Metals are made from mineral ores that are typically mined and forged from the ore state. They are opaque, fusible, ductile, strong, and lustrous materials. During working, smelting changes the chemical makeup of the mineral ores. With the removal of oxygen, the material changes from its "ore state" to its "metallic state." Metals are good conductors of heat and electricity. Common metals are iron, copper, silver, and gold.
The corrosion process is electrochemical process similar to the action of a battery. Any time two different metals are in contact, either physically, as a part of an alloy (a mixture of metals, such as sterling silver), or through an electrolyte (any solution containing ions, such as water), they form a galvanic cell based on the differences in electrical potential. The anode (less noble of the two metals) gives up electrons (negatively charged ions, seen above as red dots) to the cathode (more noble of the two metals). At the same time, positively charged metal ions (seen above as blue dots) are stripped from the anode and go into solution. The surface of the cathode is protected from corrosion at the expense of the anode. This process can proceed at any scale, and does not require liquid water - water vapor alone is sufficient.
Metal alloys contain a mixture of two or more metals. Alloying two metals alters the working properties of the metals such as melting point, color, ductility, surface characteristics, brittleness, etc. Common alloys are sterling silver, brass, bronze, steel, wrought iron, and pewter.
More Resources (pdf)
Caring for Copper Alloys
Caring for Firearms
Caring for Iron Alloys
Caring for Silver
Notes on Treating Tools
Pinhole Corrosion in Sprinkle Systems