'Rust' in the Bible

Therefore thus says the Lord Yahweh: Woe to the bloody city, to the caldron whose rust is therein, and whose rust is not gone out of it! take out of it piece after piece; No lot is fallen on it.

Then set it empty on its coals, that it may be hot, and its brass may burn, and that its filthiness may be molten in it, that its rust may be consumed.

She has wearied [herself] with toil; yet her great rust doesn't go forth out of her; her rust doesn't [go forth] by fire.