What is Corrosion?

Corrosion is one of the biggest problems in the metal industry. Corrosion is the process by which a material undergoes a change or wears away due to a chemical and electrochemical reaction that starts at the material's surface and affects deeper layers. In short, it is the process of natural chemical reactions that occur on metal surfaces. The chemical structure formed at the end of this process is called rust.

One of the most common problems faced by manufacturers in the metal industry is rusting. Protecting raw and processed metals from corrosion during storage and transportation is a critical process for producers. Especially during overseas shipments, where high humidity speeds up the rusting process, it's crucial to preserve the parts as best as possible during transport. Even in well-packaged parts, some air may remain inside the packaging, which can initiate the rusting process. To avoid this risk, it is always better to coat the parts with a protective layer.

In addition to causing an unattractive appearance on the surface, rust also complicates the processing of the material. Over time, rust can penetrate deeper into the material, shortening its lifespan. Corroded materials need mechanical or chemical treatments to be used, resulting in additional costs for labor, material loss, and time. Therefore, protecting the material from corrosion from the start is more advantageous.

Corrosion can occur in two different ways: chemical and electrochemical.

1.Electrochemical Oxygen Corrosion
This type of corrosion occurs as a result of oxidation-reduction reactions (redox reactions). An atom that is electrically neutral may lose or gain electrons due to certain effects. In aqueous environments, the reactions that occur when electrons are lost (oxidation) or gained (reduction) are called "electrochemical reactions." All corrosion reactions that occur in water, air, and underground environments are electrochemical. This can also be called wet corrosion.

Corrosion Cell Structure and Reactions
Whether electrochemical corrosion occurs on a micro or macro scale, it can be modeled with a corrosion cell. A corrosion cell consists of the anode, cathode, conductive medium (electrolyte), and the conductive connection between the anode and cathode. If one of these four components is missing, corrosion does not occur. Corrosion occurs at the material acting as the anode. The electrolyte can be as simple as slight moisture inside a crack, a film layer, or even sweat.

Corrosive events arise when the oxygen in the air interacts with water on the metal surface. Under a water droplet, metal ions begin to dissolve. At the edges of the droplets, OH- ions formed from oxygen in the air react with the dissolved metal, and first form metal hydroxides (Me(OH)3), which then turn into rust (MeO(OH)).

Anode: The metal that undergoes corrosion (oxidation)
Me → Me+n + ne-

Cathode: The surface where the reaction using the electrons released at the anode takes place (reduction)
O2 + 2H2O + 4e- → 4OH-

Electronic Conductor: The metallic conductor that carries the electrons from the anode to the cathode.

Electrolyte: The conductive solution that provides an ionic connection between the anode and cathode.

Anodic Reaction: Metal atoms lose negative charge and turn into positively charged metal ions, producing electrons.
Me → Me+n + ne-

Cathodic Reaction: The function of the cathodic event is to consume the electrons produced in the anodic reaction.
Me+n + ne- → Me

2.Chemical Corrosion (Dry Corrosion)
This occurs when metals and alloys oxidize in a gas environment. In atmospheric conditions, the most important corrosive substances are O2, H2S, and halogens, leading to the formation of oxides and sulfides on the metal surface. However, corrosion caused by the surrounding humid air is outside the scope of this description. The most common example is the oxide layers formed on iron and steel at high temperatures.

Corrosion reactions for different metals:

Iron Corrosion Reactions:
Fe+2 + 2OH- → Fe(OH)2 (k)
4Fe(OH)2 + O2 + 2H2O → 4Fe(OH)3(k)
2Fe(OH)3 → Fe2O3.H2O(k) + 2H2O

1. Fe2O3.H2O is the main component of red-brown rust, and it can form a mineral called hematite.
2. Fe3O4.H2O is typically green but may appear dark blue in the presence of organic complexes.
3. Fe3O4 is magnetite, which is black.

Aluminum Corrosion:
4Al(k) + 3O2(g) → 2Al2O3(k) (white-gray in color)

Zinc Corrosion:
2Zn(k) + O2(g) → 2ZnO(k)

Copper Corrosion:
2Cu + H2O ↔ Cu2O + 2H+
Cu2+ + H2O ↔ CuO + 2H+

Copper rust is green.

Atmospheric Corrosion
Atmospheric corrosion is a common type of corrosion. Research shows that approximately 80% of metallic structures are exposed to atmospheric corrosion. Many steel structures such as poles, bridges, railings, railroads, warehouses, and vehicle bodies are constantly affected by the atmosphere. Iron and steel corrode when exposed to oxygen and moisture in the air, forming rust. Rust, which is physically soft and permeable, does not protect metals from further corrosion. Instead, it creates a favorable environment for the condensation of water vapor and the absorption of sulfur oxides on the metal surface. Even in naturally clean atmospheric conditions, corrosion can occur when water vapor is present. Industrial pollution, especially sulfur oxides, is a major factor in atmospheric corrosion. These oxides combine with water vapor in the air to form acids and other chemicals.

Salt particles carried by the wind are common in coastal areas and contribute to corrosion when they settle on metal surfaces.

Types of Corrosion by Structure

1.Uniform Corrosion: Corrosion that occurs evenly across the entire surface of the material. It is the most common and results in the greatest material loss.

2.Pitting Corrosion: Localized corrosion that leads to small pits or holes in the material.

3.Crack Corrosion: Corrosion that occurs in cracks or narrow spaces on the metal surface.

4.Galvanic Corrosion: Occurs when two metals with different electrochemical potentials are in the same electrolyte.

5.Intergranular Corrosion: Corrosion that occurs at the grain boundaries of a material.

6.Delamination Corrosion: Occurs when corrosion affects layers of the material.

7.Fatigue Corrosion: Corrosion that occurs due to dynamic loads that cause cracks.

8.Erosion Corrosion: Corrosion caused by the relative movement between the metal and the corrosive environment.

9.Selective Corrosion: Occurs when one element in an alloy corrodes preferentially.