Metals

Iron and steel

The most important alloying element for iron is carbon—because it combines to form a diverse range of alloys, including wrought iron through to steels and cast iron.

In cast iron, carbon can exist as discrete phases or minute areas of graphite, in a variety of physical forms. Because of the differences in the hardness and chemical reactivity of the various phases, steels and cast iron are subject to localised corrosion—one phase is selectively corroded while another is protected. This is a form of internal galvanic corrosion.

The addition of other metals such as nickel and chromium result in the wide range of stainless steels, which are hard and chemically durable alloys. These alloys corrode to form protective coatings of chromium oxide/nickel oxide, and transform iron into a much less reactive metal with a much wider range of uses.

Adding elements such as molybdenum further improves the corrosion-resistance properties of the stainless steel alloys in chloride solutions.