Corrosion basic concepts
- Posted by: César Boró Martín
- Category: Materials
It can be said that corrosion affecting materials is a harmful effect that appears in the different sectors of the industry. Therefore, it is convenient to know this phenomenon in order to avoid it as much as possible. Firstly, we should use a classic definition about corrosion in which it is established as the degradation mechanism of a material, usually metal, or its properties due to its interaction with the environment in which material is surrounded. The main importance regarding corrosion lies in two fundamental aspects: the structural integrity of all types of elements, both industrial ones and common ones in our daily life, with the consequent danger for people, and the economic damage that this entails. As an example, it is worth mentioning that in a country like United States, the direct costs caused by corrosion represent several percentage points in their Gross Domestic Product (GDP).
It is necessary to know the basic fundamentals of electrochemistry to properly understand the problem we face. Corrosion occurs because of electrochemical reactions. The first main component present in this scenario is a liquid, named electrolyte, which contains ions and, precisely for that reason, it has the capability to conduct an electric current by means of ions flow. The electrolyte causes an exchange of electrons, which occurs in different places. It should be noted that an electrolyte can greatly promote the corrosion process in case of having a high content of ions. The ions that flow toward the anode are named anions, and those that flow toward the cathode are name cations. In the case of electrons, they go through the metal parts from one location to another one of them.
These electrochemical reactions are also named oxidation/reduction reactions because of corrosion one is composed of both. Oxidation takes place in the anodes and reduction in the cathodes. Focusing the part of electrons flow, it is convenient to know that electrons that go out from the anode circulate through the metal up to reach the cathode. Once there, the arriving electrons are consumed in the reduction reactions. For these reactions to happen, it is necessary that there is a complete electric circuit, or they will tend to stop.
From a thermodynamic point of view, it should be noted that in corrosion reactions, the energy goes in form of electricity and that the flow of energy determines the direction of these reactions. The amount of energy in the metals in which oxidation/reduction reactions happen is greater than the energy level in the substances named corrosion products. The processes that occur in a natural way tend to decrease the total amount of energy in the system where they happen.
At this point, it is necessary to discuss about electrical potential of metal, which indicates the energy difference between the components of the system. The anode, what is the negative electrode, is originally in an energetic state higher than the cathode or positive electrode. Regarding all exposed so far, it is possible to point out that electrons move from a higher energy area to another area with less energy. The greater the potential difference, that is, the energy level between the anode and the cathode, the more tendency the corrosion takes place.
A usual way to consider the electrical potential consists of galvanic series, which are composed of lists of metals, ordered according to their potential, from more active or oxidizable ones to less active or less oxidizable, being each galvanic series for a specific environment. The galvanic series most commonly used is for metals in sea water.
Another important parameter is the impact of the area of the anode and the cathode in the development of the corrosion reaction. Taking into account that the total electrochemical reaction is the same in both electrodes, anode and cathode, the effect in which is smaller will be more intense than in the larger electrode. This aspect could be graphically seen by means of an Evans diagram about an anode and a cathode with equal surfaces, as shown below:
Some phenomena are especially relevant, as passivation, which is the case of attenuation of chemical reactivity in a metal when certain circumstances occur. Passivation can take place because of several reasons. One of the most common is the formation of an oxide layer on the surface of a metal, as in the typical case of aluminum. Another one happens when a metal or alloying component is added in order to achieve a passiving layer, as in the chromium-iron alloys.
These concepts that apparently are only theoretical (potential difference between anode and cathode, the need for a complete electrical circuit, areas of the electrodes, etc.), have a high importance in practice because the professional involved will have some criteria about what methods should be considered in order to avoid, as far as possible, this undesired phenomenon.