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Galvanic corrosion

Discussion in 'off topic' started by matt j, Mar 21, 2020.

  1. matt j

    matt j pfm Member

    I'm no chemist but I suspect I already know the answer to this and it is 'nothing'.

    My question is what can I do to separate two parts that have suffered galvanic corrosion, in this case stainless and aluminium?

    I regularly have to change a knackered fan wheel from it's motor, the clever people who made it used an aluminium hub on the fan and the motor shaft is s/s. They know they f*cked up because the new replacement fan they send has a s/s hub, unfortunately the only way I've managed to find to separate them also writes off a perfectly good (and expensive) fan motor as damaging the shaft is inevitable so the customer has to stump up for the cost of a motor as well.

    At the minute it is an extremely tedious, exhausting and time consuming job because it has to be done in situ and it requires drilling down the aluminium hub shaft in ever increasing drill sizes until a big enough channel has been made to allow access for a flat bladed screwdriver to be driven in and used as a chisel to split them apart, it usually takes multiple channels as it doesn't completely split from one side. It took me 2 hours of drilling yesterday whilst knelt down and torso inside a a commercial baking oven, absolutely back breaking, I'm still sore today and I'm used to this sh*t!

    I suspect there isn't an easy way to do this but I thought I'd ask.
  2. Mike P

    Mike P pfm Member

    Certainly worth applying a good smear of copper grease.
  3. Sonority

    Sonority pfm Member

  4. stevec67

    stevec67 pfm Member

    Once stuck, you can dissolve most corrosion by leaving it in vinegar for a day or two. More brutal is to put the thing in strong caustic solution, the ally will dissolve and be destroyed, the stainless will survive. Other sort metals will be toast also. Coca cola will dissolve most corrosion too. Or use heat in the usual way if feasible.
  5. stevec67

    stevec67 pfm Member

    Better to prevent it with coppergrease of course, but you know this and by the time o get the call it's too late.
  6. Vinny

    Vinny pfm Member

    If I am imagining correctly, can you not use a hole saw to cut the hub off the shaft, leaving just a small ring of the centre of the hub that you can then use a hacksaw to split and remove? Should be quick and easy that way, if it works at all.

    Might need s simple jig to keep things centred, based on something like this - https://www.ebay.co.uk/i/1734139016...MI7b-Jk9Kr6AIVk_hRCh15jQzHEAQYBCABEgIadvD_BwE

    Even seized that bad, a good soak in something like WD40 for 20-30 minutes and a good quality 3-legged hub/bearing puller should work so long as there is some feature on the hub to hook one onto.

    How big is the hub? Can you get/cut the blades off easily, then use a nut splitter?
  7. ian123running

    ian123running pfm Member

    I once neglected a tandem and had a very similar experience getting the al alloy seat post out of the steel frame. Ruined the frame tube in the process and had to get the frame repaired professionally. Now I regularly grease it. Hard lesson learned!
  8. matt j

    matt j pfm Member

    Prevention is a non starter as these are put together by the manufacturer and I only get called when they're broke which can be years down the line.

    Access is the key issue here, soaking in any sort of chemical isn't an option as I just turn up to fix it on the day, apart from that it's vertical so keeping any chemicals confined to the fan hub would prove troublesome. Unfortunately it has to be done in situ as the motor shaft passes through the oven cabinet from the back of the oven and the hole is only a couple of mm bigger than the motor shaft so the fan hub has to be completely removed to allow the motor to be withdrawn.

    Blow torching it seems to have no effect, I just do it out of frustration in a "take that ya bastid" kind of scenario when I've had enough.

    A hole saw would just leave the same issue of getting the remainder seized section off the shaft. I'll get some pictures of the remainder of yesterday's effort, it was a particularly tough one as I do a few and they vary but even the "easier" ones are a complete nightmare.
  9. matt j

    matt j pfm Member

    It's easier with a bike, I've turned the frame upside down and poured chemicals into the base of the seat tube to corrode out the ally post, obviously though you have all the time in the world to solve that, I have to do this in real time!
  10. Vinny

    Vinny pfm Member

    A hole saw need only leave a collar of a very few mm thick, dead easy to cut and remove - half inch shaft, three-quarter hole saw...….eighth to cut through. If there is a keyway, you could hit that with a hole saw and the hub would fall off.

    If you can drill it, you can use a hole saw, it'd be the way that I'd go.
  11. Gervais Cote

    Gervais Cote Predator

    Metaflux Torsion Spray : best product ever to disassemble rusty parts. Just spray, wait 10 minutes and pull out.
    Tony Lockhart likes this.
  12. Bob McC

    Bob McC Living the life of Riley

    It’s not rust.
    hifilover1979 likes this.
  13. Vinny

    Vinny pfm Member

    The old trick was to use a blow lamp on the hub and should work well enough for you. Probably need to use a hub puller after applying heat although, if there is enough room, twatting it the wrong way to start it moving could also work.

    It works for countless interreference fits in gas turbines - pour liquid nitrogen on one part or fit a heater blanket to the other mating part.
  14. matt j

    matt j pfm Member

    Pics of the aftermath





  15. matt j

    matt j pfm Member

    You've still got to get a saw in though to cut the remainder, which isn't possible.
  16. Vinny

    Vinny pfm Member

    Nut splitter. It would relieve the tension in the joint and break the "glue" formed by the corrosion.

    To be honest, faced with that from the start, I'd saw on a diameter, across the hub, as far as the shaft and then split it. Skip drilling entirely.

    Presumably the motor shaft is semi-gefooked on that one?
    sean99 likes this.
  17. stevec67

    stevec67 pfm Member

    Is the motor shaft knackered after all this pain, or just the hub?
  18. Gervais Cote

    Gervais Cote Predator

    Best way to go if you don’t want to break the shaft.
  19. matt j

    matt j pfm Member

    Yes the shaft is a write off, you can see where I've wrecked it with running the drill down it, in situ with hand tools it's impossible to be accurate enough to "cut" the hub without scoring the shaft, then of course when wailing on the splitting tool untold damage is done.

    RE: the splitter. The hub is tool long and is stuck down its length, even when I've chiselled out a whole piece the remainder is still stuck and has to be beaten off, you might be able to tell from the pics, even though that chunk has come off, the rest was still stuck in place.
  20. Vinny

    Vinny pfm Member

    Not sure of price, availability etc., but small hydraulic splitter are available. If you can find and justify one that would fit, that would take a few minutes to do the job.

    The idea of a splitter is not to remove the thing in one go but to put 2-3-4 expanded sections into the hub so that the hole that was, is opened out.

    You just need a splitter with a wide tool, not something 5-10-15mm wide.

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