I put some aluminium foil and rusted copper(namely a Japanese 10 yen coin) in saturated NaCl solution. After some minutes tiny amount of small bubbles were observed on the surface of the foil. After several hours there appeared white cloudy thing at the bottom of the container. I think this cloudy thing is aluminium hydroxide, but how could this possible? In textbook it is written that aluminium does not react in neutral environment.

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    $\begingroup$ Does the book begin and end with this statement, or does it say something else about Al? $\endgroup$ Sep 22, 2022 at 6:50
  • $\begingroup$ Also, look up electrochemistry. BTW, "rust" is usually used for the red oxide of iron exclusively, while "corrosion" is a more general term, applied to other metals, such as copper, as well as to iron. "Tarnish" is a surface coating on Ag, which can also be removed with the above reaction. $\endgroup$ Sep 22, 2022 at 19:34
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    $\begingroup$ @DrMoishePippik Thank you for letting me know definition of rust ,corrosion, and tarnish. I didn't know these words well. For electrochemistry, I know Al is more likely to be ionized than Cu thanks to the fact that standard electrode potential is lower in Al. But in this book it is written that Al is coated with Al2O3 and this barrier is strong and prevents further corrosion. $\endgroup$ Sep 22, 2022 at 21:26
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    $\begingroup$ Ah! @satorukurita, that does show understanding of the question, and belongs in it (i.e., edit the question). And you (and the text) are correct, as long as the Al2O3 coating remains intact. If it is physically damaged, or Na+ ions react with it (forming a non-adherent aluminate), then the Al can react. $\endgroup$ Sep 23, 2022 at 1:42
  • $\begingroup$ @DrMoishePippik Thank you for explaining why Al2O3 might not be a perfect coating of Al.I thought both theory are plausible in this case. $\endgroup$ Sep 23, 2022 at 12:23


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