The white powder that comes out of the iron might be calcium carbonate. It is a mineral residue formed due to the reaction of hard water with heat, also called limescale. Most steam irons come with the "Anti-Calc" feature. It does not let this powder settle down in your iron. To get rid of the powder, it is better to pour distilled or RO water instead of tap water. Also, you can add a little vinegar too to adjust the pH.