There are a total of 11 Iron Chests. Iron Key is dropped by Key Orbling in Southern Forest.
Iron Chest #1
Item: Knight Sword
Location: En - New District, west side of town at the garbage dump.
Iron Chest #2
Item: Magic Belt
Location: Southern Forest - Waterfront, far east path near the entrance to Kina's old house.
Iron Chest #3
Item: Elixir
Location: En - Old District, inside Owen's shop, bedroom.
Iron Chest #4
Item: Pearl M
Location: En - New District, inside Mini Toy Box.
Iron Chest #5
Item: Peridot S
Location: En - Harbor, on top of the tower. You can climb using the ladder.
Iron Chest #6
Item: Heal Stone x3
Location: West Vence, inside Innocent Eyes bar, behind Cas.
Iron Chest #7
Item: 4000G
Location: Central Vence, inside City hall, observation desk.
Iron Chest #8
Item: Elixir
Location: East Vence, in front of the house near the second gondola pier.
Iron Chest #9
Item: Remedy
Location: West Vence, next to a house accessible by gondola at the southwest pier.
Iron Chest #10
Item: Elixir
Location: West Vence, in front of a house accessible by gondola at the far east pier.
Iron Chest #11
Item: Potion L x3
Location: West Vence, inside a house accessible by gondola at the far north pier.