineralogy, on a LFR.

The anodised layer might have a high hardness, not far off diamond. But anodising does wear, so much for being almost one of the hardest (most abrasion resistant) materials - evidenced by looking at an aluminium sheave

Aluminium boat hulls do not need to be anodised as they are not subject to abrasion - other than the abrasive characteristics of sea water. Aluminium hulls are subject to corrosion despite the abrasive resistant nature of the thin layer of aluminium oxide - chemical and physical influences are not necessarily related.

Jonathan

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I wonder if hydrogen peroxide (or potassium permanganate if you wanted a beautiful purple component!) would have a similar effect to anodizing for small components? Exposing the metal to an oxidizing agent ought to increase the thickness of the oxide layer - providing the pre-existing oxide layer formed by exposure to air doesn't isolate the metal from the peroxide. This is a question; I don't know whether it would work or not, and put it out for more knowledgeable people than I to answer.

AntarcticPilot said:

I wonder if hydrogen peroxide (or potassium permanganate if you wanted a beautiful purple component!) would have a similar effect to anodizing for small components? Exposing the metal to an oxidizing agent ought to increase the thickness of the oxide layer - providing the pre-existing oxide layer formed by exposure to air doesn't isolate the metal from the peroxide. This is a question; I don't know whether it would work or not, and put it out for more knowledgeable people than I to answer.

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Thermodynamically yes hydrogen peroxide will rapidly attack aluminium but for the reason you give the existing aluminium oxide will prevent it happening. If there were some chlorine ions in the solution of Hydrogen peroxide that could attack the oxide and then the peroxide would reoxidise the nascent aluminium. In practice I doubt you would see any reaction.

Wow, this got more response than I expected.Thanks everyone. I learned a lot from the replies. Seems like I might have been lucky and replacement may be a good option too :-( I had just had a different surprise boat-part bill so thought I would see if these were savable. They also need new bushings. I change halyards (all modern synthetic rope but I think one could do with an upgrade) from deck but I can only work on these with the mast down.

I'm not sure if they were anodised and I would have been hesitant if they were. The uncorroded parts of the metal were dark (still are, except where I emeried off the edges of damage) but a few chipped off bits were exactly the same colour. I am pretty sure these are over 20 and they may even be 40 years old.

The corrosion is much bulkier than the metal and jammed them up - I was surprised to get back to obviously the original surface (can still see casting and machining marks in unworn parts) with just some rough patches and pitting - fortunately the pitting is not on parts that touch the rope.

Refueller, your pic shows it exactly. Bottom left.

AntarcticPilot, I agree there is more effective chemistry but I would worry about damaging the underlying metal - and I have citric lying around the house and no sodium hydroxide! I love your idea of a purple permanganate coating but I don't think I can make it work at home.

(deleted - accidental duplicate)

AntarcticPilot said:

I wonder if hydrogen peroxide (or potassium permanganate if you wanted a beautiful purple component!) would have a similar effect to anodizing for small components? Exposing the metal to an oxidizing agent ought to increase the thickness of the oxide layer - providing the pre-existing oxide layer formed by exposure to air doesn't isolate the metal from the peroxide. This is a question; I don't know whether it would work or not, and put it out for more knowledgeable people than I to answer.

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I did some anodising, acid, battery charger and recall that you need clean aluminium in order to anodise. One of the warnings related to touching the pieces as with a clear coating you will see the finger prints 'through' the anodising). You have to remove the existing, naturally forming, oxide layer first. My instructions came from a cross section of UTubes. I don't think I'm more knowledgeable - a little bit of knowledge can be very, very dangerous.

I had no success with colouring the pieces but did produce, what I thought was, a decent anodised coating which was quite robust (it was part of the rode and the coating had to be robust).

Later once I'd proved that my ideas worked I had them made professionally and professionally anodised and then decided I preferred them in Duplex stainless! This went wrong, lost in translation - I'd used 'my' professionally made components and they were black anodised as 'examples' or 'samples' (same manufacturer who makes Ronstan's LFRs) and the processor of the Duplex stainless, different company, thought I wanted black Duplex - and duly painted then black - not the polished I was expecting.

The paint on the Duplex versions chipped and looked tatty even though the Duplex itself was robust.

Jonathan

As I mentioned I don't recall the acid used, hydrochloric or sulphuric - both are, very, nasty to (mis)handle. If there was the choice citric is easy (and can be easily sourced - I used to sell it in Tonnage quantities as the powder). Concentrated HCl and H2SO4 are easily accessible here in Oz but I recall, recesses of my memory, it might be restricted in the UK.

Anodising is quite easy - just have decent HazMat kit - goggles and gloves, ban pets and children. You also need to plan how you will dispose of the acid when you have finished.

Jonathan

There was a member here, 5-10 years ago, who wrote an article for PBO on anodising (and he successfully coloured items). If you had access to a digital form of PBO you might find the article. I confess I've forgotten his name.

Fr J Hackett said:

Thermodynamically yes hydrogen peroxide will rapidly attack aluminium but for the reason you give the existing aluminium oxide will prevent it happening. If there were some chlorine ions in the solution of Hydrogen peroxide that could attack the oxide and then the peroxide would reoxidise the nascent aluminium. In practice I doubt you would see any reaction.

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Shame - I loved the idea of purple "anodizing"! But I know the natural oxide layer is extremely tenacious and hard, so I guess you're right. Maybe I ought to try it sometime, just for fun. Maybe if the aluminium was pickled first in caustic soda and then immediately dropped into the permanganate (or peroxide) solution?

AntarcticPilot said:

Shame - I loved the idea of purple "anodizing"! But I know the natural oxide layer is extremely tenacious and hard, so I guess you're right. Maybe I ought to try it sometime, just for fun. Maybe if the aluminium was pickled first in caustic soda and then immediately dropped into the permanganate (or peroxide) solution?

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Likely to get a very vigorous explosive reaction think thermite reaction but even if it worked then I doubt there would be any color to the anodising. The permanganate colour comes from the highly oxidised state of the manganese which would end up being reduced to the MnO2 lower state a mucky brown black always assuming there isn’t a complex aluminium manganese oxide formed and it’s to early of a Saturday morning to try and work that out.

Fr J Hackett said:

Likely to get a very vigorous explosive reaction think thermite reaction but even if it worked then I doubt there would be any color to the anodising. The permanganate colour comes from the highly oxidised state of the manganese which would end up being reduced to the MnO2 lower state a mucky brown black always assuming there isn’t a complex aluminium manganese oxide formed and it’s to early of a Saturday morning to try and work that out.

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Yes, I guess you're right - that makes sense; I hadn't though about the colour of the reaction products. I don't think the reaction would be that bad - it would be in solution, so you could control the permanganate concentration. Powdered aluminium with solid permanganate would probably make a nice rocket fuel though!

Neeves said:

I did some anodising, acid, battery charger and recall that you need clean aluminium in order to anodise. One of the warnings related to touching the pieces as with a clear coating you will see the finger prints 'through' the anodising). You have to remove the existing, naturally forming, oxide layer first. My instructions came from a cross section of UTubes. I don't think I'm more knowledgeable - a little bit of knowledge can be very, very dangerous.

Click to expand...

You can't remove the oxide film from aluminium in the air, well you can but it reforms in a millisecond actually less. I think the process is more about removing any contaminants and providing a physically clean surface. I know bugger all about anodising but I know a bit about chemistry and aluminium.

Taking a step back, what are the sheaves used for? Back in the day metal sheaves were often specified for use with wire halyards.

Nowadays wire halyards are no longer needed - or indeed generally recommended - as dyneema cored ropes are a better alternative. For these robust “plastic“ (precise spec varies by size & load requirements) sheaves are generally better.
So might be worth swapping the sheaves anyway.

An old crewmember was an optician. He took my traveller cars which were blocked with corrosion and cleaned them in his ultrasonic bath, normally used for cleaning glasses. Worked well.

AntarcticPilot said:

Yes, I guess you're right - that makes sense; I hadn't though about the colour of the reaction products. I don't think the reaction would be that bad - it would be in solution, so you could control the permanganate concentration. Powdered aluminium with solid permanganate would probably make a nice rocket fuel though!

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I did a bit of research and there are over 100 known Manganese/Aluminium silicate minerals - and quite a few are pretty colours (mainly pink or orange)! So maybe there's hope for a nicely coloured result from oxidizing aluminium with permanganate solutions. Maybe add sodium silicate to the mix?

Manganese is surprisingly common in rocks!