Mabus
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Magnesium not corroded in sodium hydroxide solution
I recently took apart an old FDD (Floppy Disk Drive) and discovered that the structure is made of a light metal, very brittle. Since I felt it was too
light, I wasn't sure if it was made of aluminium or aluminium-magnesium alloy or just magnesium. So I decided to test it by putting small chunks in a
solution of sodium hydroxide. I also added an aluminium wire for comparison. In just a few seconds, it started to bubble on both, meaning it was one
of the above. Since magnesium reacts slowly with hot water and aluminium does not, I added a few more pieces into another beaker with hot water, along
with a few magnesium chips for comparison. The magnesium began to slowly bubble, while the FDD body metal did not. So I then decided to add a few
magnesium chips in the sodium hydroxide solution for a better comparison. A few small bubbles formed on the surface, vanished and then nothing. No
reaction. I agitated the beaker, nothing happened. The aluminium wire still bubbles in the solution, meaning there's still sodium hydroxide there.
Why hasn't the magnesium reacted?
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Bert
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The quick and cheap test for Magnesium/high Magnesium alloys used by scrap yard workers is to clean the surface with sandpaper or a grinder, then
brush some VINEGAR onto the freshly cleaned metal. Surface will bubble if there is a high % Magnesium in alloys-
Rapopart’s Rules for critical commentary:
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Anatol Rapoport was a Russian-born American mathematical psychologist (1911-2007).
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unionised
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"Why hasn't the magnesium reacted?"
Because magnesium doesn't react with alkalis.
It will react with hot water- but the product of that reaction is magnesium hydroxide and the presence of hydroxides inhibits the reaction.
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Mabus
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I just added vinegar to my magnesium flakes. It bubbled immediately and it reacted vigorously.
Nothing happened to my FDD metal. No reaction. I guess it's just some light aluminium alloy.
I always had the impression that magnesium is attacked by alkali hydroxides, since it can slowly dissolve in water.
I wonder, does magnesium pasivate in alkali medium?
Oh and the the magnesium I used for reference is very pure, pure enough to be used in Grignard reactions.
EDIT: Oh, never mind.
EDIT 2: Surprisingly I haven't found any source that describes the behavior of magnesium metal in alkali solutions, whether it reacts or not.
[Edited on 5-10-2014 by Mabus]
[Edited on 5-10-2014 by Mabus]
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DrMario
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It's not directly magnesium that dissolves in water, but magnesium hydroxide. This is trivial to prove, as the gas created during the reaction is
hydrogen.
As for Mg in alkaline solutions, I tried this right now: prepared a ~0.5M solution of KOH in water, and put a piece of magnesium in it. At least at
room temperature nothing at all is happening. Granted, my magnesium isn't "very pure", only slightly purer than 2N.
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Bert
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Speaking from my experience in pyrotechnics-
Aluminum is usually best protected by a mildly acidic environment if water is used in processing- A small % of boric acid may be used for this purpose
in water bound compositions, particularly if nitrates are included.
High % Magnesium/Aluminum alloys DO NOT benefit from this. Nor do they take a strongly basic environment well- Basic environment + Aluminum +
nitrates = strips the Aluminum oxide coating, generates ammonia and may lead to an accelerating decomposition, thermal runaway and a big old
industrial accident if sufficient quantities are involved. If one is foolish enough to add chlorates to the above... Chlorine dioxide. Guaranteed bad
time at the fireworks factory.
Pure Magnesium sees little use, and it needs to be processed WITHOUT WATER, then protected from atmospheric O2 and water vapor in storage. A light pre
coating of a drying oil such as boiled linseed oil with sufficient time allowed for drying of the oil film before mixing of is the bare minimum. A
coating of plastic may be better...
Rapopart’s Rules for critical commentary:
1. Attempt to re-express your target’s position so clearly, vividly and fairly that your target says: “Thanks, I wish I’d thought of putting it
that way.”
2. List any points of agreement (especially if they are not matters of general or widespread agreement).
3. Mention anything you have learned from your target.
4. Only then are you permitted to say so much as a word of rebuttal or criticism.
Anatol Rapoport was a Russian-born American mathematical psychologist (1911-2007).
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Dan Vizine
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My vague recollection is that an aqueous solution of silver nitrate is used to identify Al containing some Mg. Wipe some on a cleaned area. If Mg is
present it will turn dark.
There aren't many Al alloys lighter than Al. To the best of my limited knowledge, only Li and Mg alloys commonly are.
[Edited on 8-10-2014 by Dan Vizine]
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blogfast25
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Quote: Originally posted by Dan Vizine  |
There aren't many Al alloys lighter than Al. To the best of my limited knowledge, only Li and Mg alloys commonly are.
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Al/Be alloys like AlBeMet, ranging from Al/Be 80/20 to 38/62.
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Dan Vizine
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They do exist, but Be alloys are not commonly encountered. In fact, they are very rare in everyday life due to the premium prices they command.
Scandium/Aluminum is another light alloy, rarely seen, and for the same reason. Although it is very slightly heavier than Al.
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DrMario
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| Quote: Originally posted by Dan Vizine | They do exist, but Be alloys are not commonly encountered. In fact, they are very rare in everyday life due to the premium prices they command.
Scandium/Aluminum is another light alloy, rarely seen, and for the same reason. Although it is very slightly heavier than Al. |
They do make bicycle frames from scandium-alloyed aluminium. Usually they just call them "scandium frames". Way too expensive, considering that the
no-fatigue-limit characteristic of aluminium still remains. I would much rather get a Ti frame, if I could afford it.
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