Most marketed metals today are alloys of 2 or more metals that when melted together, form a more desirable product, with more desirable characteristics. Since earlier times, metals such as bronze, brass, and iron were smelted from their ores for various implements such as tools and weapons. Today, man has perfected this operation to considerable accuracy, making possible the production of complex alloys.
Most structural metals today are made from iron and fractional amounts of other elements such as carbon, manganese, chromium, vanadium, and tungsten. Steel is by far the most produced and is made from smelted iron ore with a carbon content between 0.2% and 2.1%, along with small amounts of other elements depending on the desired outcome. Think of carbon as a hardening agent that is added or reduced accordingly.
When iron ore is first smelted (pig iron), it contains a relatively high carbon content, which must be reduced to appropriate ratios. Elements such as manganese add tensile strength while chromium increases hardness and melting temperature. In this way, various additions can contribute valuable characteristics to the final product. Stainless steel for example has an 11% to 26% chromium content that gives it its renowned attributes.
Structural steel is steel used as structural members in construction, and as such, is characterized by its extremely high "second moment of area". The second moment of area is a property of a structural member's cross section used to predict its resistance to bending and deflection. To rephrase, the second moment of area is the cross sectional geometry with which one can calculate how well a given beam can withstand loading.
A good way to explain this is to use an "I" beam. An I beam has a considerably higher second moment of area in comparison to a beam of equal mass forming a solid square cross section. This is why I beams are often used in the construction industry. Another important aspect of structural steels – as well as other steels – is how resistant they are to corrosion. No matter how strong you make them, they are still vulnerable to chemical changes.
Stainless steel is an example of a corrosion resistant alloy, but can be inappropriate for structural purposes due to its cost. As an alternative, a method of coating steel with zinc, called galvanization, is often used to protect base metals against corrosion. Most structural steel elements are treated in this way. Zinc forms an initial physical barrier over the base metal but also acts as a sacrificial anode if this gets damaged.
When a more easily corroded element such as zinc, is coated over a given metal, it will itself corrode before the more important metal begins corroding – thus termed "sacrificial" anode. This is the basis of cathodic protection in which an anode is electrically connected to a cathode to protect the latter from corrosion. The cathode – in this case the steel – will not begin to corrode until the anode – the zinc – has first corroded away.