'Rust' in the Bible

Wherefore thus saith the Lord Jehovah: 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 upon it.

Then set it empty upon the coals thereof, that it may be hot, and the brass thereof may burn, and that the filthiness of it may be molten in it, that the rust of it may be consumed.

She hath wearied herself with toil; yet her great rust goeth not forth out of her; her rust goeth not forth by fire.