Patina (Deterioration)
Introduction
Corrosion is defined as the deterioration of a material as a result of an electrochemical attack by its environment. More generally, it can be understood as the general tendency that materials have to seek their form with the greatest stability or lowest internal energy. Whenever corrosion is caused by an electrochemical reaction (oxidation), the rate at which it takes place will depend to some extent on the temperature, the salinity of the fluid in contact with the metal and the properties of the metals in question. Other non-metallic materials also suffer corrosion through other mechanisms. The corrosion process is natural and spontaneous.
Corrosion is a chemical reaction (oxide-reduction) in which three factors intervene: the manufactured part, the environment and water, or through an electrochemical reaction.
The best known factors are the chemical alterations of metals due to air, such as the rust of iron and steel or the formation of green patina on copper and its alloys (bronze, brass).
It is an important industrial problem, as it can cause accidents (breakage of a piece) and, in addition, it represents a significant cost, since it is estimated that every few seconds five tons of steel are dissolved in the world, coming from a few nanometers or picometers, invisible in each piece but which, multiplied by the amount of steel that exists in the world, constitutes a significant amount.
Corrosion is a field of materials science that invokes both notions of chemistry and physics (physicochemistry).
Basic definitions
What causes corrosion is a massive flow generated by chemical differences between the parts involved (corrosion is an electrochemical phenomenon). A current of electrons is established when there is a potential difference between one point and another. When electrons are transferred from a chemical species and migrate to another species, it is said that the species that emits them behaves like an anode and oxidation occurs, and the species that receives them behaves like a cathode and reduction occurs.
For this to occur between species, an electrochemical differential must exist. If we separate a species and its half-reaction, it will be called electrochemical half-pair; If we put both semi-pairs together, an electrochemical pair will be formed. Each semi-pair is associated with a reduction potential (previously the concept of was used). That metal or chemical species that exhibits a more positive reduction potential will proceed as a reduction and, vice versa, that which exhibits a more negative reduction potential will proceed as an oxidation.