The article introduces the classification and influencing factors of metal corrosion, elaborates in detail the methods and importance of corrosion protection, briefly lists some commonly used chemical coatings, and simply introduces their mechanisms and roles.
With the engineering examples in the petrochemical industry, it demonstrates that the means of metal corrosion protection are not singular, but a combination of various methods to achieve the purpose of protection.
Finally, it sends a message to the technical corrosion protection industry to minimize the disastrous effects caused by metal corrosion.
I. Classification of Metal Corrosion
Metal corrosion is the destruction or alteration of metals caused by environmental effects. According to the corrosion process, it is mainly divided into chemical corrosion and electrochemical corrosion.
Based on the form and distribution of metal corrosion damage, it is mainly categorized into uniform corrosion and localized corrosion. It is also classified into high-temperature corrosion and room-temperature corrosion, dry corrosion and wet corrosion, etc., depending on the environmental conditions of the corrosion.
II. Factors Affecting Metal Corrosion
Factors affecting metal corrosion include:
- Some influencing factors in the production process
- The impact of acids, alkalis, and salts
- Relative humidity of the gas phase and the critical relative humidity of metal corrosion
- The impact of pollutants in the air
- The impact of temperature
III. Methods of Corrosion Protection
01 Improve the Nature of Metals
By choosing different material compositions to form corrosion-resistant alloys for different purposes, or adding alloy elements to metals to improve their corrosion resistance, we can prevent or slow down metal corrosion.
02 Forming a Protective Layer
Covering the metal surface with various protective layers, separating the protected metal from the corrosive medium, is an effective way to prevent metal corrosion.
Phosphating treatment of metals: After the steel product is degreased and derusted, it is immersed in a phosphate solution of specific composition. A water-insoluble phosphate film is formed on the metal surface, which is called phosphating treatment. The phosphate film is dark gray to black gray, generally 5-20 μm thick, and has good corrosion resistance in the atmosphere.
Oxidation treatment of metals: When steel products are added to a mixed solution of NaOH and heated, a blue oxide film (mainly Fe3O4) of about 0.5-1.5 μm thick is formed on the surface, achieving the purpose of corrosion resistance. This process is called bluing.
Non-metal coating: Non-metal materials such as paint, spray paint, enamel, ceramics, glass, asphalt, polymer materials (such as plastic, rubber, polyester) etc., are coated on the metal surface to form a protective layer, known as a non-metal coating, which can also achieve the purpose of corrosion prevention.
Metal Protective Layer: It is a protective coating formed by plating a metal or alloy with higher corrosion resistance on the surface of another metal product to be protected.
03 Improving the Corrosion Environment
Improving the environment is of great significance to reduce and prevent corrosion. Corrosion inhibitors are a type of chemical substance.
By adding a small amount of it to the corrosion medium, the rate of metal corrosion can be significantly reduced. Since the use of corrosion inhibitors is small, convenient and economical, it is a common means of corrosion prevention.
04 Electrochemical Protection Method
The electrochemical protection method is to take measures based on electrochemical principles on metal equipment, making it the cathode in the corrosion cell, thereby preventing or mitigating metal corrosion.
Sacrificial anode protection method: The sacrificial anode protection method uses a metal or alloy with a lower electrode potential than the protected metal as the anode, fixed on the protected metal to form a corrosion cell, and the protected metal is protected as the cathode.
Cathodic protection: This uses an external power supply to protect the metal. Connect the metal to be protected to the negative terminal to become a cathode and avoid corrosion. Also, some iron pieces are connected to the positive terminal to become an anode, let it corrode, this is actually a sacrificial anode.
Anode protection: This also uses an external DC power supply to protect the metal. But connect the metal to be protected to the positive terminal to become an anode.
IV. Mechanism and Functions of Paint
A variety of paints exist, and here we will briefly introduce a few commonly used ones.
- Zinc-rich primer
Its mechanism is based on the cathodic protective effect of metallic zinc powder on the surface of steel. Because the electrochemical activity of metallic zinc (whose standard electrode potential is -0.763 V) is more active than iron (-0.409 V), when there is a sufficient amount of zinc powder, a layer of zinc powder film forms on the surface of the steel.
It closely contacts the surface of the steel and under the effect of a corrosive medium (mainly oxygen, moisture, etc.), forms a zinc-iron corrosion cell. Zinc, acting as the anode, sacrifices itself to corrosion, thus protecting the steel as the cathode.
- Epoxy micaceous iron oxide paint
Micaceous iron oxide (MIO) is used as an anti-rust pigment. Its chemical composition is λFe2O3, named for its flaky structure similar to mica.
It arranges in layers within the paint film, effectively blocking the penetration of water, oxygen, and other corrosive media, thereby prolonging the penetration time of the media.
- Two-component aliphatic polyurethane topcoat
Two-component aliphatic polyurethane serves as a surface coating. The characteristic reaction for a typical polyurethane coating to cure and form a film involves a compound containing two or more isocyanate groups (-NCO) acting as a hardener (component 1) and a compound with two or more hydroxyl groups (-OH) as a base resin (component 2), which then gradually form a polymer through an addition reaction.
The polyurethane macromolecular structure contains both polar groups of isocyanates and flexible carbon-carbon long chains. The formed paint film is tough yet flexible with good wear resistance.
Moreover, it possesses excellent light retention, color retention, UV resistance, and anti-aging performance, demonstrating outstanding weather resistance.
V. Engineering Applications for Corrosion Protection
In the design of chemical plants, various equipment, steel structures, pipelines, pipe components, etc., must choose appropriate protection methods according to their different characteristics, minimizing the losses caused by metal corrosion.
Whether in everyday life or in industrial production, the work of metal corrosion protection plays a pivotal role and has significant impacts on the environment, economy, and safety.
Petroleum and chemical facilities, such as new oil storage facilities, pipelines, and large-scale petrochemical production devices, must be well protected against corrosion.
However, with the various causes of corrosion and many methods for corrosion prevention, this calls for a large number of professionals to put in more effort and sweat.