Aluminum and chrome in air

There is a rule: the more active the metal, the more it oxidizes in the air, is corroded. Therefore, for example, sodium is stored under kerosene. But aluminum is much more active than iron, but iron quickly rusts, and aluminum and chrome in the air, no matter how much they hold, practically do not change. Why is this happening?

Oxide and aluminum hydroxide are amphoteric, that is, they exhibit both basic and acidic properties simultaneously, reacting with both acids and alkalis

Oxide and aluminum hydroxide are amphoteric, that is, they exhibit both basic and acidic properties simultaneously, reacting with both acids and alkalis. Place a piece of aluminum in a tube with any acid, for example, with hydrochloric acid. Aluminum immediately begins to dissolve, vigorously displacing hydrogen from the acid - the aluminum salt of AlCl3 is formed. Another piece of aluminum gently dip into a concentrated solution of alkali, for example, caustic soda. Again, the metal begins to dissolve with the evolution of hydrogen. This time another salt is formed - aluminate NaAlO2.

Aluminum oxide covers the surface of aluminum with a thin and dense layer and does not allow it to be further oxidized. This is his property used in practice. For example, for cladding metals; A thin aluminum layer is applied to the metal surface, aluminum in air is immediately covered with an oxide, which reliably protects the metal from corrosion. To see this, secure a piece of aluminum wire in an inclined position above the flame of a gas burner or an alcohol lamp so that the bottom of the wire is heated. At 6600 C, the metal melts and one would expect that aluminum will start to drip onto the burner. But instead of melting, the heated end of the wire suddenly sags. You can see a thin cover, inside of which there is molten metal. This cover is made of alumina Al2O3, a substance that is durable and very heat resistant.

No less interesting are chrome and nickel, two metals covered with metal products, so that they shine and do not rust in the air. Separate a piece of the coating from the old part and leave it in the air for a few days, so that it can be covered with a film of oxide, and then carefully place it in a tube with concentrated hydrochloric acid. If it was nickel, it would immediately dissolve in acid, forming a salt of NiCl2; thus hydrogen will be allocated. If a brilliant coating of chromium, then at first there will be no changes and only then the metal will dissolve in acid with the formation of chromium chloride CrCl3. After removing this piece of coating from acid with tweezers, rinsing it with water and drying it in air, after two or three days it will be possible to observe the same effect again.

Explanation: On the surface of chromium, the finest oxide film is formed, which prevents the interaction of the acid with the metal. However, it dissolves in acid, albeit slowly. In air, chromium is again covered with an oxide film. But nickel has no such protective film.