Understanding Corrosion in Water Pipelines: A Guide for Pipeline Designers



Reviewed by Raghvendra GopalCheckmark
Last updated: September 7, 2022

What Does Absorption Mean?

Absorption is a chemical or physical phenomenon in which the molecules, atoms and ions of the substance getting absorbed enters into the bulk phase (gas, liquid or solid) of the material in which it is taken up. Absorption is the condition in which something gets mixed or absorbed completely in another substance.

In the process of absorption, a substance captures and transforms energy. The absorbent distributes the material it captures evenly through the surface. Processes involving gas or liquid that penetrates into the body of an absorbent are commonly known as absorption. Whether they are to be considered physical absorption or chemical absorption, depends on if there is any chemical reaction occurring between the solute and the solvent which is also called the absorbent.

No significant chemical reaction occurs between the absorbent and the solute when water or hydrocarbon oils are used as the absorbents, which is why such processes are commonly referred to physical absorption.

Absorption is endothermic, meaning that it is a process that has some input of energy. In absorption, energy from the absorbate is then added to the energy of the absorbent system, which increases the energy of this system. The process of absorption occurs and continues at a uniform rate throughout until all absorbate has been dissolved in the absorbent. After which the concentration of the absorbate in the absorbent is uniform.


Corrosionpedia Explains Absorption

The process of absorption is completely different from adsorption and should not be confused. In the adsorption process a liquid or gas accumulates on a liquid or solid surface and builds an atomic or molecular film. When the atoms of a substance hang onto the surface of a different liquid or solid substance this is what is referred to as adsorption.

The material that is able to stick permanently to the surface is called the adsorbate and the material that makes up the surface is called the adsorbent. However, in the absorption process, a substance goes through diffusion into a solid or a liquid to produce a solution (i.e., molecules undergoing absorption are taken up by the volume, not by the surface).

In the absorption process, a substance captures and transforms energy, which can cause metals to corrode. Therefore, in acidic environments corrosion of metal takes place because of oxidation and other reductions. The oxidation reactions occur at the anode where the metal loses its electrons to the environment and becomes absorbed in the solution in the form of positive ions to the cathode.

Electrons are lost from the anodic area and absorbed into ions at the cathodic area. There is a tendency for metal to corrode at the anodic area because the electrons are released from the anode, and cathodic reactions such as oxygen absorption, electroplating and hydrogen evolution take place which increase the corrosion reactions in harsh acidic or aqueous environments. Whereas with the use of aqueous sodium hydroxide or any other strong base as a absorbent to dissolve an acid gas, the absorption reaction is followed by a rapid and irreversible neutralization reaction in the liquid phase. Such a process can be referred to as chemical absorption or reactive absorption. The temperature at which an absorption process might occur does not affect the speed or product of the process.

The presence of absorbed hydrogen (H) within pure iron and most steels (including carbon and stainless steels) has been reported to increase their corrosion. Historically, the increased corrosion of H-charged steels has been attributed to the ability of H to destabilize the passive film formed upon the metal. In recent works it was evident that absorbed H can promote Fe dissolution even in non-passivating solutions. This shows that the effect of H on metal corrosion could be “intrinsic,” rather than through “extrinsic” mechanisms involving solely the destabilization of the passive film.


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