Types of Corrosion. The three types of corrosion

"Corrosion" is a chemical term often used in everyday life to refer to the process of total, partial, superficial or structural destruction of certain material caused by the action of the environment.

Generally, when talking about this subject, the first thing that comes to our mind is the corrosion of metals, especially iron, generating rust. However, other materials can corrode, such as polymers and structures made of reinforced concrete.

Indeed, corrosion is very present in our society and represents great economic losses, as all types of corrosion are related to a reduction in the life of a material.

There are three ways in which the medium can act on the material, degrading it; therefore, corrosion is classified as: electrochemistry, chemistry and electrolysis. See how each of them occurs:

• Electrochemical corrosion:

This is the most common type of corrosion, as it occurs with metals, usually in the presence of water. It can take two main ways:

(1) When the metal is in contact with an electrolyte

(conductive or ionic conductor solution involving anodic and cathodic areas at the same time), forming a Corrosion pile.

Example: A rust formation is an example of electrochemical corrosion. Iron oxidizes easily when exposed to moist air (oxygen (O₂) and water (H₂O)). This oxidation results in the Fe cation²⁺, forming the negative pole (which loses electrons) of the cell:

Anode: Fe_(s) → Fe²⁺ + 2e^-

Among the various reduction processes that can occur, the most significant is that of water:

Cathode: 2H₂O + 2e^– → H₂ + 2OH^–

While the Fe cations²⁺ migrate to the negative pole (cathode), the OH anions^- migrate to the positive pole (Anode) and the formation of ferrous hydroxide (Fe (OH) occurs₂).

Faith²⁺ + 2OH^– → Fe(OH)₂

In the presence of oxygen, this compound is oxidized to iron III hydroxide (Fe (OH)₃), which then loses water and turns into iron(III) oxide monohydrate (Fe₂O₃ . H₂O), which is a compound that has a reddish-brown color, that is, the rust we know:

2Fe(OH)₂ + H₂O + 1/2O₂ → 2 Fe(OH)₃

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2Fe(OH)₃ → Faith₂O₃ . H₂O + 2H₂O

(2) When two metals are linked together by an electrolyte, they form a galvanic cell.

For example, if we place a copper and an iron plate, both dipped in an aerated neutral electrolyte and brought into contact, forming an electrical circuit, each plate will become an electrode. Iron will be the anode, oxidizing and losing electrons that migrate to the cathode (copper plate), which in turn is reduced. The anode will wear out, forming rust at the bottom of the container.

• Chemical Corrosion:

It is the attack of some chemical agent directly on a certain material, which may or may not be a metal. It does not require the presence of water and there is no transfer of electrons as in electrochemical corrosion.

Examples:

* Solvents or oxidizing agents can break down polymer macromolecules (plastics and rubbers), degrading them;

* Sulfuric acid corrodes metallic zinc;

* Reinforced concrete in buildings can corrode over time by polluting agents. In its constitution there are silicates, calcium aluminates and iron oxide that are decomposed by acids, as shown in the following reaction:

3CaO.2SiO₂.3H₂O + 6HCl → 3CaCl₂ + 2SiO₂ + 6H₂O

• Electrolytic corrosion:

It is a electrochemical process that occurs with the external application of an electrical current. This process it's not spontaneous, unlike the other types of corrosion mentioned above. When there is no insulation or grounding, or they are deficient, they form leakage currents, and when they escape to the ground, small holes are formed in the installations.

Examples: This happens in water and oil pipelines, in telephone lines and in gas stations.

By Jennifer Fogaça
Graduated in Chemistry

Would you like to reference this text in a school or academic work? Look:

FOGAÇA, Jennifer Rocha Vargas. "Types of Corrosion"; Brazil School. Available in: https://brasilescola.uol.com.br/quimica/tipos-corrosao.htm. Accessed on June 28, 2021.