Corrosion of iron nails in water
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Masters! Is the corrosion of iron nails in water hydrogen evolution corrosion or oxygen absorption corrosion? What substances are formed, and why do the iron nails turn brown? Shouldn’t ferrous hydroxide be formed instead? Ferrous hydroxide is white and insoluble in water, which does not match the actual properties of iron oxide.
Whether it is hydrogen evolution corrosion or oxygen absorption corrosion, it is always iron that loses electrons; hydrogen ions gain electrons to form hydrogen gas. In the absence of hydrogen ions, oxygen gains electrons to form hydroxide ions, resulting in the formation of ferrous hydroxide. Under conditions of water and oxygen, ferrous hydroxide rapidly converts into ferric hydroxide, with the white precipitate changing to gray-green before eventually turning reddish-brown. Some of the ferric hydroxide loses water to produce iron oxide and water
It all became clear now – Master, what do you do?
Under acidic conditions, a hydrogen evolution reaction occurs; under neutral or slightly alkaline conditions, an oxygen absorption reaction takes place. In either case, iron loses electrons to become ferrous ions, which then hydrolyze to form ferrous hydroxide, which is subsequently oxidized to ferric hydroxide, and dehydration leads to the formation of iron oxide.