Jeremiah 21:4 describes God turning weapons against Judah; here the unbreakable iron from the north is the instrument of that judgment — both show Babylon as God's tool.

In Jeremiah 1:18, God makes Jeremiah an iron pillar; here the enemy from the north is unbreakable iron — same symbol of strength, different subjects.

Isaiah 45:9 uses the clay-potter image to parallel the rhetorical question here: both show human inability to challenge God's sovereign plan.