'Rust' in the Bible

‘Therefore, thus says the Lord God,“Woe (judgment is coming) to the bloody city,To the pot in which there is rustAnd whose rust has not gone out of it!Take out of it piece by piece,Without making any choice.

“Then set the empty pot (Jerusalem) back on the coalsSo that it may be hotAnd its bronze may glowAnd its filthiness may be meltedAnd its rust (scum) may be consumed.

“She has wearied Me with toil,Yet her great rust has not left her;Her thick rust and filth will not be burned away by fire [no matter how hot the flame].