Concept of corrosion

Corrosion is an electrochemical technique of deterioration of materials. In many circumstances, especially when liquids are present, chemistry is involved. During corrosion, electrons from different metal surface portions move to other places via an ion-conducting environment. The chemistry of corrosion is straightforward, but the specifics are anything. The same is true for the impact.

In reality, the economic impact of corrosion is far more than most people believe. Utilities, notably water and sewer systems, have the most significant economic burden, followed by motorised vehicles and transportation.

Because metallic corrosion is a continuous electrochemical process, it is critical to understand the fundamental nature of electrochemical methods to suppress corrosion and decrease its influence on structures effectively.

What exactly is Corrosion?

Corrosion in an aqueous and atmospheric environment is an electrochemical process that transports electrons between a metal surface and a liquid electrolyte solution, resulting in substrate deterioration. Corrosion happens due to metals’ proclivity to react electrochemically with oxygen, water, and other chemicals in the atmosphere. In this context, the word anode refers to the piece of the metal surface that is rusting. In contrast, cathode refers to the area of the metal surface that consumes the electrons produced by the corrosion process.

Electrochemical Reactions 

An electrochemical reaction is defined as a reaction in which electrons are transferred. It is also a reaction involving oxidation and reduction. Because both processes are commonly mixed in one piece of metal (e.g. zinc), corrosion consists of a minimum of one chemical reaction, and one reduction reaction is not apparent.

A piece of zinc submerged in acid solution is corroding. At some point on the surface, Zn is converted to Zn ions, which lose electrons. These electrons go through the solid conducting metal to different places on the metal surface where hydrogen (H) ions are converted to hydrogen gas, as shown by the equation below.

Anodic reaction: Zn(s)Zn2++ 2e–

Cathodic reaction: 2H++2e– H2(g)

These equations depict the characteristics of a zinc electrochemical reaction. Electrons are transported during such a reaction, or, to put it another way, an oxidation reaction occurs concurrently with a reduction event.

Overall reaction: Zn+2H+Zn2++H2(g)

As a result, anodic and cathodic reactions co-occur at the same rate in electrochemistry. On the other hand, Corrosion occurs only in places that serve as anodes.

Reactions and Corrosion Cells

Most corrosion processes are distinguished by separating the oxidation and reduction stages on the metal. This is feasible because metals are conductive, allowing electrons to pass from the anodic to the cathodic areas.

Although water is required to transport ions to and from the metal, a thin coating of adsorbed moisture can be sufficient.

A corrosion system can be thought of as a short-circuited electrochemical cell with an anodic process similar to,

Fe(s)Fe2+(aq.)+2e–

and the cathodic processes may involve the reduction of oxygen gas,

O2+2H2O+4e–4OH–

Alternatively, proton reduction,

H++e–12H2(g)

or the ion reduction of a metal,

M2++2e–M(s)

Here , M represents the metal.

Many factors influence which sections of the metal function as anodes and cathodes, as seen by the uneven corrosion patterns that are regularly observed. Atoms in stressed areas, such as those created by forming or machining, frequently have greater free energies and so tend to become anodic.

Corrosion Control

Corrosion requires both the cathodic and anodic stages to occur, so preventing any one will prevent corrosion. The most apparent method is to cover the object with paint or another protective coating to inhibit both processes. Even if this is done, there will most certainly be areas where the coating is damaged or does not penetrate, especially if there are holes or screw threads. A more advanced way is to impart a tiny negative charge to the metal, making the reaction more difficult to occur:   MM2++2e–

Method 1 of Protection: Sacrificial Coatings

Applying a coating of a highly active metal is one method of giving this negative charge. A typical method of preserving steel against corrosion is to cover it with a thin layer of zinc, a process known as galvanising. Because zinc is less noble than iron, it corrodes selectively. When this sacrificial coating dissolves, electrons concentrate in the iron, making it cathodic and limiting its disintegration.

The contrast created by coating iron with a less active metal is intriguing. An excellent example is the typical tin-plated can (on the right). All is okay as long as the tin covering is intact, but even minor exposure of the underlying iron to the wet environment causes corrosion. The electrons generated by the iron flow into the tin, causing the iron to become more anodic, and the tin is now actively supporting iron corrosion! You’ve probably noticed how quickly tin cans degrade when left outside.

Cathodic Protection is the second method of protection.

A more advanced technique is to maintain a constant negative electrical charge on a metal, preventing it from dissolving as positive ions. This technology is known as cathodic protection because the entire surface is driven into a cathodic state. An external direct current power supply (often employed to safeguard oil pipelines and other subterranean infrastructure) or the corrosion of another, more active metal, such as a piece of zinc or aluminium buried nearby, can be the source of electrons.

Conclusion

In this post, we looked at corrosion as an electrochemical phenomenon that causes materials and structures to decay. The electrochemical processes of corrosion are thoroughly described. Because we can avoid corrosion if we fully grasp the electrochemistry of corrosion, corrosion product producers and consumers should pay close attention to the electrochemical processes that cause corrosion. Corrosion science is a complicated discipline that is still evolving from the basic conventional definition of “destruction of metals by oxidation and its prevention” to “degradation of a material involving one or more chemical and/or electrochemical processes and its foresight.” This latter definition includes a wide variety of conditions as well as all material types (ceramics, organics, composites), not simply metals. Corrosion is primarily concerned with the electrochemical aspects of physical-chemical oxidation and reduction processes occurring at the surface of materials, as well as the effect of the reactivity of materials’ surfaces in relation to their environment, time, pH, temperature, pressure, and electrolyte composition. It is virtually always an irreversible heterogeneous response of a substance with the environment, which typically (but not always) leads to the material or its qualities degrading.