So I’ve got an aluminium seatpost that has corroded itself fast into a steel frame.

Some crazy folks on YouTube have remedied this problem by filling the seat tube and post with caustic soda solution and dissolving the seat post.

I work with sodium hydroxide, and understand that it’s not particularly nice, but i’m willing to give it a shot.

Anyone here tried this? If so, any tips?

Mechanical means preferable to chemical ones I think

But if you have tried all the mechanical one then go for it. Sodium Hydroxide has a strong affinity for Aluminium so should dissolve it enough to weaken it pretty easily.

Just wear gloves

Thanks.

I meant to say, I have tried various mechanical means, this is pretty much last chance saloon I think.

Never tried but it should work as Sodium Hydroxide will dissolve Aluminium and Aluminium Oxide as it’s amphoteric. It will take 371cm3 of pH13 NaOH to dissolve 1g of aluminium. So may take a fair amount. As iron (Steel) does not have amphoteric properties it should not react.
But I guess it may be easier to find a way of getting some leverage.

Yes – I have used sodium hydroxide to dissolve an aluminium seatpost from a steel frame.

I removed all the components from the frame first, and poured sodium hydroxide solution in through the bottom bracket to fill the seattube/seatpost. I changed the solution once a day and it probably took 4-5 days in total. I needed to block the seatpost end of the seatube by wrapping a latex glove over it after the end of the seatpost dropped out as there was still more seatpost attached inside the frame.

Be very careful with the sodium hydroxide solution though – follow the instructions when mixing it (water first, then add sodium hydroxide I think). Also wear rubber gloves and don’t get it on your skin or in your eyes. Put a decent sized drip tray or washing up bowl underneath to catch any leaks/spillages. It is very nasty stuff so make sure pets/kids/family/wildlife can’t get at it – leave the bike frame with the solution in somewhere secure.

Bear in mind that the sodium hydroxide will also strip the paint. I ended up powder coating my frame afterwards!

Not used sodium hydroxide to get a seatpost out but i had one stuck in an a steel frame. I ended up cutting the seatpost close to the seat tube, then using a hack saw blade removed from the saw, to cut the tube length ways. Took me a couple of days to do it (not sawing all the time) but it came out fine once the post was slit. No damage to the frame and no mess.

Thanks guys, great advice.

I use sodium hydroxide a lot (I’m a chemist) so am fairly comfortable with the risks/precautions. Was more interested in how to go about it.

I did spend an hour or so cutting a slit with a hacksaw blade, but lost patience.
I had started to cut into the frame too.

Perhaps I’ll give that another go though…

Thanks again.

Are you sure its not an ali frame and steel post – that would be a disastrous but also hilarious mistake to make!

Oh and get plain sodium hydroxide crystals. Some of the drain cleaners have aluminium filings in to agitate / heat the solution. You obviously don’t need any added aluminium!

Are you sure its not an ali frame and steel post – that would be a disastrous but also hilarious mistake to make!

The seat post would no longer be stuck fast though.

Might knacker your paint.

Where do you work, just out of interest as a fellow (al)chemist.

Jonba- a pharmaceuticals lab in Cambridge.

Oh, and the paint job has seen better days anyway!