Generally speaking, anti-corrosion methods can be divided into two categories: one is to correctly select anti-corrosion materials and other anti-corrosion measures; the other is to select reasonable process operation and equipment structure. Strictly abiding by the process regulations of chemical production can eliminate the corrosion phenomenon that should not occur, and even if good corrosion-resistant materials are used, and the operation process does not corrode regulations, it will cause serious corrosion.
At present, the commonly used anti-corrosion methods in chemical products are as follows:
01 Correct material selection and design
It is the most effective method to understand the corrosion resistance of different materials and choose the anticorrosion material correctly and reasonably. As we all know, there are many varieties of materials, and different materials have different corrosion rates in different environments. The material selection personnel should choose materials with low corrosion rate, low price, physical and mechanical properties, etc. that meet the design requirements for a specific environment, so that the equipment can obtain economical , Reasonable service life.
02 Adjust the environment
If the various factors that cause corrosion in the environment can be eliminated, the corrosion will be terminated or slowed down, but most environments are uncontrollable, such as moisture in the atmosphere and soil, oxygen in seawater, etc. cannot be removed, and the chemical production process is also Impossible to change at will. However, some local environments can be adjusted, such as removing oxygen from the boiler inlet water (adding deoxidizers sodium sulfite and hydrazine, etc.) to protect the boiler from corrosion; for example, removing moisture before the air enters a closed warehouse, which can also avoid storage. rusted metal parts.
In order to prevent scaling and perforation of heat exchangers and other equipment caused by cooling water, alkali or acid can be added to the water to adjust the pH value to the optimal range (close to neutral); alkali or ammonia are often added in the refining process to keep the production fluid in a neutral state. Sexual or alkaline. When the temperature is too high, it can be cooled on the wall of the device, or the inner wall of the equipment can be lined with refractory bricks for heat insulation, etc. These are all methods used under the premise of changing the environment and not affecting the product and process. If permitted, it is recommended to use a moderate medium instead of a strong corrosive medium in the process.
03 Add corrosion inhibitor
Usually, adding a small amount of corrosion inhibitor in a corrosive environment can greatly slow down the corrosion of metals. We generally divide it into three categories: inorganic, organic and gas-phase corrosion inhibitors, and their corrosion inhibition mechanisms are also different.
3.1 Inorganic corrosion inhibitor
Some corrosion inhibitors will slow down the anodic process and are called anodic corrosion inhibitors, which include oxidants (chromates, nitrites, iron ions, etc.) or anodic film formers (alkali, phosphoric acid, etc.) that promote anode passivation salts, silicates, benzoates, etc.), which react mainly in the anodic region, promoting anodic polarization. Generally, anodic corrosion inhibitors will form a protective film on the surface of the anode. In this case, the corrosion inhibition effect is good, but there are also certain risks, because if the dose is insufficient, the protective film will be incomplete, and the exposed bare metal at the film defect will be caused. The smaller the area, the higher the anode current density, and the more likely to perforate.
Another type of corrosion inhibitor reacts at the cathode, such as calcium ions, zinc ions, magnesium ions, copper ions, manganese ions, etc., and the cathode generate hydroxide ions to form insoluble hydroxides, which cover the cathode surface in the form of a thick film. , thus blocking the diffusion of oxygen to the cathode and increasing the concentration polarization. In addition, there are also mixed corrosion inhibitors that block both the anode and the cathode at the same time, but the amount to be added generally needs to be determined through experiments.
3.2 Organic corrosion inhibitor
Organic corrosion inhibitors are adsorption-type, which adsorb on the metal surface to form an invisible film with a thickness of several molecules, which can block the anodic and cathodic reactions at the same time, but the influence on the two is slightly different. Commonly used inorganic corrosion inhibitors include organic compounds containing nitrogen, sulfur, oxygen and phosphorus. The adsorption type can be divided into electrostatic adsorption, chemical adsorption and π bond (unpositioned electron) adsorption according to the different molecular configurations of organic matter. Organic corrosion inhibitors are developing rapidly and have a wide range of uses, but their use will also have some disadvantages, such as contaminated products, especially food. As a harmful substance, it may also inhibit the desired reaction, such as the slow removal of the film during pickling, etc.
3.3 Gas phase corrosion inhibitor
This type of corrosion inhibitor is a highly volatile substance containing corrosion inhibitor groups. It is generally used to protect metal parts during storage and transportation and is mostly used in solid form. Its steam is decomposed by water in the atmosphere to form effective corrosion inhibitor 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.
04 Cathodic protection
Cathodic protection is a method of reducing or eliminating metal corrosion by relying on external direct current or sacrificial anodes to make the protected metal a cathode. Because before the application of cathodic protection, there are cathodic areas and anodic areas on most metal structures that cause corrosion. If all the anodic areas can be turned into cathodic areas, the entire metal component will become a cathodic area, which will eliminate corrosion. the goal of. For a specific project, there are many issues that should be considered before choosing a cathodic protection system:
4.1 Required total protection current
For cathodic protection, the total current required must be known.
This can be done with a temporary test rig to determine current requirements. 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 more economical to use impressed current protection.
4.2 Changes in the environment
In soils with poor air permeability, metals are relatively easy to polarize. In soils where oxygen can easily reach the surface of the structure, a large current is required to polarize the structure. In addition, the places with the lowest soil resistivity are the anodes that are most suitable for installing four acoustic anodes or an impressed current system. The movement of water has a significant effect. If the water is still, the protection current can take a smaller value. Conversely, turbulent water washes over the surface of the structure and therefore requires extremely strong mechanical depolarization.
4.3 Electrical 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 layer member, and only a small amount of current can reach the inner layer member, so the outer layer member forms an electrical shield. At this time, the number and configuration of cathodes should be approximately equal to each part of the protected structure to make the current distribution more uniform.
4.4 Economic factors
When using cathodic protection, consideration should be given to whether cathodic protection is economically justified. If cathodic protection is an economical solution to corrosion problems, the cathodic protection system chosen should be the lowest cost one, taking into account design and installation costs, power costs, and system maintenance costs.
4.5 Protection life
When designing, the expected lifespan of the protected structure should be known. Where the cathodic protection is actually applied, the design life of the cathodic protection system should be the same as the life of the protected structure. If the life is too low, the protection effect is not good. If it is too high, it will increase the cost and cause waste.
4.6 Influence of Stray Current
Before designing a cathodic protection system, it is necessary to know whether 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 protected structures, usually more severe than those caused by other environmental factors. Therefore, when designing cathodic protection, the location of the anode system should be well selected to avoid stray currents as much as possible.
Temperature changes the resistance of the medium, since the resistance of soil and water generally decreases with increasing temperature. The electrical resistance of tropical seawater is much lower than that of the same seawater in cold regions.
4.8 Sacrificial anode materials
Materials suitable for sacrificial anodes include aluminum, magnesium, and zinc. The anode material can be cast into sacrificial anodes of many different shapes to meet the needs of cathodic protection designs.
4.9 Impressed current anode
Anodes used in impressed current cathodic protection systems should preferably have a practical minimum 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 consume more, they come 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 strong corrosive media, due to the large consumption of electric energy and shielding materials, it is generally not used.
05 Anodic protection
Using the equipment as the anode, passing current from the outside will generally accelerate the corrosion, and the corrosion current will increase with the polarization of the anode. But for metals that can be passivated, another situation occurs. When the potential rises with the current and reaches the passivation potential, the corrosion current drops rapidly, even tens of thousands of times. area so far. Using this principle, the equipment to be protected is used as the anode, and the current is introduced to keep the potential in the middle of the passivation zone, and the corrosion rate can be kept at a very low value. The current passed in represents the corrosion rate of the equipment.
Alloy components that can promote passivation are added to the base metal, and when the added amount reaches a certain proportion, a material with excellent corrosion resistance is obtained. For example, adding more than 12% chromium to iron is called stainless steel; adding nickel to chromium steel can expand the passivation range and improve mechanical properties; and add 14% silicon to iron to obtain high-silicon with excellent acid resistance Iron, wait.
In addition, adding trace amounts of cathode noble metals with low overvoltage to some active metals can promote passivation. For example, stainless steel and titanium are active in sulfuric acid at certain concentrations and temperatures. For example, adding 0.1~0.15% palladium or platinum to the base metal will distribute into numerous micro-cathodes on the surface of the alloy, promoting the operation of localized corrosion cells. The cathodic 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 passivation agent or a film-forming agent before contacting the operating environment, and a stable and dense passivation film is formed on the surface, and the corrosion resistance is greatly increased. It is different from the corrosion inhibitor method in that it does not need to add corrosion inhibitors in the future use environment. After the aluminum is anodized, the surface can form a denser film than that generated in the atmosphere. It has excellent corrosion resistance in a mild corrosive environment, and this is also the principle of bluing on the surface of steel parts.
08 Metal plating and cladding
A thin layer of more corrosion-resistant metal can be used to protect the steel base layer. The commonly used method is electroplating, which is generally plated with 2~3 layers, only a few tens of microns thick, so there are inevitably micropores, and the solution can penetrate into the micropores, which will constitute a plating-bottom corrosion battery. If the coating is a precious metal, its potential is higher than that of iron, and it will become a cathode, which will accelerate the corrosion of the underlying iron.
Therefore, this type of coating is not suitable for strong corrosive environments but can be used in environments such as the atmosphere and water. The slowly generated corrosion products can block the micropores, increase the resistance, and obtain a certain service life. If an inexpensive metal is used, the polarity of the corrosion cell is opposite to the above, so that the steel can be cathodically protected and can maintain a long life. In addition to electroplating, hot dipping (fusion dipping), flame spraying, vapor plating and overall metal sheet cladding are also commonly used. The latter has no micropores, strong corrosion resistance and longer life, but the price is slightly higher.
Protecting metallic structures in the atmosphere with organic coatings is the most widespread means of corrosion protection. The coating covers the metal surface and forms a porous film after drying. Although it cannot completely isolate the metal from the medium, it increases the diffusion resistance and solution resistance of the micropores and reduces the corrosion current.
In a moderate environment, such as the atmosphere, seawater, etc., the metal at the bottom of the micropore corrodes slowly, and the corrosion products can block the micropore, which has a long service life, but it is not suitable for strong corrosive solutions, because the metal corrosion rate is fast, and accompanied by The generation of hydrogen can cause the paint film to crack.
10 Rubber sheet lining
Corrosion-resistant linings are unvulcanized, pre-vulcanized or vulcanized rubber sheets or sheets designed to prevent equipment corrosion. The technology of forming a continuous insulating covering layer with a lining rubber sheet on the working surface of metal or other materials is called rubber lining technology. Lining rubber can be divided into soft rubber, hard rubber and semi-rigid rubber, generally made of NR, CR and SBR according to the conditions of use. The manufacture of linings includes surface treatment of metal substrates, processing of lining rubber sheets, cutting, lamination and vulcanization. Lining rubber sheet is widely used as chemical anti-corrosion and anti-mechanical wear material as chemical equipment lining and mining, metallurgical mud pump, flotation machine, mill, cement mill for building materials industry and other equipment linings.
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