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PostPosted: Thu May 26, 2011 10:35 am 
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Quick question, i have a stuck seat post so i was going to try the caustic soda method on it...but i was going to spray it on instead of dumping it down the tube because i don't think i can bung it up sufficiently to stop the stuff going everywhere.

The question is this...will it damage the paint? i don't think it will but i'd like a second opinion,cheers in advance.


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PostPosted: Thu May 26, 2011 11:08 am 
Anglian Deputy AEC
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Copied from the Classic Lightweights website.

Quote:
Seatpin removal
Author: John Conway

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.


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PostPosted: Thu May 26, 2011 12:36 pm 
SotS Winner
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It may well damage the paint, it's very nasty stuff. It is very,very effective in the alloy post in steel frame situation though. I very recently used a caustic soda based product called Drano - I don't know what the strength is but it worked very fast - I was done in about 1/2 an hour - of course I was removing a post that was the right size for the frame - I just needed to dissolve the corrosion from the outside of the post - If you're unlucky enough to have a too-large post stuck in your frame it could be a much longer job.


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PostPosted: Thu May 26, 2011 1:40 pm 
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I have a suggestion to improve this. Assuming your seat tube is hollow through to the bottom bracket.....
Saw seatpost to within an inch of the frame. Then put a cork or rubber bung in the top of the seatpost ( the kind used in home wine baking might do). Mount the frame in a stand or makeshift holder UPSIDE DOWN. Get a funnel and tubing (e.g brewing equipment type) and feed into seatpost through bottom bracket. Pour your solution in. The fact that the frame is upside down and you are using a funnel should minimise risk of getting soda on the paintwork. Also you can empty the solution from the frame without moving it by using the bung. Lastly the section in which the bung is should be less eaten by the soda , giving you something to grab and pull out at the end.


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PostPosted: Thu May 26, 2011 1:49 pm 
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I've done the dump a load of caustic soda down the seat tube method and it's very difficult to bung the BB area up - it does go everywhere. Raymond's method sounds like a good bet. Not sure about spraying, might be difficult to get enough down the seat tube for it to have an effect.

Also, don't do it if the frame's aluminium. You probably already knew that.


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PostPosted: Thu May 26, 2011 1:58 pm 
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yeah its a steel frame, i was thinking it would just take a misting of super strong concentrate to get rid of the reacted stuff that is jamming it in there...the more i read the stupider it seems.

I might give the flipped upside down method a try if i do get round to it...the saddle is at the right height for me though and having had a caustic soda skin burn in the past i'm kindof putting it off.


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PostPosted: Thu May 26, 2011 7:13 pm 
Dirt Disciple

Joined: Mon Oct 12, 2009 5:43 pm
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Location: Bradford-on-Avon, Wiltshire
If you fill the bottom bracket shell with Plasticine it will stop any caustic soda from escaping from the bottom.


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PostPosted: Fri May 27, 2011 10:01 am 
retrobike rider / Gold Trader
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Caustic soda is only good for steel and Titanium frames, as it doesn't corrode them. The only issue with a ti frame may be if it has alloy riv nuts for the bottle cages or an alloy insert for the seatpin.
I will be completing my 93 team ti build soon, and I face the issue of a buried seatpost about 6 inches inside the tube, plus it is sat on top of alloy riv nuts, and there is an alloy shim glued inside the top of the seat tube!
A very delicate job in my case.


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PostPosted: Sat Oct 20, 2018 4:02 pm 
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Benandemu wrote:
Caustic soda is only good for steel and Titanium frames, as it doesn't corrode them. The only issue with a ti frame may be if it has alloy riv nuts for the bottle cages or an alloy insert for the seatpin.
I will be completing my 93 team ti build soon, and I face the issue of a buried seatpost about 6 inches inside the tube, plus it is sat on top of alloy riv nuts, and there is an alloy shim glued inside the top of the seat tube!
A very delicate job in my case.


Apologies for resurrecting an utterly ancient thread - but are you quite sure that Titanium is not corroded by caustic soda? I've got a Ti frame with the remnants of a broken seatpin in the seat-tube, and it appears to be the only solution to remove....

I'm also slightly concerned that the bottle cage rivets in the seat-tube might be alloy and not Ti.... (great point about that by the way). How does one tell?


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PostPosted: Mon Oct 22, 2018 8:18 am 
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Sorry if I'm hijacking the thread, but for those of afraid using the soda solution, I've done freeing a corroded alu seatpost up from an old steel race mtb frame, here's how:

I've was lucky enough to have about 4-5 inches of seatpost left outside of the frame. So I've removed the clamphead, put the now headless sp. end into a vice and fixed it very strongly in it, and was spraying freezer spray into it via the now opened top hole. I've used the frame as the "lever" to turn it slowly in either direction until -with a frightening screeching loud sound - the corroded surface between the seatpost & seattube gave up. Then after a bit of struggle I've pulled the frame constantly turning and rotating while the seatpost has remained fixed in the vice until the job was done. Sure, I was a bit skeptic about success before starting this method but it was well worth it.

Make sure the part of the alloy seatpost inside the seattube got sprayed very well because it needs to be shrinked more than the steel frame material to lose contact from the chemical corrosion bond. If one attempt is not enough, try it again as the shrinking sometimes needs more time, don't give it up.


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