Posted: Monday 21st September 2020
I have finished extracting the alloy seat pin from my tandem frame via the “dissolving method”. If any one would like information on this, I am happy to give them the benefit of my experience. All in all a good one I might add.
I was interested in removing the seatpin using coke, ie phosphoric acid. I expect that there would be some etching of the steel of the frame but on the other hand some benefits of rust proofing!
The dissolving method which I was attracted to in an early issue of Lightweight News uses caustic soda to dissolve the recalcitrant alloy seat pin. In my case there was no access via the bottom bracket as the frame was a welded frame with the seat tube sealed. All other methods of removing the corroded/frozen pin had been exhausted and I was also exhausted. Well there were other possibilities, but they were more destructive.
Firstly I decided that the seat pin could be sacrificed.
1. I cut the top off the pin leaving about 1 cm above the seat clip. In retrospect about 3 cms would have been wiser!
2. I determined that there was an end to the pin and that the seat tube did not need to be filled with any chemical. Your situation may be different.
3. I made an approximately 4N solution of caustic soda by adding 40 gms of Caustic soda grains(Crystals) to 250 mls (a standard cup) of water. Important: use rubber gloves and a plastic container. Do not use glass because the material will ruin the glass and you will have to throw it away (Recycle). I put the cut off piece of the seat pin in this solution and observed the cutting rate. It was about 0.5 mm in 24 hrs.
4. I tried a more aggressive solution of about 6N, 60 gms in 300 mls of water. This was too aggressive, it all bubbled and frothed up and chewed away at the seat post at a frightening rate. I settled on a 5N solution, 200 gms of caustic soda to 1 litre of water. Safety: Add caustic soda to the water, and not the other way around. Mind your eyes. Wear protective gloves and eyewear.
5. I then poured a solution down the hollow seat post and left it. Periodically it would froth up to the top of the seat post. If it overflows remove the overflow immediately by diluting the overflow with water. This stuff make an effective paint stripper.
6. Every 24 hrs empty, flush and refill. What to do with the residue is an environmental hazard problem. Diluted, it could probably go into the sewer. Do not put it into the stormwater drains. The residue is normally a black solution of some aluminite. I have not quite worked it out but it is very caustic and corrosive to most things except steel.
7. After 10 days of this treatment, the pin was all gone except for a 2 cm section at the top. I knew that the job was finished when I went to flush it out one morning and the residue was clear. No activity. I just pulled out the 2cm shell with a pair of pliers. It was paper thin at this time. YMMV My pin was Al Alloy 25.8+mm dia forced into a 25.8 tube without any grease I suspect with at least 250 mm immersed. Wall thickness was at least 3mm. It may have been quicker if I had woken up earlier to regular flushing and refreshing the solution.
It goes without saying to keep the stuff/process away from children and animals including birds.
Hope that this may be helpful to someone who is desperate. I was desperate.