Abstract:Corrosion action and passive mechanism of magnesium alloy in the fluoride solution were studied by means of scanning electron microscopy(SEM), energy dispersive X-ray spectroscopy(EDS), and electrochemistry methods. The results show that an insoluble MgF₂ film is generated on the surface of magnesium alloy activated in the hydrofluoric acid. And the mass of the deposited MgF₂ film may reach a constant value, when the mass ratio of Mg/F on the magnesium alloy surface is fixed at 11.3׃1. The activated magnesium alloy gains a ‘passivation state’ in a mixture of sulfuric acid and hydrofluoric acid at a volume ratio of less than 1.2. At the same time the mass of magnesium alloy is maintained as a function of the time. When the ratio is above 1.4, the mass of magnesium alloy rapidly decreases. The passive film formed through adsorption of HF₂⁻ (or H₂F₃⁻, H₃F₄⁻) ions by the deposited MgF₂ film can protect the magnesium alloy from corrosion in fluoride solution, but not in non-fluoride solutions. The passive state is maintained for activated magnesium alloy in an acidic sulfuric nickel solution with added fluoride. If fluoride and carbonate are added to the acidic sulfuric nickel solution, a replacement reaction between magnesium alloy and solution takes place.

Key words: magnesium alloy; passive film; corrosion; fluoride; nickel sulfate