While noble or precious metals like Gold and Platinum, that are found in nature in their metallic state, show a high corrosion resistance and remain almost intact when exposed to the weather or to the action of an electrolyte (soil and water), common and less precious metals that are extracted from their ores like iron experience severe corrosion in many environments.
Although many different kind of corrosion exist, the common mechanism of action can be identified in the oxidation of metal that occurs in so-called “anodic areas” of the surface by an oxidizing agent, which is reduced at cathodic areas. Three conditions are necessary for corrosion to happen:
- A difference in polarization potential between two parts of the metal;
- A metallic path connecting each others;
- An common electrolyte in contact with the two parts;
The formation of anodic and cathodic area is naturally occurring in any metal due to microscopic differences in metallic structure, mechanical or heating treatment, internal stresses and many other causes, leading to what is called “generalized corrosion”.
Anodic and Cathodic areas can be imposed by inhomogeneity in the surrounding electrolyte (corrosion by differential aeration), by the irregular shape of the metal which causes difference in the diffusion of oxidizing agents (for example, crevice corrosion) or by difference in the composition of the outer layer (pitting corrosion).
Furthermore, when coupling different metals, the polarization is imposed by the natural difference in equilibrium potentials, so that the most active (electronegative) metal is shifted to a more positive potential and thus corroded at increased rate (galvanic corrosion).