Except for the Mark I suit that Stark and Yinsen made while prisoners, no suit has been made with iron. From Mark III onwards, the suit has been made from titanium or titanium-gold alloy to counter issues like corrosion and freezing at higher altitudes. As for the later armors such as the Mark XLVI used in Captain America: Civil War’s earlier fights or Endgame’s unique armor, better known as the Bleeding Edge, it is made from vibranium nanotechnology.