Corrosion is a slow and continuous consumption of a material, an electrochemical phenomenon, that leads to a progressive deterioration of the characteristics and physical properties of the element. It represents a true regression of metals to their initial state, a natural and unstoppable process that, however, can be slowed down and prevented through some specific treatments applied to the different materials.
In the following paragraphs, we will better understand the causes, effects, and possible preventive treatments of corrosion.
What Causes Corrosion?
Using more technical language, corrosion is aphenomenon that arises from a physico-chemical interaction of the material with its surrounding environment. This interaction causes the consumption of the material, a process that ultimately results in what is commonly called rust.
When a ferrous material comes into contact with atmospheric oxygen or substances containing oxygen, it tends to return to its original state. This process is accelerated in the presence of water and salts, substances that trigger chemical reactions.
This phenomenon leads to the formation of rust, a powdery and porous iron oxide. Generally, two types of rust can be identified:
White rust, when oxidation affects the protective layer of the material;
Red rust, when corrosion damages the metal.
Both are the visible result of the progressive deterioration of the material, a phenomenon that has repercussions on the specific product and the structure to which it belongs.
In general, humidity and water are components that accelerate the process and, therefore, for example, proximity to the sea or adverse weather conditions worsen the situation. However, corrosion is linked to various phenomena. In the next paragraph, we report all possible cases.
What Are the Types of Corrosion?
A first macro distinction between types of corrosion is that between:
Dry corrosion, caused mainly by air and the substances it carries. It, therefore, concerns structures and products located in outdoor environments;
Wet corrosion, resulting from the presence of an electrolyte, for example, water. It can affect both external structures (for example, in maritime areas) and internal structures (for example, in tanks and pipelines). In this second case, the decay cannot be monitored daily and, for this reason, maintenance work is carried out regularly.
Corrosion can manifest in two main ways:
Generalized corrosion occurs in the absence of protective surface treatments;
Localized corrosion initiates at specific points of the structure and manifests with perforations on the surface.
What Mechanisms Cause Corrosion?
The main mechanisms of corrosion activation are divided into:
Electrochemical corrosion, deterioration triggered by substances present in water or water vapor;
Galvanic corrosion, a phenomenon resulting from contact between metals with different composition characteristics;
Mechanicochemical corrosion, caused by causes of both electrochemical and mechanical nature.
In detail:
Electrochemical corrosion depends on an electron shift: materials possess an electrical potential that translates into the ability to receive or donate electrons. In the presence of a difference in electrical potentials and an acidic solution, an electron transfer occurs. The material that has donated its electrons oxidizes.
Galvanic corrosion occurs when two materials, with different nobility, are placed in contact in the presence of a third element called an electrolyte. This phenomenon is, therefore, linked to the potential difference between the two elements and the amount of oxygen present in the environment. Generally, less noble materials like iron, nickel, and zinc tend to corrode more easily compared to more noble materials like copper, silver, and stainless steel. This activation mechanism can be contained if the choice falls between materials of similar nobility.
Regarding mechanicochemical corrosion, it can occur due to friction (application of frictional forces), stress corrosion (application of a constant structural load over time), and fatigue (application of a variable load over time).
What Metal is Most Resistant to Corrosion?
Metals are classified according to a nobility scale that represents their resistance to oxidation, to moisture-driven corrosion, and to acids. Therefore, noble metals are used in metallurgy, technology, and production but are generally expensive due to their rarity. Among these are ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, and gold.
It is important to emphasize that some metals, although not considered noble, are very resistant to corrosion. For example, titanium.
Furthermore, when talking about metals resistant to corrosion, the reference to stainless steel cannot be missed. Its name suggests that it can resist all types of corrosion. In reality, even this material can deteriorate if in contact with others or with the external environment, although over very long periods. Corrosion of steel can be prevented or slowed down through some treatments like passivation.
How to Combat Corrosion
Technological evolution and studies on the matter have brought interesting developments related to various mechanisms aimed at combating corrosion. These are coatings and processes that protect materials from oxidation and, therefore, slow down their destruction.
To achieve optimal results, it is advisable to perform thorough cleaning of the surfaces, such as pickling, before proceeding with the mentioned treatments.
Moreover, it is a best practice to always consider the final application of the product.
For example, screws represent a particular case because, coupled with nuts or to allow the operation of tightening tools, they must be treated with a thin but sufficient coating to protect the product.
Or again, a metal that is conceived and thus treated for indoor use, if placed outdoors and near the sea coast, will undergo rapid degradation and corrosion.
It is therefore crucial to consider all factors to maintain and protect the composition and characteristics of the material over time.
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