Metallic Corrosion
Metallic corrosion is an electrochemical process, where an electrical current is transferred between an anode and a cathode of different pieces of metal. The metal at the anode reacts with the environment (atmosphere, fluids or solids) and corrodes, thereby releasing electrons that travel to the cathode and electrochemically bond with it. The corrosion process changes the material composition and/or surface properties of the metals, thus leading to failures, loss of aesthetics and eventually to safety concerns for the structures. The consequence of the corrosion process depends on the environment, the particular metal species and other variables such as presence of corrosion inhibitors and/or surface treatments.
Metallic corrosion is, generally, inconspicuous and requires careful monitoring or proactive strategies, in the form of corrosion protection systems, to minimize the damages, repair costs and risks to the safety of persons and structures. Corrosion control, in practice, involves two distinctive strategies: (1) protection systems, if possible, to eliminate the possibility of corrosion altogether and (2) monitoring systems that detect, as soon as possible, any forthcoming corrosion attack. Both strategies require (1) an understanding of the environment, and corrosivity thereof, in which the metals are placed, and (2) appropriate selection of metallic alloys and/or surface treatments that are considered sufficiently corrosion-resistant for the particular environment.
Metallic corrosion is a complex phenomenon that involves three separate phases, namely energetics, kinetics and thermodynamics, whichin turn control the electrochemical reactions related to oxidation and reduction between the metal and its environment.
The energetics of corrosion is related to the electrical potential (or driving force), which can be measured in terms of the anodic and cathodic polarization curves, that must exist for a successful corrosion attack. The potentials of the different metals can be measured as potentials relative to a standard electrode for that particular environment.
The kinetics of corrosion is related to the surface composition and the rate of oxidation and reduction reactions at the metal-solution interface. The corrosion rate (or speed) of a metal is a function of environment, temperature, pH and the presence of corrosion inhibitors, accelerators and/or pollutants.
The thermodynamics of corrosion is related to the thermodynamically favored states of oxidation and reduction of the metals in question, according to which the chemical energy is converted into electrical energy and vice versa. The thermodynamics of corrosion plays a major role in controlling the energetics and kinetics of corrosion.
In addition to their individual contributions, the energetics, kinetics and thermodynamics of corrosion are also interdependent. A situation, for example, where an increase in temperature results in an elevated electrochemical potential leading to a particular corrosion rate can only happen if the thermodynamic conditions allow the oxidation of the particular metal at the given temperature. The understanding of these variables and the relevant electrochemical reactivity is essential to assess the corrosion performance of the various metallic systems.
In conclusion, corrosion of metals is a complex process that requires proper understanding of the environment and the corrosion behavior of metals in order to minimize the damages, repair costs and risks to the safety of individuals and structures. Although corrosion of metals is highly unavoidable, correct selection ofalloys, surface treatments and corrosion protection systems can lead to increased life span and safety of structures.
