Corrosion is a natural phenomenon. It is the return of a material from a high-energy state to a low-energy state, so it cannot be completely stopped. We can only control the corrosion rate through reasonable material selection and effective anti-corrosion measures to make it within the allowable and controllable range.

1. Common methods of controlling corrosion include:

1.1 Reasonable material selection and structural design;

1.2 Change the environment (control and treatment of corrosive media, also called process anti-corrosion measures);

1.3 Electrochemical protection.

But not under any conditions we have three methods to choose from. In more cases, we have no way to change the corrosive environment, nor do we have the conditions for electrochemical protection, and the choice of structure cannot fundamentally solve the problem. At this time, a reasonable selection of materials is often the most effective and convenient method.

The choice of anti-corrosion materials is sometimes affected by the characteristics of the materials. For example, when manufacturing equipment, it is often required to use metal materials with high strength, good temperature resistance, and good machining performance. However, the corrosion resistance of metal materials often fails to meet the requirements, or the price of materials with good comprehensive properties is too high. At this time, people thought of a more effective method, covering a more corrosion-resistant material on the surface of the material that needs to be protected to form a layer of anti-corrosion layer, and use its corrosion resistance and barrier effect to solve the corrosion problem. The purpose of effectively controlling corrosion can be achieved by spending a higher cost to select materials with better comprehensive performance.

2. The protective effect of the anti-corrosion layer on the substrate is mainly reflected in the following three aspects:

2.1 Isolation: Isolate the base material from the corrosive medium to achieve the purpose of anti-corrosion;

2.2 Corrosion inhibition effect: with the help of the internal components of the coating to react with the metal, a passivation or protective film is formed to improve the protective effect of the anti-corrosion layer;

2.3 Electrochemical protection: Add active metal materials to the paint to protect the sacrificial anode and slow down corrosion.

3. There are several types of anti-corrosion methods using anti-corrosion layer technology:

3.1 Coating anti-corrosion;

3.2 FRP lining and tape anti-corrosion;

3.3 Lining with rubber or plastic;

3.4 Corrosion-resistant metal lining;

3.5 Metal spray coating and plating;

3.6 Enamel or glass lined.

4. The anti-corrosion layer should have the following properties:

4.1 Stable resistance to chemical media (including good water resistance);

4.2 Have sufficient mechanical properties such as impact resistance, bending resistance, and wear resistance;

4.3 When used for anti-corrosion under general corrosive conditions, sufficient electrical insulation is required;

4.4 Good adhesion and bonding force to the substrate;

4.5 It has good cathodic disbonding resistance and good aging resistance and temperature resistance;

4.6 The coating should not have an adverse effect on the base material and the contacted medium, and it should not cause harm to the human body and the environment during construction;

4.7 Good storage and construction stability;

4.8 Easy to repair.

Because the coating has anti-corrosion, it has simple construction, wide adaptability (applicable to anti-corrosion in the atmosphere, water, general liquid, soil and other environments), easy on-site construction, suitable for large-area and complex-shaped equipment, and the comprehensive cost of anti-corrosion is relatively It has the advantages of low temperature, short coating period and better anti-corrosion effect, so it has become the most widely used anti-corrosion technology in various anti-corrosion measures.

However, coating anticorrosion also has some shortcomings: the thickness is thin, the resistance to mechanical impact is not strong, there is a certain life limit, and the temperature resistance is limited, and it is not suitable for strong corrosive environments.

So today we focus on introducing relevant knowledge in this area.

5. Classification and characteristics of commonly used anti-corrosion coatings

5.1 Definition and interpretation of paint

Definition: A line of liquid or solid materials that can form a solid coating film with protection, decoration, or special properties (such as insulation, anti-corrosion, and signs) when applied to the surface of an object.

Among them, liquid coatings account for the vast majority, and solid coatings are a new type of coating that has developed rapidly in recent years. It has the characteristics of environmental protection, short construction period, and excellent performance, and it is continuously expanding the scope of use. In liquid coatings, organic coatings have an absolute advantage in the field of coatings due to their excellent performance and good use performance.

Since most of the earliest paints used vegetable oils and natural paints as the main raw materials, they are also called paints. However, with the advancement of science and technology, more and more coatings are directly synthesized using chemical raw materials, which have no contact with "oil", and many coatings are already water-soluble coatings or solid coatings, so the term "paint" has become Can not represent all the paint.

5.2 Basic composition of paint

Regardless of the type and performance of the coating, it is composed of three or four basic ingredients, and its functions are shown in the following table:

5.3 Requirements for anti-corrosion coatings

5.3.1 There must be good adhesion

The adhesion of the coating, especially the wet adhesion, can replace the water and oxygen on the interface and improve the protective performance of the coating. Therefore, coatings with good adhesion generally have better anti-corrosion ability. Anti-corrosion coatings used as primers often require stronger adhesion. Therefore, epoxy and alkyd coatings are often used alone as primers.

5.3.2 Strong anti-penetration ability

Strong resistance to penetration can effectively prevent the penetration of corrosive media. In addition to the inherent properties of the coating resin, the anti-penetration ability is related to the inherent properties of the coating resin. In order to improve the anti-penetration ability, fillers such as mica and iron oxide are often added to improve its performance.

5.3.3 Stability to corrosive media

Not only is it required that the film-forming substance of the coating has good stability, but the added additives, fillers, etc. should also be stable and not chemically react with the medium or undergo physical changes.

5.4 Classification method of paint

There are thousands of types of coating products, and there are many classification methods. Common classifications are:

5.4.1 According to whether there are pigments, it is divided into two types: varnish and color paint.

5.4.2 Divided by form: water-based paint, solvent-based paint, solid powder paint, high solid content paint.

5.4.3 Divided by purpose: architectural paint, wood paint, marine paint, drinking water warehouse paint, etc.

5.4.4 According to the construction process: primer, putty, intermediate paint, top paint, finishing paint.

5.4.5 According to the effect of use: insulating paint, anti-rust paint, anti-fouling paint, anti-corrosion paint, fire-resistant paint.

5.4.6 According to the main film-forming substances of the paint, it is divided into natural resin paint, asphalt paint, phenolic paint, epoxy paint, and polyurethane paint.

5.5 The performance characteristics, uses and selection principles of commonly used anti-corrosion coatings

5.5.1 Anti-corrosion coatings have different anti-corrosion performance, applicable conditions, construction characteristics, service life, and price level due to different main film-forming substances. Therefore, you should have a certain understanding of the characteristics of various coatings before choosing coatings. The following table shows the characteristics and main uses of commonly used synthetic resin coatings:

5.5.2 Each coating has different performance characteristics and its applicable environment is also different. The following table is a recommended table for the selection of coatings under common corrosive environments in the petrochemical industry.

5.5.3 The selection of anti-corrosion coatings must consider the use environment and construction conditions. For example: In the anticorrosion of buried pipelines, bitumen coatings with excellent water resistance should be preferred; high-viscosity, one-time thick anti-corrosion coatings should be used in severe corrosive environments; perchloroethylene, nitrocellulose paint, etc. should be quickly volatile Dry paint is not suitable for spray application; chlorosulfonated polyethylene is not suitable for construction by rolling back and forth.

5.5.4 The selection of anti-corrosion coatings should also consider the compatibility of the coatings. There should be good compatibility between the primer and the substrate, and there should be no mutual dissolution or biting between the primer and the topcoat. For example: Chlorosulfonated polyethylene can be used as primer and topcoat on the surface of concrete, but it is not suitable for primer on the surface of steel; epoxy resin paint can be used as primer in most cases, but It is not suitable for topcoat in the open air, because epoxy coatings have poor UV resistance; coatings with good corrosion resistance but poor adhesion (for example: perchloroethylene) can only be used as topcoats, and primers must be selected for adhesion. Excellent coatings (epoxy, alkyd).

5.5.5 The selection of paint varieties should also consider comprehensive economic factors. Sometimes the paint price is low but the service life is short but it is not economical, because the construction cost of re-corrosion may be much higher than the difference of the paint cost in some occasions. Generally speaking, when designing anti-corrosion coatings for bridges, iron towers, and large structures, it is necessary to ensure more than 10 to 20 years, and more than 5 years for general chemical equipment. It is difficult to achieve a long-term service life of general anti-corrosion coatings in severely corrosive environments, mainly because of insufficient technical reliability and economically uneconomical.

5.6 Key points of paint construction control

After the paint variety is determined, the quality of the coating is the key factor that determines the anti-corrosion effect and life of the coating. Among the various factors that affect the quality of the coating, the surface treatment of the substrate and the condition control during the painting construction process (including the ratio, thickness, pinhole, temperature, and humidity) have an impact on the quality of the coating accounted for 80%.

Therefore, to control the anti-corrosion quality and life of coatings, we must focus on the following key links:

5.6.1 Surface treatment before painting

The purpose of surface treatment is to ensure that the surface of the substrate achieves the necessary cleanliness, porosity and roughness.

To achieve the necessary cleanliness is to remove the rust, oxide scale, oil, dust, water, etc. on the surface of the substrate to ensure the basic adhesion between the coating and the substrate, and to prevent impurities from mixing into the formation of pores and pinholes. A certain degree of porosity and roughness can improve coating adhesion. However, excessive roughness will result in insufficient local thickness, prone to pinholes, and increased paint consumption.