Why Metals Corrode?
This is a very good question indeed!
Corrosion is the disintegration of metal through an unintentional chemical or electrochemical action, starting at its surface. All metals exhibit a tendency to be oxidized, some more easily than others. A tabulation of the relative strength of this tendency is called the galvanic series. Knowledge of a metal's location in the series is an important piece of information to have in making decisions about its potential usefulness for structural and other applications.
The driving force that causes metals to corrode is a natural consequence of their temporary existence in metallic form. To reach this metallic state from their occurrence in nature in the form of various chemical compounds (ores), it is necessary for them to absorb and store up for later return by corrosion, the energy required to release the metals from their original compounds. The following pictures illustrate the similarity in color between pale green malachite, a common copper ore mineral, and the corrosion products on a brass plate (70% copper) exposed to a humid environment.
Malachite, a common copper ore mineral
Corroded brass plate
The thermodynamic or chemical energy stored in a metal or that is freed by its corrosion varies from metal to metal. It is relatively high for metals such as magnesium, aluminum, and iron, and relatively low for metals such as copper, silver and gold. the following Table lists a few metals in order of diminishing amounts of energy required to convert them from their oxides to metal. (reference)
The high reactivity of magnesium and aluminum expressed as energy in Table 2.1 is paralleled by the special efforts that were historically required to transform these metals from their respective ores. The industrial process to produce aluminum metal on a large scale, for example, was only invented at the end of the 19th century and objects made of this metal where still considered to be a novelty when the 2.85 kg aluminum cap was set as the last piece of the Washington Monument in 1884.
A typical cycle is illustrated by iron. The most common iron ore, hematite, is an oxide of iron. The most common product of the corrosion of iron, rust, has a similar chemical composition and color. The energy required to convert iron ore to metallic iron is returned when the iron corrodes to form the original compound. Only the rate of energy change may be different.
Corroded ashtray with a typical rust color
The energy difference between metals and their ores can be expressed in electrical terms that are in turn related to heats of formation of the compounds. The difficulty of extracting metals from their ores in terms of the energy required, and the consequent tendency to release this energy by corrosion, is reflected by the relative positions of pure metals in a list, which is discussed later as the electromotive series.