The second law of thermodynamics is an expression of the tendency over time differences in temperature, pressure and chemical potential will equili­brate in an isolated physical system. In other words, every material tends to reach the maximum of disorder, in order to minimize its potential energy. With regard to corrosion, it means that leaving the crystalline network under the action of an electric field, metal ions yields energy. According to thermodynamics, almost all metals have negative free energy, suggesting their reactivity in environments where they are exposed.

Corrosion reactions are electrochemical in nature, based on mass and charge transfers. Reactions can be split into partial oxidation and reduction reactions. The potential is the propensity to exchange electrons: the metal donating electrons is oxidized, while the metal receiving electrons is reduced. The stability of elements in a given medium is predicted by the correspond­ing Pourbaix diagram, where predominant phases are defined in agreement with thermodynamics. However, reaction kinetics play a major role in the evolution of the system (such as changes in pH, potential or temperature).

For example, if iron is introduced into hydrogenated water at 300°C (with­out any dissolved oxygen) at pH 7 and a potential of -700 mVSHE, cations Fe2+ are dissolved in the water (Reaction [2.1]). This anodic reaction (oxi­dation of metal) is coupled to the cathodic reaction (reduction of water) described by Reaction [2.2]. Then, dissolved cations Fe2 + can join oxidant ions OH — (Reaction [2.3]), to form ferrous hydroxide Fe(OH)2.

Cations can be released in the water due to dissolution. When the satura­tion in ferrous hydroxide Fe(OH)2 or ferrous cation Fe2+ is reached, accord­ing to the Pourbaix diagram, magnetite Fe3 O4 forms based on Schikorr Reaction [2.4].

3Fe(OH)2 -» Fe3O4 + H2 + 2H2O [2.4]

Finally, passivation is the process of building a protective layer of oxide isolating the surface of the material from the aggressive environment. Some corrosion inhibitors help the formation of such layers. Figure 2.1 shows energy-dispersive x-ray (EDX) analysis of the oxide formed on stainless steel exposed to water at 360°C (-600 mVSHE, pH325°C = 7.2). The passive film is the 50 nm-thick layer containing a significant level of chromium at the surface of the metal.