Go to a home brew-shop and buy some Citric Acid powder - make up a solution - enough to fill the iron and perhaps two tablespoons of citric acid. Put it in the iron and leave it overnight.
Citric Acid is also good for descaling kettles - coffee machines and dishwashers etc
Amazon also sell citric acid - despite the word acid in the name it's quite harmless
Good Luck..., March 2017