Sciencemadness Discussion Board

Oxide layer on Copper

Romix - 14-3-2024 at 16:11

It is known that copper oxidizes on air after contact with acids, after thoroughly washing off acid residues with water.
I've heard that it's because acid removes the oxide layer that protects Cu from corrosion and therefore copper getting covered in oxides.
But why isn't it corrode when you polish copper ingot or saw it in half? In theory, the oxide layer is also removed, right?


[Edited on 15-3-2024 by Romix]

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Texium - 14-3-2024 at 16:40

Copper doesn’t form a protective oxide layer. You may be thinking of aluminum, or wrongly applying a property of aluminum to copper. In your case, you probably just didn’t fully wash off the acid. There could still be microscopic amounts clinging to the surface even if you think you were thorough.

Romix - 14-3-2024 at 16:59

Quote: Originally posted by Texium  
Copper doesn’t form a protective oxide layer. You may be thinking of aluminum, or wrongly applying a property of aluminum to copper. In your case, you probably just didn’t fully wash off the acid. There could still be microscopic amounts clinging to the surface even if you think you were thorough.

Ok, thanks.
Next time after contact with acid and washing it off I'll put copper bar in a weak basic solution of KOH. If your explanation is correct, oxidation products should not form!

This also happens with copper after keeping it in weak organic acids for ages, like lactic, citric or acetic. After washing off the acids of PCBs, copper oxidizes on air very quickly. If there were acids left on a boards they won't be able to oxidize metal that quick. I think your explanation is wrong mate.

The ingot on a picture, been in a weak solution of AR with traces of gold in it.
If acids were left, oxidation products would be water soluble, but they are not!

[Edited on 15-3-2024 by Romix]

Precipitates - 14-3-2024 at 18:10

Was the copper dried thoroughly after washing?

I've found wet fine copper powder (produced from sodium hypophosphite + copper nitrate), filtered and rinsed with deionised water, to oxidise quickly, but that the shininess can be restored with a solution of acetic acid and sodium chloride. The oxidation was mainly confined to the top layer of the copper powder, the bulk precipitate remained unoxidised.



Sir_Gawain - 14-3-2024 at 18:23

The reason copper oxidizes so fast after being in acid is that the acid etches off a thin layer of copper. This leaves the surface rough with a very high surface area, which quickly oxidizes.

woelen - 15-3-2024 at 00:30

The presence of other metal ions and certain anions also can change the redox properties of copper (or more generally, of many metals). This has to do with complex formation. E.g. the presence of chloride ion makes oxidation of copper easier. It facilitates formation of a chloro complex of copper(II), which is formed more easily than copper(II) oxide.
I once did an experiment with copper metal in hydrochloric acid. The acid is colored green/yellow quickly, if oxygen is allowed to get in contact with the acid. When a little thiourea is added, then the copper dissolves even faster, with production of hydrogen! Here, the thiourea dramatically alters the redox properties of copper metal, because of very easy formation of a copper(I) complex with thiourea.

@Precipitates: Are you sure that the powder you get from a copper(II) salt with hypophosphite is copper metal? I have read in multiple places that the reddish material you get from copper(II) and hypophosphite is copper(I) hydride, a covalent compound, which is relatively stable, compared to many other metal hydrides.

Texium - 15-3-2024 at 09:09

Quote: Originally posted by Romix  
The ingot on a picture, been in a weak solution of AR with traces of gold in it.
If acids were left, oxidation products would be water soluble, but they are not!
This is an incorrect assumption. Aqua regia contains chloride, which with copper on exposure to air will form insoluble copper oxychloride. Copper that has been exposed to HCl and then rinsed will oxidize in air faster than copper that has been exposed only to HNO3, in my experience. My explanation is consistent with woelen and Sir_Gawain's as well. The pitted surface of the acid-etched copper can cling to chloride ions which catalyze its more rapid oxidation.

I propose another experiment: take three similar pieces of copper. Rough up the surface of each with a wire brush. Submerge one in distilled water, one in HCl, and one in dilute HNO3 for ~10 sec, then rinse them and leave them exposed to the air.

[Edited on 3-15-2024 by Texium]

Keras - 16-3-2024 at 00:24

I thought the alteration of copper in air was mainly due to the formation of copper carbonate? A bit like silver, whose tarnishing is not caused by oxygen, but atmospheric sulphur dioxide or hydrogen sulphide.

Precipitates - 16-3-2024 at 01:59

Quote: Originally posted by woelen  

@Precipitates: Are you sure that the powder you get from a copper(II) salt with hypophosphite is copper metal? I have read in multiple places that the reddish material you get from copper(II) and hypophosphite is copper(I) hydride, a covalent compound, which is relatively stable, compared to many other metal hydrides.


I probably produced copper(I) hydride initially, but for me it quickly decomposed. From my notes:

"Very gradually there was formation of Cu powder, which slowly darkened over time."

If I repeated the experiment at a low temperature, I could probably isolate the copper(I) hydride without decomposition, and test against HCl (aq), which we know copper won't react with.

[Edited on 16-3-2024 by Precipitates] To remove trivia.

[Edited on 16-3-2024 by Precipitates]