Caustic Soda to melt stuck seatpost: COMPLETED!! (see p.2)

[samc]

Note: melting is now complete, see this post for conclusions >>

I bought a ('94) Lavadome a while back and the frame is incredible to ride. BUT it's got a long alloy seatpost well and truly stuck which I've been trying to shift because it's at the wrong height. I've tried everything (including epic leverage on the saddle, slicing it from inside, filling it with ice and heating the steel, leaving it and using shims...) and have got to the last resort; melting it out with caustic soda.

So I thought I'd put some pics / vids of my progress here in case anyone else wants to try it.



Here's a video with me pouring it in:
http://www.youtube.com/watch?v=NJVAU0GXMcY&feature=youtu.be

(Clicky)


Things I've learnt:

- It's brilliant fun
- Ignore the advice on the caustic soda label re: strength - you need it way stronger than it says; at least 2 parts soda to 5 parts water
- it's seriously exothermic; I measured 100 degrees celcius off the frame (!)
- don't drink heavily before you try it

Note: melting is now complete, see this post for conclusions >>

 

[type49]

Might be thinking wrong here but.... as alloy expands more than steel, surely heating it up will make it even tighter in the frame than it was originally...???

 

[We_are_Stevo]

Not going to be anything left to be any sort of tight!

Not advisable on alloy frames of course

 

[danson67]

surely heating it up will make it even tighter in the frame than it was originally...???

Probably, but thankfully only briefly while the seatpost turns to hydrogen and sodium aluminate (gas and white powder :shock

2 Al + 2 NaOH + 2 H2O → 2 NaAlO2 + 3 H2

All the best,

 

[samc]

Might be thinking wrong here but.... as alloy expands more than steel, surely heating it up will make it even tighter in the frame than it was originally...???

You're right about the expansion properties, but I wasn't heating it up - it's an exothermic reaction so the alloy was heating itself up! I wasn't trying to remove it at the same time so it made no difference; the plan is to melt the alloy away enough, then let it cool and clean it up before trying to remove it.

 

[FluffyChicken]

Might be thinking wrong here but.... as alloy expands more than steel, surely heating it up will make it even tighter in the frame than it was originally...???

You're right about the expansion properties, but I wasn't heating it up - it's an exothermic reaction so the alloy was heating itself up! I wasn't trying to remove it at the same time so it made no difference; the plan is to melt the alloy away enough, then let it cool and clean it up before trying to remove it.

Adding the solid to the water is exothermic. How did you do it, make the solution or just fill it with pellets/powder and add some water ?

I would recommend having Citric Acid (solid) to hand in case you spill it on the floor, a bit of that and it'll neutralise the solution.

Though dilute it slightly and pour down you drain to clear it out. It is drain unblocker (a bit weaker still and it's oven cleaner)


*EDIT, don't stick the citric acid in the frame, while the sodium hydroxide will react with the aluminium and not the steel, acids will.

 

[walleater]

Reminds me of a mate taking his GT steel MTB into Halfords to get the seatpost removed.....they also used caustic soda but neglected to remove any of the other aluminium parts off the frame, so the CS seeped through the frame and wrecked his fork as well as various other things. That ended up a very expensive repair bill for Halfords!

 

[We_are_Stevo]

Used to use caustic soda to decoke my Fizzie exhaust way, way BiTD; also used to be quite amusing whenever someone tried it on their ally Allspeeds!

 

[samc]

Adding the solid to the water is exothermic. How did you do it, make the solution or just fill it with pellets/powder and add some water ?

I would recommend having Citric Acid (solid) to hand in case you spill it on the floor, a bit of that and it'll neutralise the solution.

I mixed it first by tipping the crystals slowly into the water and stirring, and then plopped it in bit by bit. It gets hotter and hotter over about 5 minutes, and stays absolutely boiling for ages after. I think because the frame's steel it seems more obvious because it stores the heat nicely.

Re; neutralising, it seems to be amazingly good at diluting; ie. it happens instantly on contact with the slightest bit of water so it's really easy to wash away. No worries in terms of neutralising it, probably wouldn't put the stuff in my mouth though, given the option!

 

[samc]

Reminds me of a mate taking his GT steel MTB into Halfords to get the seatpost removed.....they also used caustic soda but neglected to remove any of the other aluminium parts off the frame, so the CS seeped through the frame and wrecked his fork as well as various other things. That ended up a very expensive repair bill for Halfords!

Ouch! They must've left it unattended for a while or something; it does happen pretty slowly.

*soaks bike in CS*
*goes off for lunch*