Anti-corrosion methods in general can be divided into two categories: first, the correct selection of anti-corrosion materials and other anti-corrosion measures; The second is to choose a reasonable process operation and equipment structure. Strict compliance with the process procedures of chemical production can eliminate the corrosion phenomenon that should not occur, and even if the use of good corrosion resistant materials, no corrosion procedures in the operation process will cause serious corrosion.

At present, the commonly used corrosion prevention methods in chemical production are as follows:

1, correct material selection and design

It is the most effective way to understand the corrosion resistance of different materials and select the anti-corrosion materials correctly and reasonably. As we all know, there are many varieties of materials, different materials in different environments in the corrosion rate is also different, material selection personnel should be for a specific environment to choose low corrosion rate, cheaper, physical and mechanical properties to meet the design requirements of the material, so that the equipment to obtain economic and reasonable service life.

2, adjust the environment

If the various factors causing corrosion in the environment can be eliminated, the corrosion will be stopped or slowed down, but most of the environment is uncontrollable, such as the water in the atmosphere and soil, the oxygen in seawater, etc., can not be removed, and the chemical production process can not be changed at will. But some local environment can be adjusted, such as boiler water first remove oxygen (adding deoxidizer sodium sulfite and hydrazine, etc.), can protect the boiler from corrosion; If the water is removed before the air enters the closed warehouse, the stored metal parts can also be prevented from rusting.

In order to prevent the cooling water from causing scaling and perforation of heat exchangers and other equipment, alkali or acid can be added to the water to adjust the PH value to the best range (near neutral); In refining process, alkali or ammonia is often added to keep the production fluid neutral or alkaline. When the temperature is too high, it can be cooled in the wall, or lined with firebrick heat insulation in the inner wall of the equipment, etc. These are the methods used under the premise of changing the environment and not affecting the product and process, under the premise of allowing, it is recommended to use a moderate medium instead of a strong corrosive medium in the process.

3, add corrosion inhibitor

Usually, adding a small amount of corrosion inhibitor in the corrosive environment can greatly slow down the corrosion of metals, we generally divide it into inorganic, organic and gas phase corrosion inhibitors three categories, the corrosion inhibition mechanism is also different.

3.1 Inorganic corrosion inhibitor
Some corrosion inhibitors will slow down the anode process, called anodic corrosion inhibitors, which include oxidants that promote anode passivation (chromate, nitrite, iron ions, etc.) or anodic film forming agents (alkali, phosphate, silicate, benzoate, etc.), which mainly react in the anode region to promote anode polarization. General anode corrosion inhibitor will generate a protective film on the anode surface, in this case the corrosion inhibition effect is better, but there is also a certain risk, because if the dose is insufficient, it will cause the protective film is incomplete, the bare metal area exposed to the film defect is small, the anode current density is large, and it is more likely to perforate.
Another type of corrosion inhibitor is the reaction at the cathode, such as calcium ions, zinc ions, magnesium ions, copper ions, manganese ions and the cathode produce hydroxide ions, forming insoluble hydroxide, which covers the surface of the cathode in a thick film, thus blocking oxygen diffusion to the cathode and increasing concentration polarization. In addition, there are also mixed corrosion inhibitors that block the anode and cathode at the same time, but the amount of addition generally needs to be determined by testing.

3.2 Organic corrosion inhibitor
Organic corrosion inhibitors are adsorptive, adsorbed on the metal surface to form an invisible film several molecules thick, which can block the anode and cathode reactions at the same time, but the influence on the two is slightly different. Organic compounds containing nitrogen, sulfur, oxygen and phosphorus are commonly used as inorganic corrosion inhibitors, and their adsorption types can be divided into electrostatic adsorption, chemical adsorption and π bond (unlocated electron) adsorption depending on the molecular configuration of organic matter. The development of organic corrosion inhibitors is very fast, and the use of it is very wide, but the use of it will also produce some shortcomings, such as contaminated products, especially food, corrosion inhibitors may be beneficial to this part of the production process, but into another part of the harmful substances, it may also inhibit the required reaction, such as pickling so that the film removal speed is too slow, etc.

3.3 Vapor phase corrosion inhibitor
This type of corrosion inhibitor is a highly volatile substance, containing corrosion inhibition groups, generally used to protect metal parts in storage and transportation, and is mostly used in solid form. Its steam is decomposed by atmospheric water into effective corrosion inhibition groups, which are adsorbed on the metal surface to achieve the purpose of slowing down corrosion. In addition, it is also an adsorbent corrosion inhibitor, and the protected metal surface does not require rust removal treatment.

4, cathodic protection

Cathodic protection is a method of reducing or eliminating metal corrosion by relying on external direct current or sacrificial anode to make the protected metal become a cathode. Because before the application of cathodic protection, there are cathode and anode regions on most corroded metal structures, if all the anode regions can be turned into cathode regions, and the entire metal component becomes cathode, the purpose of eliminating corrosion can be achieved. For a specific project, there are many issues that should be considered before choosing a cathodic protection system:

4.1 Total protection current required
For cathodic protection, the total current required must be known.
This can be used to determine the current requirement by a temporary test device. If the required protection current is not large (less than 1.5~2A), it is best to choose sacrificial anode protection, if the required protection current is large, the use of impressed current protection is more economical.

4.2 Environment Changes
In soils with poor permeability, metals are relatively easy to polarize, and in soils with oxygen easily reaching the surface of the structure, a larger current is required for the structure to polarize. In addition, the area with the lowest soil resistivity is the one most suitable for installing a four-tone anode or an impressed current system. The movement of water has a significant effect, if the water is still, the protection current should be smaller. In contrast, turbulent water can scour the surface of the structure and therefore requires an extremely strong mechanical depolarization.

4.3 Shielding of electricity
Electrical shielding is easy to occur to the components with small spacing, complex structure and cathodic protection. The current from the distant cathodic protection power supply is easily absorbed by the outer component, and only a small amount of current can reach the inner component, so the outer component forms an electrical shield. At this time, the number and configuration of the cathode should be as far as possible to be roughly equal to the distance between the various parts of the protected structure, so that the dispersion of the current is more uniform.

4.4 Economic Factors
When using cathodic protection, consider whether it is economically viable. If cathodic protection is an economical solution to corrosion problems, then the cathodic protection system chosen should be the lowest cost, taking into account design and installation costs, power supply costs, and system maintenance costs.

4.5 Protection Life
When designing, you should know the expected service life of the protected structure. In the actual application of cathodic protection, the design life of the cathodic protection system should be the same as the life of the protected structure, the life is too low, the protection effect is not good, too high will increase the cost and cause waste.

4.6 Influence of stray current
Before designing a cathodic protection system, it is necessary to know if there is stray current in the area. It is mainly from electrified railway, mining machinery, electric welding and other DC power supplies. Stray currents cause rapid corrosion of the protected structure, usually more severe than corrosion caused by other environmental factors. Therefore, when designing cathodic protection, the location of the anode system should be well selected to avoid stray current as much as possible.

4.7 Temperature
Temperature will change the resistance of the medium, because the resistance of soil and water usually decreases with the increase of temperature. The resistance of tropical water is much lower than that of the same water in cold regions.

4.8 Sacrificial anode material
Suitable materials for sacrificial anodes are aluminum, magnesium, and zinc. Anode materials can be cast into a variety of different shapes of sacrificial anodes to meet the needs of cathodic protection design.

4.9 Impressed current anode
Anodes for impressed current cathodic protection systems should preferably have a realistic lowest corrosion rate at the output current. Scrap steel tubes, rods and similar scrap materials can be used as anodes for impressed current protection systems, although more consumption, but from a wide range of sources. In short, cathodic protection is more suitable for less corrosive media, such as seawater, soil, neutral salt solution, etc., in highly corrosive media, due to the consumption of electric energy and screen materials is too large, generally not used.

5, anode protection

Taking the device as the anode, passing current from the outside will generally accelerate the corrosion, and the corrosion current increases with the anode polarization. However, for metals that can be passivated, another situation will occur, when the potential rises with the current, after reaching the passivation potential, the corrosion current drops rapidly, and can even drop tens of thousands of times, and then rises with the potential, the current does not change until the dull area. Using this principle, the equipment that needs to be protected is taken as the anode, and the current is introduced to keep the potential in the middle part of the passivation zone, and the corrosion rate can be maintained at a very low value, and the current through indicates the corrosion rate of the equipment.

6, alloying

After adding the alloy component that can promote passivation to the base metal, the material with excellent corrosion resistance can be obtained when the amount of addition reaches a certain proportion. If more than 12% chromium is added to iron, it is called stainless steel; The addition of nickel to chromium steel can expand the passivation range and improve the mechanical properties. If 14% silicon is added to iron, high silicon iron with excellent acid resistance is obtained, and so on.

In addition, adding trace cathode precious metals with ultra-low voltage to some active metals can promote passivation. For example, stainless steel and titanium are active in sulfuric acid at certain concentrations and temperatures, such as adding 0.1-0.15% palladium or platinum to the base metal, which will be distributed on the surface of the alloy into a large number of microcathodes, promoting the operation of the local corrosion battery, the cathode current quickly increases, quickly reaches the passivation zone, and the metal corrosion resistance is enhanced.

7, surface treatment

The metal is treated with passivating agent or film forming agent before contact with the environment, and a stable and dense passivating film is generated on the surface, and the corrosion resistance is greatly increased. The difference between it and the corrosion inhibitor method is that it does not need to add corrosion inhibitors in the future use environment, aluminum is anodized, and the surface can generate a more dense film than that generated in the atmosphere, such a film has excellent corrosion resistance in a mild corrosion environment, and the blue surface of steel parts is also the principle.

8, Metal coating and coating layer

The steel substrate can be protected by a thin layer of more corrosion-resistant metal. The commonly used method is electroplating, generally plated 2 to 3 layers, only tens of microns thick, so there are inevitably microholes, the solution can penetrate the microholes, will constitute the plating - bottom corrosion battery. If the coating is a precious metal, its potential is higher than that of iron, and it will become the cathode, which will accelerate the corrosion of the underlying iron.

Therefore, this kind of coating is not suitable for strong corrosion environment, but can be used in atmosphere, water and other environments, slowly produced corrosion products can plug the microhole, increase the resistance, and obtain a certain service life. If cheap metal is used, the polarity of the corrosion battery is the opposite of the above, so that the steel is cathodic protection and can maintain a longer life. In addition to electroplating, it is also commonly used in hot dip plating (melt dip plating), flame spray plating, steam plating and integral metal sheet coating. The latter has no micropores, strong corrosion resistance, longer life, but the price is slightly higher.

9, Coating

The use of organic coatings to protect metal structures in the atmosphere is the most extensive means of anti-corrosion. The coating is covered on the surface of the metal and forms a porous film after drying, although the metal can not be completely isolated from the medium, but the value of the diffusion resistance and solution resistance through the micropores, so that the corrosion current is reduced.

In a mild environment, such as atmosphere, seawater, etc., the metal corrosion at the bottom of the microhole is slow, and the corrosion products can plug the microhole and have a long service life, but it is not suitable for strong corrosion solutions, because the metal corrosion rate is faster, and accompanied by the production of hydrogen, which will cause the paint film to break.

10, rubber sheet lining

Anti-corrosion lining is an unvulcanized, pre-vulcanized or vulcanized rubber sheet or sheet to prevent corrosion of equipment. The technique of lining a metal or other material work surface with a rubber sheet to form a continuous insulating coating is called the lining technique. Lining rubber can be divided into soft rubber, hard rubber and semi-hard rubber, generally according to the conditions of use with NR, CR and SBR manufacturing. The manufacture of the lining includes the surface treatment of the metal substrate, the processing of the lining rubber sheet, cutting, laminating and vulcanization. As a chemical anti-corrosion, anti-mechanical wear material, the lining rubber plate is widely used as the lining of chemical equipment and the lining of mining, metallurgical mud pump, flotation machine, mill, cement mill and other equipment used in building materials industry.