I am analyzing a reaction between water sulfate copper and aluminum. A chunk of $\ce{Al}$, e.g., aluminum foil. I realize that a layer of $\ce{Al2O3}$ forms on the surface of the $\ce{Al}$, passivizing it, and I realize that the $\ce{NaCl}$ (or rather just the $\ce{Cl^-}$) cleans the aluminum oxide off the surface of the chunk of aluminum, but I don't understand why. I'd appreciate greatly if somebody could offer me an explanation.

Furthermore, (I didn't exactly include this in the topic of my question,) why is it that the reaction looks like this:

$$\ce{6H2O + 3CuSO4 + 5Al \rightarrow 3H2 + Al2(SO4)3 + 3Al(OH)2 + 3Cu}$$

(or something along the lines of that) rather than:

$$\ce{3CuSO4 + 2Al -> Al2(SO4)3 + 3Cu}$$ ?

Something along these lines has to be the case, because the reaction yields gas..

  • 2
    $\begingroup$ You should split your first equation into two, one where Al reduces water, one where Al reduces Cu. Both reactions happen more or less independently, there is no absolute stochiometric relation between them. $\endgroup$ – Karl Nov 15 '16 at 21:44
  • $\begingroup$ Would that imply that clean Al would react with H2O to produce H2 gas equally as vigorously as when it's reacting with CuSO4 + H2O solution and forming H2 gas, if at the same temperature? Also, thanks for the suggested edit. $\endgroup$ – Jake Waitze Nov 15 '16 at 21:49
  • $\begingroup$ No, that's what I meant by "more or less". I would not remove the first equation, just add a third that describes the evolution of H2 from water + Al. There is surely some interesting interaction between the two parts. Btw. Al-sulfate is soluble in water. $\endgroup$ – Karl Nov 15 '16 at 22:00

Chlorides are known to be an aggressive ion for metal corrosion, this is thought to be because of the ability of chlorides to destabilise the passivation film on metals which protect the metals from corrosion. What is known as pitting corrosion can occur, where a breakdown of the passivation layer at specific points of the metal surface leads to corrosion there by other oxidising agents in the aqueous medium, leading to the formation of corrosion pits. Exposed metal surfaces due to the breakdown of the passivation layer releases metal ions, which complex with the chloride anions; the metal-chloride complex reacts with water to form hydrochloric acid and a metal-hydroxide complex, reducing the pH of the medium close to the metal surface, further accelerating corrosion. (A reference can be found here: http://sassda.co.za/stainless- steel-and-corrosion/).

Aluminium is an "amphoteric" metal which reacts with alkalis; water first oxidises Al to Al (III), producing hydrogen gas (probably the effervescence that you observe) due to the reduction of water. Al (III) ions then complex with hydroxide ions to form a soluble complex in solution. In fact, when an alloy of Cu and Al is placed in NaOH, the Al is seen to dissolve away, leaving the copper behind in a nanoporous structure, a material that has potential for catalytic applications.

As for your question as to whether Al can react as vigorously with water as copper (II) sulfate solution, I think yes. But if your Al isn't as "clean" as you think and still has a layer of passivation on the surface, sulfate ions are known to also destabilise passivation layers of metals so that might enhance the corrosion (in fact some sources claim sulfate to be a better corrosive agent than chloride).

Sorry that I don't have any references, most of this is based on literature that I have read quite a while ago. Also, I am just a beginner learner in chemistry and corrosion science, so please help to improve my answer and correct any mistakes!


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