Reactivity of Aluminium Oxide

Question

I know that Aluminium Oxide is an amphoteric substance meaning that it reacts with both acids and bases. I recently learned that Aluminium has very good corrosion resistance due to the "Aluminium spontaneously forms a thin but effective oxide layer that prevents further oxidation."

However, in the test for Nitrate Ions, an Aluminium foil is added into the solution, the Nitrate Ion is reduced by the Aluminium, but this Aluminium foil surely has Aluminium Oxide which should prevent the reaction from happening, but it happens anyway. How can this phenomenon be explained?

Source: http://www.aluminiumdesign.net/design-support/aluminium-corrosion-resistance/

Answer 1

Aluminum oxide coating can relatively easily react with either strong base or acid, dissolving in them, exposing naked elemental aluminum to reaction medium. You can read more here.
To be concise, there are many ways to make aluminum react: you can use a base, you can use an acid, and you can scratch a sample of aluminum and coat it with mercury. Mercury would form an alloy with Al, but that wouldn't affect its reactivity towards the solution of anything, it would just make it harder for oxygen to oxidize the fresh aluminum.
Answering you question: as far as I know, the nitrate test involves strong base, which serves as a reactant for aluminum oxide, dissolving it and letting aluminum to react further.