Chromium Chemistry (1): Reaction of Metal
Bulk Cr, like Al but not Fe, forms a protective oxide layer of Cr2O3. It is extremely inert due to the combination of corundum(alpha-Al2O3) structure and exchange inert d3 Cr3+ centers. As a result, it resists further oxidation and attack from acid. Some sources claim that acid can destroy the layer, but at least for me, 30% H2SO4 is of no use even upon heating. Maybe, HCl is needed as Cl- is a good ligand. However it was found that reduction can effectively remove passivation. Passing electric current through it, using it as cathode and diluted H2SO4 as electrolyte, can remove passivation immediately, and now Cr normally reacts with acid. Cr dissolves in acid, forming a blue solution of Cr2+. Without exclusion of air it is quickly oxidized into dark green Cr3+. Paraffin oil cannot stop this especially when heated, and preparation of Tutton's salt (NH4)2[Cr(H2O)6](SO4)2 thus failed.