We explain what corrosion is and what types of corrosion exist. Examples and how to avoid it. Also, relationship between corrosion and oxidation.
What is corrosion?
corrosion the degradation process of certain materials, as a consequence of an electrochemical reaction that is, oxidation-reduction, from its environment.
It is a natural, spontaneous phenomenon that affects especially (although not exclusively) metals. The speed of the reaction depends on the temperature at which it occurs, as well as the properties of the elements involved, especially their salinity.
Corrosion is a chemical process in which Three factors usually intervene: the corroded element, the environment and, generally, the water. However, there are also corrosive substances, that is, capable of causing corrosion of the materials with which they come into direct contact.
For its part, the metallurgical industry studies corrosion as an important enemy to overcome for its products, especially those exposed to the environment, in architectural or construction pieces. In fact, it is estimated that approximately five tons of steel are lost worldwide due to corrosion every few seconds.
See also: Chemical phenomena
Types of corrosion
Broadly speaking, there are two types of corrosion: chemical and electrochemical, depending on the type of materials and reactions involved:
- Chemical corrosion. It occurs when a material reacts in a corrosive liquid or gas until it completely dissolves or saturates the liquid. This can happen in different ways:
- Liquid metal attack. It occurs when a solid metal and another liquid metal come into contact, and the first is corroded in its weak points by the second.
- Selective leaching. It occurs when there is selective corrosion in metallic alloys.
- Chemical attack It occurs with aggressive chemical reactions by powerful solvents, such as those capable of dissolving polymers, generally considered resistant to corrosion.
- Electrochemical corrosion. It generally occurs in metals, when their atoms lose electrons and become ions. It can happen in different ways:
- Microbiological corrosion. When it is caused by microscopic living organisms capable of altering the chemistry of materials, such as bacteria, algae and fungi.
- Galvanic corrosion. It is the most intense of all and occurs when different metals interact with each other, and one acts as an anode and the other as a cathode, and they form what is known as a galvanic cell.
- Corrosion due to surface aeration. Known as the Evans Effect, it occurs on flat surfaces located in humid and dirty places, which foster electronegatively charged environments.
Examples of corrosion in everyday life
Some examples of corrosion in everyday life are:
- Corrosion of water pipes. It occurs especially in metallic ones, which tend to break over time and contaminate the water with small doses of rust, which give it a blackish or brown color.
- Rust on metals exposed to water. It occurs on the body of the automatic washing machine, or the doors of cars left on the beach, where the saline environment accelerates the oxidative reaction and cracks and the typical brown rust stains soon appear.
- The color of the Statue of Liberty. Its original tone would not have to be greenish, but rather copper-colored, the material from which it is made. Being surrounded by water, the humidity in the air oxidizes it and covers it with a greenish dust (copper oxide), a product of corrosion.
- Corrosion of cans. Cans that have been in the pantry for too long begin to show brown spots in some regions, an unmistakable sign that air corrosion has begun to affect them.
How to avoid corrosion?
The fight against corrosion is part of the metal industries, which have various mechanisms to avoid or slow down these disintegrating processes of metals, such as:
- Coatings Many metals are coated with polymers or plastics, for example, to prevent or minimize environmental corrosion, isolating them from their environment in a substance more resistant to these types of reactions.
- Alloys Many combinations of metals give the result greater resistance to corrosion, as occurs with zinc-alloyed steels.
- Corrosion inhibitors These are chemical substances that have the property of slowing down or preventing the natural corrosion process of certain materials, so it is enough to immerse them in a film of them to make them more resistant.
- Choice of materials The simplest solution is to choose carefully which materials to leave exposed to the environment and which not, and which to use for works exposed to the elements or the action of water, for example.
Corrosion and oxidation
Although it is not often called that way, oxidation is a corrosion reaction since in every act of oxidation an electronic exchange occurs, classifiable as electrochemical corrosion.
For this reason, metals left outdoors or submerged in water corrode since they react with the oxygen in the air or water and form layers of oxide on their surface, which prevent the chemical reaction from continuing.
However, this oxide layer is mechanically removed and the deeper layers of the metal are again exposed to oxygen, resulting in the destruction of the material completely. This process is particularly fast in saline environments, since sodium chloride acts as a catalyst for the reaction, speeding it up.
References
- “Corrosion” on Wikipedia.
- “The problem of corrosion” at the Academy of Sciences of the Region of Murcia (Spain).
- “What is corrosion and how to avoid it?” in Bricolemar.
- “Corrosion” in Chemistry Explained.
- “Corrosion (chemical process)” in The Encyclopaedia Britannica.
