Chemical Forums
Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: TTG TriCkSh0tZz on June 11, 2014, 01:23:10 AM
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Hi,
I have a piece which i believe is Aluminium with Copper plating. What i am trying to do is remove the Copper plating without harming the underlying Aluminium. From what i have read, Nitric acid will dissolve copper but will it harm the Aluminium?
Thanks
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Hint: Reactivity series
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Sorry but ive looked up reaction series and i still dont quite understand. Im very new to all this stuff
Thanks
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What don't you understand? Please be more specific. We don't dump full answers on Chemical Forums (see Forum Rules (http://www.chemicalforums.com/index.php?topic=65859.0)).
Did you see this: http://en.wikipedia.org/wiki/Reactivity_series
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Thank you, your link really helped. Problem sorted :)
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Thank you, your link really helped. Problem sorted :)
Not so.. What don't you understand? Please be more specific. We don't dump full answers on Chemical Forums (see Forum Rules).
While its good not to give away answers without letting the person who asked do any work, your forum rules probably don't say anything about misinformation do they?
Linking the activity series was very misleading. It strongly suggests that nitric acid reacts with aluminum, which is rarely the case.
Only very dilute nitric acid reacts with Al, all other conc. pasivates completely.
So, unless your acid is contaminated with sulfate or chloride anions, the Al will not dissolve.
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Only very dilute nitric acid reacts with Al, all other conc. pasivates completely.
Thank you for posting this detail, I was not aware of that.
While its good not to give away answers without letting the person who asked do any work, your forum rules probably don't say anything about misinformation do they?
No they don't, because everybody makes mistakes. They do ask you not to be rude though. If you're unsure, you can read the rules here (http://www.chemicalforums.com/index.php?topic=65859.0).
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Furthermore, I'm going to ask you to provide a source for your factoid, that more concentrated nitric passivates aluminum at all, much less your decidedly non-quantitative term "completely." Iron may form a passive coating with concentrated nitric, and various steel alloys can be passivated by a variety of means as ferrous centers are removed, leaving carbon, chromium and nickel behind. But the aluminum oxide coating is very acid liable, unless formed by chromate. Although the aluminum oxide coating is resistant to weathering, the amphoteric nature of the compound allows it to be attacked by acids or bases.
http://en.wikipedia.org/wiki/Passivation_%28chemistry%29#Aluminium
http://en.wikipedia.org/wiki/Aluminium_oxide#Properties
Which is another reason why we don't dump answers on this forum. Questions have to be complete, so we don't give bad answers. That's something everyone has to learn. Yahoo answers if full of random "Why?" questions, then the poster "votes up" an off-topic, angry rant they got as a responce. We don't do that here.
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I'm going to ask you to provide a source for your factoid.
Per this ref:http://khimiya.org/pdfs/KHIMIYA_19_3_PETRUSEVSKI.pdf (http://khimiya.org/pdfs/KHIMIYA_19_3_PETRUSEVSKI.pdf).
To quote:Metallic aluminium was found not to react with either concentrated or
diluted nitric acid.
[...]much less your decidedly non-quantitative term "completely."
I said that, because aluminum is one of the only metals that passivates completely. Is that hard to understand? It forms an oxide layer, which the acid does not penetrate, even after long periods of time (unless it's scraped off, I which case a new layer is formed.)
Iron may form a passive coating with concentrated nitric, and various steel alloys can be passivated by a variety of means as ferrous centers are removed, leaving carbon, chromium and nickel behind.
Do you have a ref. for that? I've dissolved Fe in nitric acid several times and it has never passivated.
But the aluminum oxide coating is very acid liable, unless formed by chromate.
Not necessarily, hydrochloric acid is the only acid that can penetrate it while dilute. All other acids either must be conc. or contain chloride ion impurities (which have excellent catalytic properties in the destruction of Al2O3.)
To quote:Thus, according to Glinka [4]
and Brady [5], aluminum reacts with diluted acids displacing hydrogen, while Greenwood
& Earnshaw [6] say that the protective oxide cover prevents any reaction with
diluted acids. On the other hand, the experiments show [7] that the only reaction that
occurs in real time with diluted acids at room temperature is the reaction with HCl(aq),
giving rise to hydrogen gas and aqueous solution of aluminum chloride. Therefore, the
simple chemical reaction based on the equation
2Al(s) + 6H+(aq) = 3H2(g) + 2Al3+(aq)
seems not to be operative for strong acids other than HCl! That being so (and the
results of experiments say very clearly it is so, indeed), one is faced with a widely
spread preconception about the negligible role of the spectator ions in reactions of
hydrogen displacement, at least for reactions including aluminum. This problem will
be addressed in more detail elsewhere.1
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I'm going to ask you to provide a source for your factoid.
Per this ref:http://khimiya.org/pdfs/KHIMIYA_19_3_PETRUSEVSKI.pdf (http://khimiya.org/pdfs/KHIMIYA_19_3_PETRUSEVSKI.pdf).
Kind of an obscure source, but I'm glad you posted it. Its good we have it here, for people who are more interested.
Iron may form a passive coating with concentrated nitric, and various steel alloys can be passivated by a variety of means as ferrous centers are removed, leaving carbon, chromium and nickel behind.
Do you have a ref. for that? I've dissolved Fe in nitric acid several times and it has never passivated.
I think Pauling's Chemistry probably has a reference to doing this. Its something of an obscure reaction, the iron passivated coating is very easily scratched.
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Iron may form a passive coating with concentrated nitric, and various steel alloys can be passivated by a variety of means as ferrous centers are removed, leaving carbon, chromium and nickel behind.
Ok, it looks like you were right, from the book "Absorption of Nitrous Gases".
To quote:
When iron is immersed in nitric acid of sp. gr.
not less than 1-25 (40 per cent. HN03), the iron becomes
" passive" and will not precipitate copper from a solution
of copper sulphate. By tapping or scratching the surface,
however, the metal may be again rendered active and function
in the normal way. The sp. gr. of nitric acid between 1-25TM
1-26 is usually known as the " passive break," owing to it
being the lowest concentration which will induce passivity in
iron. A very complete bibliography on passivity is given by
1
Heathcote. Recent work on the passivity of iron by Young
2
and Hogg seems to show that the passivity induced by nitric
acid is due to an equilibrium between iron and nitrogen tetrox- ide, but owing to the reversibility of the reaction N2O3 = |-[N2O4] + NO, it is impossible to state whether the passifying agent is nitrogen tetroxide, nitrous acid, or a mixture of both. The rate of reaction between nitric acid and iron falls off rapidly as the density rises above 1-26. A much greater degree of passivity seems to be obtained by exposing iron to nitrogen tetroxide than by treatment with nitric acid, although no gas film could be discovered on the metal.
Iron-silicon alloys are the most resistant to nitric acid in all conc.
Here's a link to the book, hosted on Sciencemadness: http://library.sciencemadness.org/library/books/absorption_of_nitrous_gases.pdf (http://library.sciencemadness.org/library/books/absorption_of_nitrous_gases.pdf). One of the best books I've read in a while. It has an incredible amount of information on various oxides of nitrogen and the oxiacids thereof.
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I recall learning many years ago that nitric acid was routinely transported using steel tank cars (railway type). That was possible thanks to passivation.
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The paper must deal with very pure aluminium (funny stuff, I had some really pure for semiconductor processes, one could make a dent with his nails). Alloys behave very differently, with little copper ruining the corrosion resistance of aluminium, allegedly because it makes the oxide layer porous. And what metallurgy calls "pure" aluminium like AA1050, AA1099 is already an alloy.
Having seen an aluminium-zinc (AA7049A) destroyed in the soil in one day, I wouldn't bet on this one in an acid, even nitric.
Aluminium corrosion being essentially a matter of oxide layer, previsions are difficult.
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Alloys behave very differently, with little copper ruining the corrosion resistance of aluminium, allegedly because it makes the oxide layer porous.
Strange. As far as I can tell duralumin is still quite corrosion resistant, despite containing around 4% of copper (and some other elements).
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The paper must deal with very pure aluminum.
Probably yes, scientific papers generally use the purist reagents possible to attempt to give the most accurate results.
I am currently testing how "aluminum foil" grade Al holds up in nitric acid.
I added 1 mL fuming nitric acid (~96%) to .2 grams Al. After 15 minutes nothing has happened.
I also added 1.5 mL 68% nitric acid to .2 grams Al. Nothing has happened to it either.
IIRC, Al foil is ~98.5% pure, impurities being Fe and Si - which are added intentionally to make it slightly stronger.
[EDIT] I added a small amount of chloride ion (in the form of CaCl2 anhydrous) to the second test tube. Surprisingly still no reaction could be observed within 10 minutes.
I had to leave the lab and won't be able to check on anything for a few hours.
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After 15 hours, the sample to which chloride ion was added is reacting very slowly, like one tiny bubble per five seconds.
The pure sample has undergone no reaction to my knowledge. This suggests that even impure aluminum pasivates entirely in pure fuming nitric.
My scale is not accurate enough to see if there was any loss of mass, but everything looks the same.
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So far as I remember SS316 piping & tanks were routinely passivated by successive washes of 10% NaOH followed by 33% HNO3.
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Alloys behave very differently, with little copper ruining the corrosion resistance of aluminium, allegedly because it makes the oxide layer porous.
Strange. As far as I can tell duralumin is still quite corrosion resistant, despite containing around 4% of copper (and some other elements).
It depends on the use. Upon regular skin contact, Al-4Cu is just fine. Under occasional rain as well. But put permanently outside, it won't resist rainwater - my own experience confirms docs. And in seawater or in the soil, it fails quickly.
Then you have surface treatments, that is mainly anodization. Al-4Cu are not the best alloys for that, but it does improve a lot. The older AA2017A, now AA2014 and AA2024 are used for aeroplane parts with anodization, and then they behave very well under rain. Same for Al-5Zn AA7075.
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An interesting thing about the statue of eros in Piccadilly circus London is that it was cast in an alloy of aluminium. I believe that this alloy contained some copper which explains how the statue has the dark patina that it does. It was restored some time ago and found to have surprisingly little corrosion given the effects of pollution and external exposure that might be expected.
Here is a link to an excerpt from an article looking at the technicalities involved in the restoration:
http://ihbc.org.uk/context_archive/42/clarke_dir/clarke_s2.htm
Just thought it might be of interest to this discussion.
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Just thought it might be of interest to this discussion.
Indeed very interesting! Thanks!
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An interesting thing about the statue of eros in Piccadilly circus London is that it was cast in an alloy of aluminium. I believe that this alloy contained some copper which explains how the statue has the dark patina that it does.
Successful TIG welding indicates that any copper contents is low. Dark patina is an indication of zinc or silicon in aluminium; for instance Al-Cu4% gives a clear anodization layer.
Presently, the preferred cast alloys are Al-Si13% and small variations, but maybe they weren't known then.
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Ah! that is interesting. I looked it up and found that it is 99.1% Al, 0.027% Fe, 0.6% Si, & 0.01%Cu. So yes it looks like it is the silicon as you say. Found this out here:
http://books.google.co.uk/books?id=o7yJAgAAQBAJ&pg=PA108&lpg=PA108&dq=piccadilly+eros+statue+alloy+composition&source=bl&ots=eSI9WcTFBn&sig=Z5KMw2IXLn8sbFWrlfYofhu4PEg&hl=en&sa=X&ei=v409VMeKJezd7QbKmoDYBQ&ved=0CGIQ6AEwDg#v=onepage&q=piccadilly%20eros%20statue%20alloy%20composition&f=false
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Or just the result of air pollution. The composition may have been as pure as was possible then, and 0.6% Si would have about no effect on the oxide colour; for instance metallic frames for windows contain this proportion of Si and Mg. At 13% Si, the colour is quite visible.
Very pure aluminium (this case) is one good receipe for longevity under rain, an other being 3% or 5% Mg (boat hulls), still an other 0.6% Mg and Si (window frames), and an other just a bit of Mn. As opposed, Cu is very detrimental to the corrosion resistance, and Zn is bad also - pity, since these two are the best aluminium alloy hardeners.
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I've cast a few sculptures in aluminium over the years and always found that it is really hard to cast if you don't have the right alloy. The 'right' alloy as far as the kind of thing available as scrap is old car wheels. The 'skin' of oxide generally impedes the flow but with alloy wheels it flows much better. Also, interestingly enough, it is best not to pour into red hot moulds - as in bronze casting. For some reason the aluminium flows better if the mould is less hot. I've never thought of this in terms of chemistry before but I guess that is how these things could be explained. A bit off topic I know. Sorry.