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What is corrosion and how to avoid it?

What is corrosion?

To know how to avoid corrosion, it is important to know what corrosion is and why it occurs. Corrosion is the natural process in which a metal gradually deteriorates as a result of electrochemical (or chemical) reactions with its environment.

These reactions cause refined metals to seek to achieve a form of greater stability or lower internal energy, which are usually their oxide, hydroxide or sulfide versions (this is why the metal is said to oxidize). Corrosion also occurs in non-metallic materials such as ceramics and polymers, but it is different and is often called degradation.

Corrosion is a human enemy process, since these damages degrade materials, change their color and weaken them, increasing the possibility of rupture and increasing costs for their repair and replacement.

For this reason, there are entire fields in materials science that are dedicated to preventing this phenomenon, such as corrosion engineering. The methods for the prevention of corrosion are varied and will depend on the materials affected.

First of all, it must be taken into account that not all metals corrode at the same speed, and some have the peculiarity of not corroding naturally at all, as in the case of stainless steel, gold and platinum.

This happens because there are materials for which corrosion is thermodynamically unfavorable (that is, they do not achieve greater stability with the processes that lead to this) or because they have such slow reaction kinetics that corrosion effects take time to show.

Even so, for the elements that do corrode there are a series of methods to prevent this natural process and give them a longer life span:

Galvanized

It is the method of corrosion prevention in which an alloy of iron and steel is coated with a thin layer of zinc. The objective of this method is to make the zinc atoms of the coating react with the air molecules, oxidizing and retarding the corrosion of the part they cover.

This methodology turns zinc into a galvanic or sacrificial anode, exposing it to corrosion degradation to save a more valuable material.

The latter is the methodology that protects the most, since the zinc is bonded to the metal by electrochemical processes and not only by mechanical processes such as immersion.

Paints and covers

The application of paints, metal plates and enamels is another way to add a protective layer to metals prone to corrosion. These substances or layers create a barrier of anticorrosive material that is interposed between the harmful environment and the structural material.

Other coatings have specific properties that make them corrosion inhibitors or anti-corrosives. These are added to liquids or gases first, and then they are added as a layer on the metal.

These chemical compounds are widely used in industry, especially in pipes that transport liquids; In addition, they can be added to water and coolants to ensure that they do not create corrosion in the equipment and pipes through which they pass.

Anodizing

It is an electrolytic passivation procedure; that is, the process through which a somewhat inert film is formed on the surface of a metallic element. This process is used to increase the thickness of the natural oxide layer that this material has on its surface.

This process has the great advantage of not only adding protection against corrosion and friction, but also provides greater adhesion for layers of paint and glues than bare material.

Despite having undergone changes and evolutions over time, this process is commonly carried out by introducing an aluminum object into an electrolyte solution and passing a direct current through it.

This current will cause the aluminum anode to release hydrogen and oxygen, generating aluminum oxide that will bind to it to increase the thickness of its surface layer.

Anodization generates changes in the microscopic texture of the surface and in the crystalline structure of the metal, causing a high porosity in it.

Therefore, despite improving the strength and resistance to corrosion of the metal, it can also make it more brittle, in addition to reducing its resistance to high temperatures.

Biofilms

Biofilms are groups of microorganisms that come together in a layer on a surface, behaving like a hydrogel but without ceasing to represent a living community of bacteria or other microorganisms.

Although these formations are often associated with corrosion, in recent years there has been a development in the use of bacterial biofilms to protect metals in highly corrosive environments.

In addition, biofilms with antimicrobial properties have been discovered, which stop the effects of sulfate-reducing bacteria.

Impressed current systems

In those very large structures or where the resistivity to electrolytes is high, the galvanic anodes cannot generate enough current to protect the entire surface, so a cathodic protection system by impressed currents is used.

These systems consist of anodes connected to a direct current power source, mainly a transformer-rectifier connected to an alternating current source.

This method is used mainly in freighters and other ships, which require a high level of protection over a greater surface area of ​​their structure, such as propellers, rudders and other parts on which navigation depends.

Changes in environmental conditions

Finally, the corrosion rate can be stopped or reduced with the alteration of the environmental conditions in which the metallic material is found.

The humidity and contents of sulfur, chlorides and oxygen in liquids and gases must be kept at low levels to increase the life expectancy of a material, and using less saline and / or hard water has a positive effect.

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