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Oxalic acid and...aluminum.

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Oxalic acid and...aluminum.

Old 01-15-21, 01:25 AM
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Oxalic acid and...aluminum.

It's a late night, and I got to thinking about oxalic acid. (Fill this area with your choice of bad jokes).

As is documented in depth here at BF, we use it to clean rust off our frames. It's fairly weak as acids go, but just how bad does it attack aluminum? More importantly, could this be a good thing in the right case?

I'm going off a faint memory right now, but I could swear that last year - after pulling some chromed steel parts I was OA'ing for my 1952 Raleigh - some trivial aluminum part fell in the bath, and I didn't discover it for an hour or so. Might have been a junked seatpost I had on my forever-cluttered workbench. At any rate, when I pulled it out, the oxidation that was on said part had been significantly altered. Enough so that I started to think: "Would this work to free some stuck seatposts?"

...and then I forgot about this for eight months or so.

So, as I'm always interested in potentially less-hazardous (lye) and less painful (do the twist) ways to get stuck seatposts to start spinning, and since I've thankfully freed myself of any frames meeting that description at the moment - has anyone tried pouring an OA bath down the seattube of an inverted, stuck-post frame before? How bad was the post stuck, and what happened?

-Kurt
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Old 01-15-21, 06:21 AM
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OA will turn aluminium parts dark when they are in the same bath as steel. This is not a fast process. I doubt it would be of much use removing a stuck seatpost.
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Old 01-15-21, 08:21 AM
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I have a Specialized Expedition I am working on now that has both a stuck stem and seatpost. I have been applying PB Blaster and apple cider vinegar for the past week. This weekend I will try and free it with either the seatpost in a vice method or with an impact wrench.

If anyone has tried this I would like to hear about it. I would never try the Lye method but OA and enough time seems like it's worth a try.
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Old 01-15-21, 09:49 AM
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I'm not sure OA could be considered weak. It rates higher than many others on this table although the math isn't simple enough for my aging brain.
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Old 01-15-21, 10:01 AM
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Originally Posted by clubman View Post
I'm not sure OA could be considered weak. It rates higher than many others on this table although the math isn't simple enough for my aging brain.
Nerd alert:
Without going too far down the rabbit hole, the term "weak" acid has a specific meaning, being an acid with Ka < 1. So oxalic acid is a weak acid, but the strongest of the weak acids listed in that table.

But for this discussion, it should be kept in mind that strong/weak is not the same as how concentrated it is. A high concentration solution of oxalic acid would attack aluminum more vigorously than a dilute solution of sulfuric acid.
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Old 01-15-21, 11:34 AM
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Originally Posted by Kobe View Post
I have a Specialized Expedition I am working on now that has both a stuck stem and seatpost. I have been applying PB Blaster and apple cider vinegar for the past week. This weekend I will try and free it with either the seatpost in a vice method or with an impact wrench.

If anyone has tried this I would like to hear about it. I would never try the Lye method but OA and enough time seems like it's worth a try.
If using a vice, put a good solid-axle hub in the rear.

Tight.
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Old 01-15-21, 01:05 PM
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i'd read somewhere about using ammonia to free aluminum parts. better at attacking aluminum oxidation vs rust attackers since it's not actually rust. i've not tried it, yet
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Old 01-15-21, 09:32 PM
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Originally Posted by Kobe View Post
I have a Specialized Expedition I am working on now that has both a stuck stem and seatpost. I have been applying PB Blaster and apple cider vinegar for the past week. This weekend I will try and free it with either the seatpost in a vice method or with an impact wrench.

If anyone has tried this I would like to hear about it. I would never try the Lye method but OA and enough time seems like it's worth a try.
Never had luck with PB - at least, not to eat enough of the post to make removal easy. At best, it was able to seep in the gap - enough so to help lubricate the oxidized areas, allowing the post to finally spin free.

R.J. the Bike Guy's impact socket method seems pretty good for forged posts, but stuck posts are often slammed posts where that's not an option.

Originally Posted by CO_Hoya View Post
Nerd alert:
Without going too far down the rabbit hole, the term "weak" acid has a specific meaning, being an acid with Ka < 1. So oxalic acid is a weak acid, but the strongest of the weak acids listed in that table.

But for this discussion, it should be kept in mind that strong/weak is not the same as how concentrated it is. A high concentration solution of oxalic acid would attack aluminum more vigorously than a dilute solution of sulfuric acid.
Would it be correct to assume that increasing the amount of OA wood bleach crystals to a seatpost could effectively create a fairly potent solution?

Originally Posted by thook View Post
i'd read somewhere about using ammonia to free aluminum parts. better at attacking aluminum oxidation vs rust attackers since it's not actually rust. i've not tried it, yet
Have had exactly 0% success with ammonia on stuck seatposts. Granted, the posts that I tried this method on were really bad.

-Kurt
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Old 01-16-21, 08:39 AM
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Originally Posted by cudak888 View Post
Would it be correct to assume that increasing the amount of OA wood bleach crystals to a seatpost could effectively create a fairly potent solution?
Kurt
Yes, certainly that would be the case. The pH of that solution would not be as low as the equivalent with a strong acid, but the strength (e.g. concentration) would scale with the amount of OA added.
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Old 01-16-21, 10:29 AM
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All I know about OA and aluminum is from experience. My experience was not good. I put drive side crank with rings into a bath for about 24 hours. The bolts looked very good, but the AL parts had dark splotches on them. I'll never put AL in OA again.
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Old 01-16-21, 11:53 AM
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Anyone tried hydrofluoric acid?

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Old 01-18-21, 09:38 AM
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Originally Posted by dweenk View Post
All I know about OA and aluminum is from experience. My experience was not good. I put drive side crank with rings into a bath for about 24 hours. The bolts looked very good, but the AL parts had dark splotches on them. I'll never put AL in OA again.
That lines up with what was mentioned here.

It does bring up the question of what would happen if the solution were in a steel seattube working on an aluminum post. I might have to seek out a frame with a stuck post just to give this a go for the sake of it.

-Kurt
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Old 01-18-21, 12:50 PM
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I collect vintage BMX bikes and I have had my fair share of stuck seat posts. What has worked the best for me with Aluminum is a old mechanics trick of Automatic Transmission Fluid mixed with Acetone. But it still can take a huge amount of force to twist the seat post free. And I'm not too sure how healthy that mixture is.


One thing I've been wanting to try is reverse electrolysis. Aluminum and Steel creates galvanic corrosion which is the transfer of electrons from a anode to a cathode in the presence of a electrolyte. So I can't help but wonder if you can reverse the process but I haven't read anyone trying it.
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