Iron and steel, the most commonly used metals, corrode in many media including most outdoor atmospheres. Usually they are selected not for their corrosion resistance but for such properties as strength, ease of fabrication, and cost. These differences show up in the rate of metal lost due to rusting. A modern and comprehensive document on the subject is the second edition of the classic CORROSION BASICS textbook.
Cast iron is a generic term that identifies a large family of ferrous alloys. Cast irons are primarily alloys of iron that contain more than 2% carbon and 1% or more silicon. Low raw material costs and relative ease of manufacture make cast irons the least expensive of the engineering metals. Cast irons may often be used in place of steel at considerable cost savings. The design and production advantages of cast iron include:
Low tooling and production cost
Good machinability without burring
Readily cast into complex shapes
Excellent wear resistance and high hardness (particularly white irons)
High inherent damping
Alloying elements can play a dominant role in the susceptibility of cast irons to corrosion attack. Silicon is the most important alloying element used to improve the corrosion resistance of cast irons. Silicon is generally not considered an alloying element in cast irons until levels exceed 3%. Silicon levels between 3 and 14% offer some increase in corrosion resistance to the alloy, but above about 14% Si, the corrosion resistance of the cast iron increases dramatically.