The corrosion resistance of magnesium alloys depends strongly on the amounts of the impurities Fe, Ni and Cu, and in high-purity alloys such as AZ91D the upper limits for Fe and Ni are set very low, at about 0.005% or less. Which is the correct reason?

Answer: B. These elements appear as particles nobler than Mg and form local cells with the surrounding Mg, accelerating its dissolution

Mg is one of the most active of the practical metals. Even small amounts of Fe, Ni and Cu hardly dissolve in Mg and appear as particles of metals or intermetallic compounds with a noble potential. Local cells form in which these particles are cathodes and the surrounding Mg is the anode, and in salt water the corrosion rate becomes orders of magnitude larger. In high-purity alloys, therefore, Fe and Ni are held to very small amounts and Cu to about 0.03% or less, and Mn is added to take Fe into Al-Mn-Fe compounds and reduce its harm. The option in which the elements dissolve and thicken the film and the option explaining it by the flammability of the melt both miss this local-cell mechanism. For the same reason, insulation is needed when Mg parts are in contact with steel bolts.