One way to understand the structure of metals on the basis of particles is to imagine an array of positively-charged ions sitting in a negatively-charged "gas" of free electrons. Coulombic attraction holds these oppositely-charged particles together, but the positively-charged ions are attracted to negatively charged particles outside the metal as well, such as the negative ions (anions) in an electrolyte.

For a given ion at the surface of a metal, there is a certain amount of energy to be gained or lost by dissolving into the electrolyte or becoming a part of the metal, which reflects an atom-scale tug-of-war between the electron gas and dissolved anions. The quantity of energy then strongly depends on a host of variables, including the types of ions in a solution and their concentrations, and the number of electrons present at the metal's surface. In turn, corrosion processes cause electrochemical changes, meaning that they strongly affect all of these variables. The overall interaction between electrons and ions tends to produce a state of local thermodynamic equilibrium that can often be described using basic chemistry and a knowledge of initial conditions.

Electrochemical corrosion involves two half-cell reactions

• an oxidation reaction at the anode, and
• a reduction reaction at the cathode.

For iron corroding in water with a near neutral pH, these half cell reactions can be represented as:

• Anode reaction: 2Fe => 2Fe²⁺ + 4e^-
• Cathode reaction: O₂ + 2H₂O + 4e^- => 4OH^-

There are obviously different anodic and cathodic reactions for different alloys exposed to various environments. These half cell reactions are thought to occur (at least initially) at microscopic anodes and cathodes covering a corroding surface. Macroscopic anodes and cathodes can develop as corrosion damage progresses with time.

From the above theory it should be apparent that there are four fundamental components in an electrochemical corrosion cell:

• an anode,
• a cathode,
• a conducting environment for ionic movement (electrolyte), and
• an electrical connection between the anode and cathode for the flow of electron current.

If any of the above components is missing or disabled, the electrochemical corrosion process will be stopped. Clearly, these elements are thus fundamentally important for corrosion control.

Reference: K.R. Trethewey and J. Chamberlain: "Corrosion for Science and Engineering 2nd Edn.", Longman (UK), 1995.