hi, may i know wrought iron, mild steel are usually used which in which area for increase safety ?

outdoor main gate door - mild steel or wrought iron? shud galvanized or powder coat or both?
backside kitchen door - mild steel or wrought iron? shud galvanized or powder coat or both?
kitchen window grill - mild steel or wrought iron? shud galvanized or powder coat or both?
room window grill - mild steel or wrought iron? shud galvanized or powder coat or both?
sliding window door grill - mild steel or wrought iron? shud galvanized or powder coat or both?

recently I did saw some dark grill with smooth rough surface, no paint, smooth touching. owner say easy maintain. is it galvanized and powder coated?