Corrosion and its problems in metals
Corrosion and its problems in metals

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Corrosion and its problems in metals

Corrosion and its problems in metals


Corrosion damage in the chemical, oil and gas industries has recently received a great deal of attention from contractors and employers. Therefore, one of the most important tasks of engineers is to identify the types of corrosion, the method of measuring the amount of corrosion, determining the type of corrosion and how to combat it. Water-based flowers in which oxygen and carbon dioxide are dissolved are a good environment for corrosion production. Lowering the pH and increasing the gelatinous state of these flowers, which increase the corrosion rate by increasing the gas trapped in the flower.

Definition of corrosion:

Corrosion is a natural affinity for reduced metals to return to pre-reduced compounds. Metals are often in nature in the form of compounds that are thermodynamically stable, lose their stability after regeneration, and if they are exposed to the natural environment again, they are corroded and return to their original composition.

Definition of wet and dry corrosion:

Corrosion can be divided into wet and dry. The mechanism of wet corrosion is electrochemical. Dry corrosion is called gas-solid reactions at high temperatures. More corrosion is discussed in the drilling industry. Corrosion of iron in drilling mud electrolytes and other fluids are more corrosive. The higher the electrical conductivity of the fluid, the higher the current flow and the higher the corrosion rate.

Electrochemical corrosion mechanism:

The following elements are required to form a corrosion cell:

The image below shows the electrochemical cell

Anode: A metal that is eaten and has anodic reactions on it.

The image above shows the anode in an electrochemical cell.

Cathode: The part on which cathodic reactions take place.

The image above shows the cathode in an electrochemical cell.

Interface wire: The electric current between the cathode and the anode is established through the interface wire.

The image above shows the movement of current in electrical circuits.


Some metals have the ability to form a protective layer against their surroundings. This layer is formed by corrosion products, and acts as a barrier against corrosion. This process is called forming a protective layer and increasing the potential of surface metals. Aluminum is one of the metals that can be surface. Indicates corrosion and reaching passivation.

Types of corrosion and their definitions:

1. Galvanic Corrosion

Galvanic corrosion occurs when two heterogeneous metals form a cell in an electrolyte. The potentials of two metals differ from each other in different electrolytes. The corrosion rate depends on the cathode / anode surface ratio; the larger the cathode surface, the higher the corrosion rate and anode loss.


Ways to prevent this type of corrosion are:

1- Avoid connecting metals with long distances in the electrochemical series.

2- Pay attention to the surface ratio of anode and cathode.

3- Insulation should be used in the connection between non-homogeneous metals.

4. Be careful when using coatings, especially on the anode.

5. Select anodic parts thicker or with suitable replaceability.

6. Use cathodic protection.

2. Uniform Corrosion

The surface of most metals in the corrosive environment is uniformly corroded, which is called uniform corrosion. This type of corrosion is caused by constant polarization of the cathode and anode at the metal surface due to polarization. The destructive power and importance of this type of corrosion is less than other types. Corrosion of steel in wet weather is often of this type.

* Ways to prevent this type of corrosion are:

1. Select the appropriate material and coating

2. Use of inhibitors

3- Cathodic protection

3. Pitting Corrosion

If the surface of the cathode and anode does not change, cavitation corrosion will occur. The surface of the cavity or groove is the anode. This type of corrosion is a type of localized corrosion. Due to the continuation of the corrosion process, the cavity becomes deeper and causes holes in the pipes.

* Ways to prevent this type of corrosion are:

1. Choosing the right materials according to the work environment

2. Control the amount of chlorine ion, temperature and pH of the environment

3- Using high alloy steels such as SS.316, which have better corrosion resistance due to molybdenum.

4. Use of corrosion inhibitors

4. Crevice Corrosion

This corrosion occurs in areas that have a static fluid, such as under washers, bolts, rivets, and surface deposits.

* Ways to prevent this type of corrosion are:

1- Using welding instead of nut screws or rivets

2. The grooves should be filled with solder at the location of the two metals on top of each other.

3- The design of the tanks should be such that the fluid is completely drained.

4- Cleaning the sediments from the surface of the equipment

5. Intergranular Corrosion

This type of corrosion that occurs at the grain boundaries is a type of localized corrosion. During the freezing process, the grain boundaries are the last points to be frozen, so the concentration of alloying elements and impurities is higher at the grain boundaries of metals. The difference in the concentration of the alloy between the surface and the grain boundaries causes the energy difference and as a result the tendency of the grain surface and the grain boundary to be corroded. As a result, the grain boundary anode and the grain surface are cathodic.

* An important way to prevent this type of corrosion is:

Reduction of carbon content to less than 0.02%.

6. Selective Leaching Corrosion

In this type of corrosion, one of the components of the alloy is separated from it and suffers corrosion.

* Ways to prevent this type of corrosion are:

1- Reduction of oxygen in the environment

2- Adding elements of tin, arsenic, antimony alloy to the element of rice

3. Use of copper-nickel alloys for important parts

7. Erosion Corrosion

Corrosion is exacerbated by wear due to fluid movement. This type of corrosion intensifies with the addition of solids in the fluid. Abrasion destroys the corrosion products created on the metal surface and allows the electrolyte to come in contact with the metal surface to continue corrosion. Increasing the rotation speed of drilling fluid, the amount of solids and turbulent flow of drilling fluid are the factors that intensify this type of corrosion, by reducing the rotation speed of drilling fluid and the amount of solids, or establishing a layered flow in drilling fluid, the speed of this type of corrosion Should be reduced. Most seat and valve pumps suffer from abrasion corrosion.

* Ways to prevent this type of corrosion are:

1. Selection of durable material

2- Appropriate design includes: increasing the diameter of pipes, reducing the change in size and angle of pipes, using more thickness in sensitive areas

3- Reducing the temperature, using filters to remove suspended particles, using inhibitors

4. Use the right cover

8. Stress Corrosion

Each type of subject corrosion that is exacerbated by stress is called stress corrosion. The combination of corrosion and tensile stress causes cracking and then failure of the metal. One of the characteristics of stress corrosion is the absence of uniform corrosion.

* Ways to prevent this type of corrosion are:

1- Reduction of tensile stress

2. Removal of harmful impurities from the environment

Author: persian / Date: 2017-10-26
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