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@fo105
Hi all, just managed to remove a completely jammed aluminium seatpost from a steel frame using caustic soda and thought would share experience. There seems to be a reasonable amount of misinformation floating around online which might scare people off, but this is a really great way to remove a seriously stuck seatpost, and if you are careful, with no damage to the paintwork (unlike heating).
First, saw off the top of the seatpost so that about 20mm remained sticking out. Blutack and electrical tape used to block up the remaining hole, and also bluetacked around the lip of the frame so as to keep in any solution that manages to slip down between the frame and the post (this becomes more of an issue as the post dissolves).
Remove the BB (and just about everything else from the frame) and funnelled a solution of caustic soda (sodium hydoxide pellets) in water (~300g/L) down the seat tube. use blutack to block the chainstays and downtube. I also decided to block up the top tube and the down tube at the headtube end, because I couldn't be bothered to remove the press fits of the headset. Obviously cannot block the top tube and seat stays from the seat tube end as they are blocked by the obstinate post.
When you dissolve the pellets in water, the solution will heat up significantly. Hot enough to burn so take care. When you tip the solution down the seat tube and it starts to react with the aluminium, everything will get hot, hydrogen gas will be released and bubble out of the bottom bracket shell. If you get close (although be careful not to be splashed by anything coming out of the top), you will hear a fizzing noise. Some solution may well bubble out too - nothing to worry about, just wash away with a bit of water and wipe any off that has ended up on paintwork. The water gets used up in the reaction so more solution will need to be added occasionally. Clear when there is none left as no more fizzing noise and the seat tube will begin to cool down.
After a day or two, the seatpost should be almost gone. When its finished, it should be really easy to pull out the remnants of the seatpost with a pair of pliers. If it is remotely difficult, plug the hole back up and tip in some more solution and leave for a bit longer.
Once the whole procedure is done, make sure to wash the seat tube (and possibly the headtube/seat stays really thoroughly with water - otherwise NaOH left in the frame might start to react with your new seatpost as soon as it rains! Vinegar (weak acetic acid) would also be a possibility to neutralise any remaining NaOH, then more water. Finally leaving to dry for a bit (or even better blowing through tubes with compressed air or something similar) then a good load of GT85 in the tubes to avoid rusting the insides away. Good as new for a new post.
If any caustic soda solution gets on the paintwork and stays there for a bit, it is likely to leave a white residue. This can be removed with water and elbow grease, although I found a bit of acetone made the job a lot easier. In both cases, the paint underneath is slightly dulled but should polish up fine. Generally best to wipe off any solution that spills on to the paintwork.
As others have mentioned, definitely work covering up to do this job as pretty nasty burns if contact with skin. Goggles and gloves are a must, and probably best to avoid shorts and flip flops. Tip a load of vinegar into the waste solution to neutralise it, dilute it plenty and then tip down the sink. Leave any solids in fresh water in a bucket for a few hours and then throw in the bin. If you treat the whole procedure with a sensible amount of respect, there is absolutely nothing to be afraid of regarding the chemicals you are using.
Hopefully this rambling post might be of use to somebody, or at least provide further evidence that this is a really simple and reliable way to remove a seatpost, costs considerably less that giving it to your LBS, doesn't require a new paint job afterwards and no risk of damaging the frame.
Clearly, don't use on an aluminium frame or that will disappear along with your seatpost.
If anybody has any questions or needs further encouragement, don't hesitate with a PM.