Whereas I do like the storybuilding options of treating impure metals this way, I don't think it really follows the "Feruchemy is of Balance" way of thinking. Feruchemy is not supposed to have any loss like hemalurgy.
Rather I would tend to believe that an impure metal would not be able to hold as many charges as a pure metal would. I.E. a large bar of crummy brass only holds 2 or 3 charges. the same as say a small bead of pure brass.
Agreed. If the metal is off-peak but usable, you can store maybe 1/10 the usual charges, or maybe your Feruchemy score is decreased with respect to that metal (so you can't store or tap as fast.) If it's really impure, it doesn't work at all.