Anti-corrosion methods can be generally divided into two categories: one is to correctly select anti-corrosion materials and other anti-corrosion measures; the other is to choose a reasonable process operation and equipment structure. Strictly comply with the chemical production process rules, can eliminate the corrosion phenomenon that should not occur, and even if the use of good corrosion-resistant materials, the process does not corrode the process, it will cause serious corrosion.
At present, the following anti-corrosion methods commonly used in chemical production are as follows.
01 Correct material selection and design
Understanding the corrosion resistance of different materials, the correct and reasonable choice of anti-corrosion materials is the most effective method. As we all know, there are many kinds of materials, and the corrosion speed of different materials in different environments is different. The material selection personnel should select materials with low corrosion rate, cheaper price, physical and mechanical properties, etc. to meet the design requirements for a specific environment, so that the equipment can be economically obtained. Reasonable service life.
02 Adjusting the environment
If the various factors causing corrosion in the environment can be eliminated, the corrosion will be terminated or slowed down, but most of the environment is uncontrollable, such as moisture in the atmosphere and soil, oxygen in the seawater, etc., and the chemical production process is also impossible. It is impossible to change it at will. However, some local environments can be adjusted. For example, the boiler first removes oxygen (adding deoxidizer sodium sulfite and helium) to protect the boiler from corrosion. If the air enters the closed warehouse, it can be drained before storage. The metal parts are rusty.
In order to prevent the cooling water from causing fouling and perforation of heat exchangers and other equipment, alkali or acid may be added to the water to adjust the pH to the optimum range (near neutrality); alkali or ammonia is often added to the refining process to keep the production fluid in the middle Sexual or alkaline. When the temperature is too high, it can be cooled or cooled on the wall of the vessel, or the lining of the refractory brick can be insulated on the inner wall of the equipment. These are methods used to change the environment without affecting the products and processes. Where permitted, it is recommended to use a moderate medium instead of a strong corrosive medium in the process.
03 Add corrosion inhibitor
Generally, the addition of a small amount of corrosion inhibitor to the corrosive environment can greatly reduce the corrosion of the metal. We generally classify it into inorganic, organic and vapor phase corrosion inhibitors, and the corrosion inhibition mechanism is also different.
1. Inorganic corrosion inhibitor
Some corrosion inhibitors slow down the anode process and are called anode-type corrosion inhibitors, which include oxidants (chromate, nitrite, iron ions, etc.) that promote anode passivation or anode film formers (alkali, phosphoric acid). Salts, silicates, benzoates, etc.), which react primarily in the anode region to promote anodic polarization. Generally, the anodic corrosion inhibitor will form a protective film on the surface of the anode. In this case, the corrosion inhibition effect is better, but there is also a certain risk, because if the dosage is insufficient, the protective film is incomplete and the bare metal exposed at the film defect is exposed. The area is small, the anode current density is large, and perforation is more likely to occur.
Another type of corrosion inhibitor is a cathode reaction, such as calcium ions, zinc ions, magnesium ions, copper ions, manganese ions, etc., which generate hydroxide ions with the cathode to form insoluble hydroxides, which cover the cathode surface in a thick film form. , thus blocking the diffusion of oxygen to the cathode, increasing the concentration polarization. In addition, there are mixed corrosion inhibitors that block both the anode and the cathode, but the amount of addition generally needs to be determined by testing.
2, organic corrosion inhibitor
The organic corrosion inhibitor is adsorbed and adsorbed on the metal surface to form an invisible film with a thickness of several molecules, which can block the anode and cathode reactions at the same time, but the influence on the two is slightly different. Commonly used inorganic corrosion inhibitors are organic compounds containing nitrogen, sulfur, oxygen and phosphorus. The adsorption type can be divided into electrostatic adsorption, chemical adsorption and π bond (non-positional electron) adsorption depending on the molecular configuration of organic compounds. Organic corrosion inhibitors are developing rapidly and are widely used, but they also have some disadvantages, such as contaminated products, especially foods. Corrosion inhibitors may be beneficial to this part of the production process, but entering another part will change. As a harmful substance, it may also suppress the required reaction, such as slowing the rate of film removal during pickling, and so on.
3, gas phase corrosion inhibitor
Such corrosion inhibitors are highly volatile materials containing corrosion-inhibiting groups, which are generally used to protect metal parts during storage and transportation, and are mostly used in solid form. Its steam is decomposed by the water in the atmosphere to form an effective corrosion-inhibiting group, which is adsorbed on the metal surface to reduce the corrosion. In addition, it is also an adsorptive corrosion inhibitor that does not require rust removal on the protected metal surface.
04 Cathodic protection
Cathodic protection relies on the application of a direct current or a sacrificial anode to cause the protected metal to become a cathode, thereby reducing or eliminating corrosion of the metal. Because before the application of cathodic protection, most of the corrosion-producing metal structures have cathode and anode regions. If all the anode regions can be turned into cathode regions, the entire metal member becomes a cathode, thus eliminating corrosion. the goal of. For a specific project, there are many issues to consider before choosing a cathodic protection system:
1. The total protection current required for cathodic protection must know the total current required.
This can be done with a temporary test device to determine the current requirement. If the required protection current is not large (less than 1.5~2A), it is better to use sacrificial anode protection. If the required protection current is large, it is economical to use impressed current protection.
2. Changes in the environment
In soils with poor gas permeability, the metal is relatively easy to polarize. If the oxygen easily reaches the surface of the structure, the structure needs to be polarized to require a large current. In addition, the lowest soil resistivity is the most suitable anode for a four-anode or impressed current system. The movement of water has a significant effect. If the water is stationary, the protection current can take a small value. In contrast, turbulent water washes the surface of the structure and therefore requires a strong mechanical depolarization.
3, electric shielding
For components with small spacing, complex structure, and cathodic protection, electrical shielding is easy to occur. The current from the remote cathodic protection power supply is easily absorbed by the outer member, and only a small amount of current can reach the inner member, so that the outer member forms an electrical shield. At this time, the number and arrangement of the cathodes should be as close as possible to the respective portions of the protected structure, so that the dispersion of the current is more uniform.
4. Economic factors
When using cathodic protection, consider whether cathodic protection is economically cost-effective. If cathodic protection is an economical solution to corrosion problems, then the choice of cathodic protection system should be the lowest cost, and the design and installation costs, power costs, and system maintenance costs need to be considered.
5, protection life
When designing, you should know the expected life of the protected structure. Where the cathode protection is actually applied, the design life of the cathodic protection system should be the same as the life of the protected structure, the life is too low, and the protection effect is not good. Too high will increase the cost and cause waste.
6, the impact of stray current
Before designing a cathodic protection system, it is important to know if there is stray current in the area. It mainly comes from DC power sources such as electrified railways, mining machinery and electric welding. Stray currents cause rapid corrosion of the protected structure and are usually more severe than corrosion caused by other environmental factors. Therefore, when designing cathodic protection, the position of the anode system should be well chosen to avoid stray currents.
7, temperature
Temperature changes the resistance of the medium, as the resistance of soil and water generally decreases with increasing temperature. The principle that tropical seawater resistance is much lower than that of the same seawater in cold regions is this principle.
8, sacrificial anode material
Suitable materials for sacrificial anodes are aluminum, magnesium and zinc. The anode material can be cast into a variety of different shapes of sacrificial anodes to meet the needs of cathodic protection designs.
9, applied current anode
The anode used in the impressed current cathodic protection system preferably has a practically lowest corrosion rate at the output current. Scrap steel pipes, rods and similar scrap materials can be used as anodes for impressed current protection systems. Although they are more expensive, they are widely available. 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 large consumption of electrical energy and shielding materials, it is generally not used.
05 Anode protection
With the device as the anode, current is supplied from the outside, which generally accelerates the corrosion, and the corrosion current increases with the polarization of the anode. However, for a metal that can be passivated, another situation occurs. When the potential rises with the current and reaches a blunt potential, the corrosion current drops rapidly, and can even drop tens of thousands of times. Later, as the potential rises, the current does not change until it is blunt. Until the district. Using this principle, the device to be protected is used as the anode, current is introduced, the potential is maintained in the middle of the passivation region, the corrosion rate can be kept low, and the current flowing in indicates the corrosion rate of the device.
06 Alloying
An alloy component which promotes passivation is added to the base metal, and when the amount is added to a certain ratio, a material excellent in corrosion resistance is obtained. For example, adding more than 12% chromium to iron is called stainless steel; adding nickel to chrome steel can expand the passivation range and improve mechanical properties; and if 14% silicon is added to iron, high silicon with excellent acid resistance is obtained. Iron, and so on.
In addition, a small amount of cathode noble metal with a low overvoltage is added to some active metals to promote passivation. For example, stainless steel and titanium are active in certain concentrations and temperatures of sulfuric acid. For example, adding 0.1~0.15% palladium or platinum to the base metal will distribute the surface of the alloy into numerous micro-cathodes, which will promote the local corrosion of the battery. The cathode current increases rapidly and quickly reaches the passivation zone, which enhances the corrosion resistance of the metal.
07 Surface treatment
The metal is treated with a passivating agent or a film-forming agent before being exposed to the environment, and a stable and compact passivation film is formed on the surface, and the corrosion resistance is greatly increased. It differs from the corrosion inhibitor method in that it does not require the addition of a corrosion inhibitor in later use environments. The aluminum is anodized and the surface can form a denser film than that produced in the atmosphere. It has excellent corrosion resistance in a mild corrosive environment, and the blue surface of steel parts is also the principle.
08 Metal plating and coating
A thin layer of more corrosion-resistant metal can be used on the steel bottom layer. The commonly used method is electroplating. Generally, it is plated with 2~3 layers, only a few tens of microns thick, so inevitably there are micropores, and the solution can penetrate into the micropores, which will constitute a plating-underlayer corroding battery. If the coating is a noble metal, its potential is higher than that of iron, which will become a cathode, which will accelerate the corrosion of the underlying iron.
Therefore, such coatings are not suitable for a highly corrosive environment, but can be used in the atmosphere, water and the like, and the slowly generated corrosion products can block the micropores, increase the electrical resistance, and obtain a certain service life. If a cheap metal is used, the polarity of the corroded battery is opposite to that described above, so that the steel is cathodically protected and can maintain a long life. In addition to electroplating, hot dip plating (melt immersion plating), flame spraying, steam plating, and integral metal sheet plating are also commonly used. The latter has no micropores, strong corrosion resistance and long life, but the price is slightly higher.
09 coating
The use of organic coatings to protect metal structures in the atmosphere is the most widely used means of corrosion protection. The coating covers the metal surface and forms a porous film after drying. Although the metal cannot be completely isolated from the medium, the diffusion resistance and the solution resistance of the micropores are increased to lower the corrosion current.
In a moderate environment, such as the atmosphere, seawater, etc., the microporous bottom metal is slowly corroded, the corrosion products can block the micropores, and have a long service life, but are not suitable for strong corrosive solutions because the metal corrosion rate is fast, accompanied by The generation of hydrogen will cause the paint film to rupture.
10 lining
The lining is generally a one-piece material suitable for use inside equipment that is in contact with strongly corrosive media. The storage tanks such as hydrochloric acid and dilute sulfuric acid are lined with rubber or plastic, and the steel tanks for storing nitric acid are lined with stainless steel sheets. Enamel is actually a glass lining, industrially known as enamel glass. It is highly resistant to acid and is widely used in food, medicine and other industries to ensure product quality, but it cannot burn too much equipment.
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