A comprehensive introduction to anti-corrosion equipment
In general, anti-corrosion methods can be divided into 2 categories: First, the correct choice of anti-corrosion materials and other anti-corrosion measures; the second is to choose a reasonable process operation and equipment structure. Strict compliance with chemical production process procedures, you can eliminate the phenomenon of corrosion should not happen, and even with good corrosion-resistant materials, the operating process does not corrode the rules, will cause serious corrosion. At present, chemical corrosion commonly used in the production of the following methods.
1. The correct material selection and design
Understanding the corrosion resistance of different materials, the correct and reasonable choice of anti-corrosion materials is the best way. As we all know, many varieties of materials, different materials in different environments, the corrosion rate is also different, material selection personnel should be selected for a particular environment, low corrosion rate, cheaper prices, physical and mechanical properties to meet the design requirements of the material in order to obtain economic equipment , A reasonable life.
2, adjust the environment
If the environment can eliminate the various factors that cause corrosion, corrosion will be terminated or slowed down, but most of the environment is beyond control, such as the atmosphere and soil moisture, oxygen in seawater can not be removed, and chemical production processes Impossible to change. But some local environment can be adjusted, such as boiler water to remove oxygen (adding deoxidizer sodium sulfite and hydrazine, etc.), to protect the boiler from corrosion; Another example is the air before entering the closed warehouse out of water, but also avoid storage Of the metal parts rust; in order to prevent the cooling water on the heat exchanger and other equipment cause scaling and perforation, can be added to the water alkali or acid to adjust the PH value to the best range (nearly neutral); refining process often add alkali or Ammonia maintains the production fluid neutral or alkaline. When the temperature is already too high, the cooling can be cooled in the wall, or lining the lining brick refractory insulation, and so on. These are the methods employed to change the environment without affecting the product and process. If possible, use mild media instead of highly corrosive media as recommended.
3, add corrosion inhibitor
Generally, adding another small amount of corrosion inhibitor in a corrosive environment can greatly reduce the corrosion of metals. We generally classify them into three types of inorganic, organic and vapor corrosion inhibitors, and their corrosion inhibition mechanisms are also different.
1, inorganic corrosion inhibitor
Some corrosion inhibitors slow down the anodic process, called anodic corrosion inhibitors, which include oxidizers (chromates, nitrites, ferric ions, etc.) that promote anodic passivation or anodic film-forming agents Salts, silicates, benzoates, etc.) that react mainly in the anode region to promote anodic polarization. Generally, anodic corrosion inhibitors will form a protective film on the anode surface. In this case, the corrosion inhibition effect is better, but there is a certain risk because if the dosage is not sufficient, the protective film will be incomplete and bare metal exposed at the film defect Small area, the anode current density, more prone to perforation. Another class of corrosion inhibitors in the cathode reaction, such as calcium ions, zinc ions, magnesium ions, copper ions, manganese ions with the cathode to produce hydroxide ions, the formation of insoluble hydroxide, thick film covering the cathode surface , Thus retarding the diffusion of oxygen to the cathode, increasing the concentration polarization. In addition, there are also mixed anode and cathode retardation inhibitor, but the amount generally needs to pass the test before they can be identified.
2, organic corrosion inhibitor
Organic Corrosion inhibitors are adsorbed, adsorbed on the metal surface, forming a few molecules thick non-visible film, which can retard the anode and cathode reactions, but the influence of the two slightly different. Common inorganic corrosion inhibitors are nitrogen, sulfur, oxygen and phosphorus-containing organic compounds, the adsorption type with the molecular structure of organic matter can be divided into electrostatic adsorption, chemical adsorption and π Bond (unidentified electrons) adsorption. Organic corrosion inhibitors have developed rapidly and are utilized in a wide range of applications. However, there are likewise some shortcomings associated with using them, such as contaminated products, especially foodstuffs. Corrosion inhibitors may be beneficial to this part of the production process but go to the other part for harmful substances. It may also be necessary to suppress the reaction, such as pickling to make the film too slow, and so on.
3, vapor corrosion inhibitor
Such inhibitors are highly volatile substances, containing corrosion inhibitors, are generally used to preserve the storage and transportation of metal parts, the majority of solid form applications. Its steam is removed by the water in the atmosphere to effectively inhibit corrosion groups, adsorbed on the metal surface, to achieve the purpose of reducing corrosion. In addition, it is also an adsorptive corrosion inhibitor, protected metal surface does not require rust treatment.
4, cathodic protection
Cathodic protection is based on the application of DC current or sacrificial anode. The protected metal becomes a cathode, thereby reducing or eliminating the corrosion of metals. Because before the cathodic protection is applied, most of the corrosion-producing metal structures have the cathode and anode regions. If all of the anode regions can be turned into cathode regions and the entire metal structure becomes a cathode, then corrosion will be eliminated the goal of. For a specific project, there are a number of issues that should be considered before choosing a cathodic protection system:
1, the total required protection current
Cathode protection, you must know the total current required. This can be used to determine the transient test device current requirements. If the required protection current is small (less than 1.5 ~ 2A), the best choice of sacrificial anode protection, if the required protection current is larger, the use of additional contemporary protection more economical.
2, changes in the environment
In poorly aerated soil, the metal is relatively easily polarized, and if the oxygen readily reaches the surface of the structure, the structure needs more current to be polarized. In addition, the lowest soil resistivity is the most suitable anode for four-anode or impressed current systems. Water movement has an important role, if the water is still, the protection of current should be smaller. In contrast, turbulent water can flush the surface of the structure, thus requiring very strong mechanical depolarization.
3, electrical shielding
On the lesser spacing, the structure is complex, and the cathodic protection of the components, it is easy to produce electrical shielding. The current from the cathodic protection power source at a distance 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 configuration of cathodes should be as close as possible to the distances of the various parts of the structure to provide protection, so that the current dispersion is more uniform.
4, economic factors
When using cathodic protection, consider whether cathodic protection is cost-effective. If cathodic protection is an economical solution to the problem of corrosion, the cathodic protection system of choice should be the lowest cost, taking into consideration design and installation costs, power costs, and system maintenance costs.
5, protect the life
When designing, you should know the expected life of the structure being covered. In the practical application of cathodic protection, the design life of the cathodic protection system should be the same as that of the protected structure. The service life is too small and the protective effect is not good. Too high will increase the cost and result in waste.
6, the impact of current spurs
Before designing a cathodic protection system, it is important to be aware if the area has stray currents. It mainly comes from the electrified railway, mining machinery, welding and other DC power supply. Stray currents cause the protected structure to quickly corrode, often more so than other environmental factors. Therefore, when designing cathodic protection, the location of the anode system should be well chosen to avoid stray currents.
7, temperature
Temperature will change the resistance of the medium, as the resistance of soil and water usually decreases with increasing temperature. This is the principle that tropical sea resistance is much lower than the resistance of the constant seawater in cold regions.
8, sacrificial anode material
Materials suitable for sacrificial anode are aluminum, magnesium, zinc. The anode material can be cast into many different shapes of sacrificial anodes to meet the need of the cathodic protection design.
9, plus the current anode
The anodes used for the cathodic protection system with an applied current preferably have a workable minimum rate of corrosion at the time of output current. Scrap pipes, rods, and similar scrap materials can be invoked as the anodes for the applied current protection system, albeit more expensive, but come from a wide range of sources. In short, cathodic protection is more suitable for less corrosive medium, such as seawater, soil, neutral salt solution, etc., in strong corrosive medium, due to consumption of electricity and shield material is too large, generally not used.
5, anode protection
Using the device as an anode, current is externally applied, which generally accelerates corrosion and the corrosion current increases with anodic polarization. But the passivation of the metal there will be another situation, when the potential increases with the current to reach the blunted potential, the corrosion current rapidly declining, or even down tens of thousands of times, with the potential rise, the current unchanged until the blunt District so far. Use this principle, the need to protect the device as the anode, the introduction of current, so that the potential to maintain the passivation zone in the middle, the corrosion rate can be kept low, the current access means that the corrosion rate of equipment.
6, alloying
the base metal can be added to promote the passivation of the alloy composition, when the amount reached a certain ratio, they get excellent corrosion resistance of the material. If the iron is added more than 12% of chromium, called stainless steel; chromium steel by adding nickel, can expand the passivation range, but also improve the mechanical properties; Another example is the addition of 14% of iron in iron, you get high acidity of high silicon Iron, and so on. In addition, some active metals by adding a small amount of ultra-low voltage cathode precious metal, can promote passivation. Such as stainless steel and titanium in some concentrations and temperature of sulfuric acid is active, such as the base metal plus 0.1 ~ 0.15% of palladium or platinum, the surface of the alloy will be distributed as a large number of micro-cathodes to promote local corrosion of the battery operation, Cathode current increases rapidly, quickly reach the passivation zone, so that corrosion resistance of the metal.
7, surface treatment
Metal contact with the use of the environment before using passivation agent or film-forming agent, the surface to generate a stable and dense passivation film, corrosion resistance greatly increased. It is different from the corrosion inhibitor method in that it does not need to be added with corrosion inhibitors in advanced use environments. After anodized aluminum, the surface can produce a denser film than in the atmosphere. Gentle corrosion environment has excellent corrosion resistance. Blue steel parts surface is also the principle.
8, metal coating and coating
Obtainable in a thin layer of steel on the bottom of the more corrosion-resistant metal protection. Commonly used method is electroplating, generally placed 2 to 3 layers, only a few tens of microns thick, and thus inevitably exist micropores, the solution can penetrate the micropores, will form a coating - the bottom of the corrosion cell. If the coating is a precious metal, its potential is higher than iron, will become a cathode, will accelerate the underlying iron corrosion. Therefore, this type of coating is not suitable for strong corrosive environment, but can be used in the atmosphere, water and other environments, slow corrosion products can plug the pores, the resistance increases, access to a definite life. With inexpensive metal, the polarity of the corrosive cell is reversed, giving the steel a cathodic protection and long life. In addition to electroplating, but also commonly used hot-dip plating (melt-immersed), flame spraying, steam plating and the whole sheet metal plating. The latter owed to no micropores, corrosion resistance, longer life expectancy, but the price is slightly higher.
9, coating
organic coatings to protect the metal structure of the atmosphere, is the most extensive means of preservation. The coating covers the metal surface and forms a porous film after drying. Although the metal cannot be completely isolated from the medium, the corrosion resistance is reduced by increasing the diffusion resistance and solution resistance through the pores. In mild environments, such as the atmosphere, seawater, etc., the micro porous bottom metal corrodes slowly and the corrosion products can clog the micropores, which has a long service life but is not suitable for strong corrosive solutions because the metal corrodes faster and with Hydrogen production, will make the film rupture.
10, lining
Lining is generally the entire piece of material, suitable for contact with the strong corrosive medium inside the device. Such as hydrochloric acid, dilute sulfuric acid tank will be lined with rubber or plastic, storage of nitric acid steel tank lining with stainless steel sheet and so on. Enamel is actually a kind of glass lining, which is called enamel glass in the industry. It is acid-resistant and widely used in food, medicine and other industries to ensure the quality of products, but it cannot be fired too much.
