Upward every day - 2010.01.18

The reference answers provided on this site are: (1) Effect of concentration There are roughly three situations in which the concentration of corrosion inhibitor affects the corrosion rate of metal.: (1) The corrosion inhibition efficiency increases with the increase of the corrosion inhibitor concentration. For example, in hydrochloric acid and sulfuric acid, the corrosion inhibition efficiency increases with the increase of rhodine dose. In fact, this is the case for almost many organic and inorganic corrosion inhibitors in acidic and neutral media with low concentration. However, in actual use, based on the principle of conservation, the actual dosage should be determined by comprehensive consideration of the protective effect and reduction of corrosion inhibitor consumption. There is a limit to the relationship between the corrosion inhibition efficiency and concentration of a corrosion inhibitor, that is, the corrosion inhibition effect is best at a certain concentration. If the concentration is too low or too high, the corrosion inhibition efficiency will be reduced. (2) When the dosage of corrosion inhibitor is insufficient, not only will it fail to inhibit corrosion, but it will accelerate metal corrosion or cause pitting corrosion. For example, if the amount of sodium nitrite added to salt water is insufficient, corrosion will accelerate. Practice has proved that if the dosage of sodium nitrite added to seawater is insufficient, the corrosion of carbon steel will accelerate and pitting corrosion will occur. Therefore, adding too little is dangerous. Also included in this category of corrosion inhibitors are most oxidants, such as chromate, dichromate, hydrogen peroxide, etc. For equipment protected by corrosion inhibitors for a long time, in order to form a good basic protection, first of all, the dosage of corrosion inhibitors is often 4-5 times higher than during normal operation. When using corrosion inhibitors to protect old equipment, the amount should be appropriately increased. At this time, the scale layer and iron oxide scale existing on the metal surface often consume a certain amount of corrosion inhibitors. Sometimes, using different types of corrosion inhibitors together can often achieve better corrosion inhibition effects at lower concentrations, which creates a synergistic effect. Such as the effect when chromate (anodic oxidant) and zinc salt (cathode corrosion inhibitor) are mixed. In addition, when zinc salts and polyphosphates, amines and iodides are mixed, synergistic effects are also produced. (2) The influence of temperature. The influence of temperature on the corrosion inhibition effect of corrosion inhibitors has the following three situations:: (1) The corrosion inhibition effect is very good in a lower temperature range. When the temperature rises, the corrosion inhibition effect decreases significantly. This is because when the temperature rises, the adsorption effect of the corrosion inhibitor is significantly reduced, thus accelerating metal corrosion. This is the case for most organic and inorganic corrosion inhibitors. This is the case, for example, with thiourea corrosion inhibitors in sulfuric acid. (2) It has little effect on the corrosion inhibition effect within a certain temperature range, but when it exceeds a certain temperature, the corrosion inhibition effect is significantly reduced. For example, the ability of sodium benzoate to inhibit corrosion of carbon steel in an aqueous solution of 20-80°C changes little, but in boiling water, sodium benzoate can already prevent corrosion of steel. This may be because the steam bubbles destroy the protective film of the complex formed by iron and sodium benzoate. For many corrosion inhibitors used in neutral aqueous solutions and water, their corrosion inhibition efficiency almost does not change with the increase of temperature. For precipitation film type corrosion inhibitors, they should generally be used below the boiling point of the medium to achieve better results. (3) As the temperature increases, the corrosion inhibition efficiency also increases. This may be because when the temperature rises, the corrosion inhibitor can rely on chemical adsorption to combine with the metal surface to form a thin film of reaction products, or when the temperature rises, the corrosion inhibitor tends to form a layer similar to a passivation film on the metal surface, thereby reducing the corrosion rate. Therefore, when the temperature of the medium is relatively high, this type of corrosion inhibitor has the most practical value and belongs to the usefulness of this type of corrosion inhibitor.: Dibenzyl sulfide, dibenzyl sulfoxide, iodide, etc. in sulfuric acid solution. (3) As the temperature increases, the corrosion inhibition efficiency also increases. This may be because when the temperature rises, the corrosion inhibitor can rely on chemical adsorption to combine with the metal surface to form a thin film of reaction products, or when the temperature rises, the corrosion inhibitor tends to form a layer similar to a passivation film on the metal surface, thereby reducing the corrosion rate. Therefore, when the temperature of the medium is relatively high, this type of corrosion inhibitor has the most practical value and belongs to the usefulness of this type of corrosion inhibitor.: Dibenzyl sulfide, dibenzyl sulfoxide, iodide, etc. in sulfuric acid solution. In addition, the effect of temperature on the efficiency of corrosion inhibitors is sometimes related to the hydrolysis factor of the corrosion inhibitor. For example, an increase in medium temperature will promote the hydrolysis of various sodium phosphates, so their corrosion inhibition efficiency generally decreases as the temperature increases. In addition, since the amount of oxygen dissolved in the medium temperature is significantly reduced, although the cathode reaction speed can be reduced to a certain extent, when the corrosion inhibitor used needs to participate in the formation of a passivation film by dissolved oxygen (such as corrosion inhibitors such as sodium benzoate), the corrosion inhibition efficiency will decrease when the temperature rises. (3) Effect of flow speed The flow state of corrosive medium has a considerable impact on the effectiveness of corrosion inhibitors. There are roughly two situations:: (1) When the flow rate increases, the corrosion inhibition efficiency decreases. Sometimes due to the increase in flow rate, corrosion will even be accelerated, turning the corrosion inhibitor into a corrosion initiator (such as triethanolamine and potassium iodide in hydrochloric acid). (2) When the flow rate is accelerated, the corrosion inhibition efficiency increases. When the corrosion inhibitor affects the protective effect due to poor diffusion, increasing the medium flow rate can make the corrosion inhibitor diffuse to the metal surface more easily and evenly, which helps to improve the corrosion inhibition efficiency. The impact of medium flow rate on corrosion inhibition efficiency will also show opposite changes at different concentrations. For example, sodium hexametaphosphate, zinc chloride (4: 1) When used as a corrosion inhibitor for circulating cooling water, when the concentration of the corrosion inhibitor is above 8×10-6, the corrosion inhibition efficiency increases with the increase of the medium flow rate. ； When it is below 8×10-6, it decreases as the medium flow rate increases.

Factors that affect the corrosion inhibition effect of corrosion inhibitors include: The surface state of the protected material, the compatibility of the material and the corrosion inhibitor used, the characteristics of the protective layer formed by them, ambient temperature, pressure, pH value, humidity, content and composition of the corrosive medium, etc.

“"Corrosion inhibitors" are additives that slow down metal corrosion. There are many factors that affect the corrosion inhibition effect. In addition to the composition and structure of the corrosion inhibitor, the properties of the medium, the type of metal and the surface state, it is also related to factors such as the concentration of the corrosion inhibitor, the operating temperature and the movement speed of the medium. The factors that affect the corrosion inhibition effect of corrosion inhibitors are complex and can be divided into four aspects: materials, environment, corrosion inhibitor addition concentration, and equipment structure and mechanical factors. ① The performance and surface state of metal materials. Most corrosion inhibitors are highly specific to the corrosion inhibition of metals. In the same corrosive medium, the same additive has different effects on different metal materials. For example, sulfate is corrosive to carbon steel in water, but has a corrosion inhibition effect on pitting corrosion and stress corrosion of stainless steel in water with chloride ions. The problem of using corrosion inhibitors in an environment containing two or more metals is more complicated. In this case, two or more corrosion inhibitors can be used in an appropriate proportion to protect all metals. ② Environmental factors 1) The composition of the medium is obviously an extremely important influencing factor. Corrosion inhibitors should be selected based on the material-environment combination. The properties of the corrosion inhibitor must be compatible with the medium, that is, not only can it be dispersed in the medium, but it should not react with the medium to neutralize chlorination and reduction, thereby causing the corrosion inhibitor to fail. For example, if a solution containing ethylene glycol is used as antifreeze in a car water tank, chromate should not be used as a corrosion inhibitor, because the two will undergo an oxidation-reduction reaction and become ineffective. 2) pH value of the medium Almost all corrosion inhibitors have a pH range for effective corrosion inhibition. In neutral media, strict control of its pH value is an important condition to ensure the long-lasting effectiveness of corrosion inhibitors. For example, sodium nitrite is ineffective when the pH is <5.5~6.0 ； Polyphosphate is used when pH=6.5~7.5 ； Chromate is less sensitive to pH value, but it is generally better to use it around pH=8.5. 3) Temperature When using corrosion inhibitors, great attention should be paid to the influence of temperature. The influence of temperature on corrosion inhibition can be divided into three situations. a The corrosion inhibition rate is high. When the temperature rises, the corrosion inhibition efficiency decreases significantly. This is because the adsorption effect of this type of corrosion inhibitor decreases with increasing temperature, and most corrosion inhibitors fall into this category. Common ones include thiourea in sulfuric acid, TIB-5 and Shen 1-D in hydrochloric acid. The corrosion inhibition efficiency does not change much within b. When the temperature exceeds a certain limit, the corrosion inhibition efficiency drops significantly. The c rate increases with increasing temperature. For some corrosion inhibitors, it may be that the increase in temperature is conducive to the formation of a film layer on the metal surface, or is conducive to the formation of a chemical adsorption film between the corrosion inhibitor and the metal surface. Potassium iodide in sulfuric acid. This is the case for corrosion inhibitors such as alkaloids and nitrogenous bases in sulfuric acid. In addition, as the temperature increases, the solubility of the gas in the solution decreases, causing hydrolysis of some corrosion inhibitors, which will also have an indirect impact on the corrosion inhibition efficiency. 4) Microorganisms When microorganisms exist in a corrosive environment, since the microorganisms will affect corrosion and corrosion inhibition from the following three aspects, they may cause the corrosion inhibitor to fail. a Microorganisms will participate in the corrosion process, causing the generation of a large amount of corrosion products and pitting corrosion. b The growth and accumulation of flocculent fungi will hinder the flow of the medium and prevent the corrosion inhibitor from being evenly dispersed on the metal surface. c Microorganisms will directly destroy the corrosion inhibitor, and the corrosion inhibitor may become a nutrient source for microorganisms. ③ The influence of corrosion inhibitor concentration: All corrosion inhibitors have a minimum concentration value. Only the corrosion inhibitor concentration greater than this minimum concentration value will have a certain corrosion inhibition efficiency. There are three different situations in which corrosion inhibitor concentration affects corrosion inhibition efficiency. 1) The corrosion inhibition efficiency increases as the concentration of the corrosion inhibitor increases. This is the case for many corrosion inhibitors in acidic and neutral solutions with low concentration. 2) When the corrosion inhibitor concentration reaches a certain value, the corrosion inhibition efficiency reaches a maximum value. 3) When the concentration of corrosion inhibitor is insufficient, uniform corrosion or pitting corrosion will be accelerated. Most oxide film type corrosion inhibitors, such as chromate, nitrite, and hydrogen peroxide, are dangerous when used in insufficient amounts. Therefore, oxide film type corrosion inhibitors are also called dangerous corrosion inhibitors. The following issues should also be noted in the control of corrosion inhibitor concentration: 1) For long-term protection equipment, the amount of corrosion inhibitor added for the first time is generally 4 to 5 times greater than the regular operating concentration to facilitate the establishment of a stable protective film. 2) The amount of corrosion inhibitor required to protect old equipment is larger than that of new equipment, because the surface rust and scale layers consume corrosion inhibitors. 3) By using different types of corrosion inhibitors together, it is possible to achieve higher corrosion inhibition efficiency with a lower corrosion inhibitor concentration. For example, the combination of zinc salt and chromate, amine and iodide, zinc salt and polyphosphoric acid corrosion inhibitor can achieve this synergistic effect. ④ Influence of equipment structure and mechanical factors 1) The existence of dead corners and gaps makes it difficult for the corrosion inhibitor to come into contact with all metal equipment, affecting the corrosion inhibition effect in local areas. 2) Under environmental conditions that cause stress corrosion, corrosion inhibitors that are effective for uniform corrosion may not necessarily be effective for stress corrosion. 3) The influence of the flow state of the medium on the corrosion inhibition efficiency is relatively complex. The corrosion inhibition efficiency of some corrosion inhibitors decreases as the flow rate increases; some increases as the flow rate increases. Some corrosion inhibitors have different effects on flow rate when their concentrations are different. Therefore, the evaluation data of corrosion inhibitors under static conditions cannot replace the data under flow conditions.

The dosage of corrosion inhibitors in different media as well as the temperature and movement speed of the medium can all affect the effectiveness of corrosion inhibitors. (1) Influence of corrosion inhibitor dosage. There are roughly three situations in which the amount of corrosion inhibitor affects metal corrosion.: a. The corrosion rate of metal decreases as the dosage of corrosion inhibitor increases. This is the case for most organic and inorganic corrosion inhibitors in acidic and neutral media with low concentration. In actual use, the protective effect should be combined with comprehensive benefits to reasonably determine the dosage of corrosion inhibitors. b. There is a limit to the relationship between the concentration of corrosion inhibitor and the metal corrosion rate. That is, the corrosion inhibition effect is best at a certain concentration. If the concentration is too low or too high, the corrosion inhibition efficiency will be reduced. Therefore, when using this type of corrosion inhibitor, care must be taken not to overdo it. c. Insufficient dosage of corrosion inhibitor will accelerate metal corrosion. Most oxidants such as chromate, dichromate, hydrogen peroxide and sodium silicate belong to this type of corrosion inhibitor. It is dangerous to add too little of this type of corrosion inhibitor and great care must be taken. Generally speaking, for facilities that need to be protected by corrosion inhibitors for a long time, in order to form a good basic protective film. The first corrosion inhibitor dosage is often 4-5 times higher than during normal operation. When using corrosion inhibitors to protect old equipment, a certain amount of additional corrosion inhibitors will be consumed due to the presence of scale and iron oxide on the metal surface, and the dosage should be increased appropriately. (2) Effect of temperature. a. As the temperature increases, the corrosion inhibition efficiency decreases significantly. This is because when the temperature rises, the adsorption effect of the corrosion inhibitor is significantly reduced and corrosion intensifies. This is the case for most organic and inorganic corrosion inhibitors. b. Within a certain range, the corrosion inhibitor does not change as the temperature increases. The corrosion inhibition efficiency of some inorganic corrosion inhibitors used in neutral aqueous solutions and water almost does not change as the temperature increases. For precipitation film type corrosion inhibitors, they should generally be used below the boiling point of the medium to achieve better effects. c. As the temperature increases, the corrosion inhibition efficiency also increases. This may be because when the temperature rises, the corrosion inhibitor can rely on chemical adsorption to combine with the metal surface to form a film of reaction products. Or when the temperature is high, the corrosion inhibitor can easily form a layer of passivation film on the metal surface, thereby reducing the corrosion rate. Therefore, this type of corrosion inhibitor has the most practical value when the medium temperature is high. (3) The effect of medium flow speed on corrosion inhibition. aThe flow rate increases and the corrosion inhibition rate decreases. In most cases, increasing the flow rate of the medium will result in a decrease in corrosion inhibition efficiency. Sometimes, due to the increase in flow rate, corrosion will even be accelerated, turning the corrosion inhibitor into a corrosion initiator. b. When the flow rate increases, the corrosion inhibition efficiency increases. When the corrosion inhibitor affects the protective effect due to poor diffusion, the corrosion inhibitor can diffuse to the metal surface more easily and evenly, which helps to improve the corrosion inhibition efficiency. c The impact of medium flow rate on corrosion inhibition efficiency will also show opposite changes at different concentrations.

Factors that affect the protective effect of corrosion inhibitors include: ①Vapor pressure (the greater the vapor pressure, the more volatile the corrosion inhibitor and the better the protection effect. However, if the vapor pressure is too high, the long-lasting protection will not be strong) ； ②Adsorption energy and how strongly the gas phase corrosion inhibitor binds to the metal surface ； ③The extent to which rust inhibitors block the electrochemical reaction of the corrosion process.

This post was last edited by lzylike on 2010-1-17 22:10 (1) The influence of corrosion inhibitor dosage. There are roughly three situations in which the amount of corrosion inhibitor affects metal corrosion.: (2) Effect of temperature. (3) The effect of medium flow speed on corrosion inhibition.

Factors such as temperature, pH, metal ions, silicate stabilizers, etc.

Factors affecting corrosion inhibition 1. Effect of concentration There are roughly three situations in which the concentration of corrosion inhibitor affects the corrosion rate of metal.: ①The corrosion inhibition efficiency increases with the increase of corrosion inhibitor concentration. In fact, this is the case for almost many organic and inorganic corrosion inhibitors in acidic and neutral media with low concentration. However, in actual use, starting from the principle of conservation, the protection effect and reduction of corrosion inhibitor consumption should be comprehensively considered to determine the actual amount of corrosion inhibitor. There is a limit to the relationship between corrosion inhibition efficiency and concentration. That is, the corrosion inhibition effect is best at a certain concentration. If the concentration is too low or too high, the corrosion inhibition efficiency will be reduced. This is the case for example with diethylene glycol sulfide in hydrochloric acid. When the concentration is greater than 150mg/L, corrosion is faster than without corrosion inhibitor and becomes a corrosion initiator. Therefore, attention must be paid to this problem and the corrosion inhibitor should not be excessive. ②When the dosage of corrosion inhibitor is insufficient, it will not only fail to inhibit corrosion, but will accelerate metal corrosion or cause pitting corrosion. If the dosage of sodium nitrite added to seawater is insufficient, the corrosion of carbon steel will accelerate and pitting corrosion will occur. Therefore, adding too little is dangerous. This type of corrosion inhibitor also includes most oxidants, such as chromate, heavy lead salt, hydrogen peroxide, etc. For equipment protected by corrosion inhibitors for a long time, in order to form a good basic protection, first of all, the dosage of corrosion inhibitors is often 4 to 5 times higher than during normal operation. When using corrosion inhibitors to protect old equipment, the dose should be increased appropriately. At this time, the scale layer and iron oxide scale existing on the metal surface often consume a certain amount of corrosion inhibitors. Sometimes, using different types of corrosion inhibitors together can often achieve better corrosion inhibition effects at lower concentrations, which creates a synergistic effect. 2. The influence of temperature. The influence of temperature on the corrosion inhibition effect of corrosion inhibitors has the following three situations:: ①The corrosion inhibition effect is very good in the lower temperature range. When the temperature rises, the corrosion inhibition effect decreases significantly. This is because when the temperature rises, the adsorption effect of the corrosion inhibitor is significantly reduced, thus accelerating metal corrosion. Most organic and inorganic corrosion inhibitors are used in this situation. ②It has little effect on the corrosion inhibition effect within a certain temperature range, but when it exceeds a certain temperature, the corrosion inhibition effect is significantly reduced. For many corrosion inhibitors used in neutral aqueous solutions and water, their corrosion inhibition efficiency almost does not change with the increase of temperature. For precipitation film type corrosion inhibitors, they should generally be used below the boiling point of the medium to achieve better results. ③As the temperature increases, the corrosion inhibition efficiency also increases. This may be because when the temperature rises, the corrosion inhibitor can rely on chemical adsorption to combine with the metal surface to form a thin film of reaction products, or when the temperature rises, the corrosion inhibitor tends to form a layer similar to a passivation film on the metal surface, thereby reducing the corrosion rate. Therefore, this type of corrosion inhibitor has the most practical value when the temperature of the medium is high. In addition, the effect of temperature on the efficiency of corrosion inhibitors is sometimes related to the hydrolysis factor of the corrosion inhibitor. For example, an increase in medium temperature will promote the hydrolysis of various sodium phosphates, so their corrosion inhibition efficiency generally decreases as the temperature increases. In addition, since the amount of oxygen dissolved in the medium temperature is significantly reduced, although the cathode reaction speed can be reduced to a certain extent, when the corrosion inhibitor used needs to participate in the formation of a passivation film by dissolved oxygen (such as corrosion inhibitors such as sodium benzoate), the corrosion inhibition efficiency will decrease when the temperature rises. 6.3. Effect of flow speed The flow state of corrosive medium has a considerable impact on the effectiveness of corrosion inhibitors. There are roughly three situations:: ①When the flow rate increases, the corrosion inhibition efficiency decreases. Sometimes due to the increase in flow rate, corrosion will even be accelerated, turning the corrosion inhibitor into a corrosion initiator (such as triethanolamine and potassium iodide in hydrochloric acid). ②When the flow rate is accelerated, the corrosion inhibition efficiency increases. When the corrosion inhibitor affects the protective effect due to poor diffusion, increasing the medium flow rate can make the corrosion inhibitor diffuse to the metal surface more easily and evenly, which helps to improve the corrosion inhibition efficiency. ③The impact of medium flow rate on corrosion inhibition efficiency will also show opposite changes at different concentrations.

　　 Chemical substances or compounds that can prevent or slow down the corrosion of materials when present in the environment (medium) in appropriate concentrations and forms, so corrosion inhibitors can also be called corrosion inhibitors. Its dosage is very small (0.1% ~ 1%), but the effect is significant. This method of protecting metal is called corrosion inhibitor protection. Corrosion inhibitors are used in neutral media (boiler water, circulating cooling water), acidic media (hydrochloric acid to remove scale, acid leaching solutions for rust removal of plated parts before electroplating) and gaseous media (vapor phase corrosion inhibitors). 　　There are many classification methods for corrosion inhibitors, and corrosion inhibitors can be classified from different perspectives. 　　(1) According to the chemical composition of the product, it can be divided into inorganic corrosion inhibitors, organic corrosion inhibitors and polymer corrosion inhibitors. 　　①Inorganic corrosion inhibitors Inorganic corrosion inhibitors mainly include chromate, nitrite, silicate, molybdate, tungstate, polyphosphate, zinc salt, etc. 　　②Organic corrosion inhibitors Organic corrosion inhibitors mainly include phosphonic acid (salt), phosphine carboxylic acid, mercaptobenzothiazole, benzotriazole, sulfonated lignin and other heterocyclic compounds containing nitrogen and oxygen compounds. 　　③Polymer corrosion inhibitors Polymer corrosion inhibitors only include polymer chemicals such as polyethylene, POCA, polyaspartic acid and other oligomers. 　　(2) According to the classification of the parts where corrosion inhibitors control electrochemical corrosion, they are divided into anodic corrosion inhibitors, cathodic corrosion inhibitors and mixed corrosion inhibitors. 　　① Anodic corrosion inhibitors Anodic corrosion inhibitors are mostly inorganic strong oxidants, such as chromate, molybdate, tungstate, vanadate, nitrite, borate, etc. Their function is to react with metal ions in the anode area of ​​the metal surface to generate an oxide or hydroxide oxide film to cover the anode to form a protective film. This inhibits metal dissolution into water. The anode reaction is controlled and the anode is passivated. Silicates can also be classified into this category, and they also achieve corrosion inhibition by inhibiting the anodic process of corrosion reactions. 　　Anodic corrosion inhibitors require a high concentration so that all anodes are passivated. Once the dosage is insufficient, pitting corrosion will occur in the unpassivated areas. 　　②Cathodic corrosion inhibitors are chemicals that inhibit electrochemical cathode reactions and are called cathodic corrosion inhibitors. 　　Zinc carbonates, phosphates and hydroxides, and calcium carbonates and phosphates are cathodic corrosion inhibitors. Cathodic corrosion inhibitors can react with water and the cathode area on the metal surface. The reaction products are deposited on the cathode to form a film. As the film thickens, the reaction of the cathode releasing electrons is blocked. In practical applications, since calcium ions, carbonate ions and hydroxide ions are naturally present in water, only soluble zinc salts or soluble phosphates are added to the water. 　　③Mixed corrosion inhibitors Some surface-active organic corrosion inhibitors containing nitrogen, sulfur or hydroxyl groups have two polar groups with opposite properties in their molecules, which can be adsorbed on clean metal surfaces to form a monomolecular film. They can form films on both the anode and the cathode. It prevents the diffusion of water and dissolved oxygen in water to the metal surface and plays a corrosion inhibitor role. Mercaptobenzothiazole, benzotriazole, cetylamine, etc. belong to this type of corrosion inhibitor. 　　(3) Classification based on the type of protective film generated. In addition to water treatment agents with neutralizing properties, the corrosion inhibition mechanism of most corrosion inhibitors for water treatment is to form a metal protective film on the metal surface that is in contact with water to isolate the metal and water to achieve the purpose of corrosion inhibition. According to the type of protective film formed by the corrosion inhibitor, corrosion inhibitors can be divided into oxide film type, deposition film type and adsorption film type corrosion inhibitors. 　　①Oxide film-type corrosion inhibitors chromate, nitrite, molybdate, tungstate, vanadate, orthophosphate, borate, etc. are all regarded as oxide film-type corrosion inhibitors. Chromate and nitrite are both strong oxidants and can react with metals without the help of dissolved oxygen in water to form a dense oxide film in the anode area of ​​the metal surface. The remaining ones, either because they have weak oxidizing ability or because they are not oxidants themselves, need the help of oxygen to form an oxide film on the metal surface. Since these oxide film-type corrosion inhibitors achieve corrosion inhibition by inhibiting the anodic process of corrosion reactions, these anodic corrosion inhibitors can react with metal ions at the anode to form oxides or oxychlorides. The deposition covers the anode to form a protective film. Taking chromate as an example, it reacts at the anode to form Cr(OH)3 and Fe(OH)3. After dehydration, it becomes a mixture of CrO3 and Fe2O3 (mainly γ-Fe2O3) to form a protective film on the anode. Therefore, they are sometimes called anodic corrosion inhibitors or dangerous corrosion inhibitors, because once they are insufficiently dosed (when inhibiting alone, the dose required to treat 1L of water is often as high as hundreds or even thousands of milligrams), they will cause pitting corrosion, making the originally less serious corrosion problem more serious. Chloride ions, high temperature and high water flow rate will destroy the oxide film. Therefore, during application, the concentration of the corrosion inhibitor must be appropriately changed according to the process conditions. Silicates can also be roughly classified into this category because they mainly achieve corrosion inhibition through the anodic process of inhibiting corrosion reactions. However, it does not form a film through an adsorption mechanism by interacting with the metallic iron itself, but rather by the corrosion products of silicon dioxide and iron. 　　②Precipitated film-type corrosion inhibitors: Carbonates, phosphates and hydroxides of zinc, and carbonates and phosphates of calcium are the most common precipitated film-type corrosion inhibitors. Because they are deposited into films by the reaction of zinc and calcium cations with carbonate, phosphate and hydroxide anions in water at the cathode area of ​​the metal surface, they are also called cathodic corrosion inhibitors. Cathodic corrosion inhibitors can react with relevant ions in water, and the reaction products are deposited on the cathode to form a film. ； Taking zinc salt as an example, it produces Zn(OH)2 precipitation at the cathode and acts as a protective film. The combined use of zinc salt and other corrosion inhibitors can have a synergistic effect. When orthophosphate is present, Zn3(PO4)2 or (Zn, Fe)3(PO4)2 will precipitate and adhere tightly to the metal surface, resulting in better corrosion inhibition effects. In practical applications, since calcium ions, carbonates and hydroxides exist naturally in water, it is generally only necessary to add soluble zinc salts (such as: zinc nitrate, zinc sulfate or zinc chloride to provide zinc ions) or soluble phosphates (e.g.: Sodium orthophosphate or polymerized sodium phosphate that can be hydrolyzed to sodium orthophosphate to provide phosphate), therefore, these soluble zinc salts and soluble phosphates are usually called deposited film corrosion inhibitors or cathodic corrosion inhibitors. In this way, soluble phosphates (including polymeric phosphates) are both oxide film-type corrosion inhibitors and deposited film-type corrosion inhibitors. In addition, some phosphorus-containing organic compounds, such as organophosphoric acid (salts), organophosphate esters and organophosphorus carboxylic acids, can also be included in this type of corrosion inhibitor, which is probably related to their eventual hydrolysis into orthophosphate. Since the precipitation-type corrosion inhibition film is not directly combined with the metal surface and is porous, it often does not adhere well to the metal surface, and the corrosion inhibition effect is not as good as that of the oxidation-type film. 　　③Adsorption film-type corrosion inhibitors Adsorption film-type corrosion inhibitors are mostly organic corrosion inhibitors. They have polar genes and can be adsorbed by the surface charge of metals, forming a monomolecular film in the entire anode and cathode areas, thereby preventing or slowing down the corresponding electrochemical reactions. For example, some nitrogen-, sulfur-, or hydroxyl-containing, surface-active organic compounds have two groups with opposite properties in their molecules. ； Hydrophilic and lipophilic groups. The molecules of these compounds are adsorbed on the metal surface with hydrophilic groups (for example, amino groups), forming a dense hydrophobic film to protect the metal surface from water corrosion. Amines called "membrane amines" such as tallow amine, cetylamine and octadecylamine are common adsorption film-type corrosion inhibitors in water treatment. Mercaptobenzothiazole, benzotriazole and * * Triazoles, these azoles, are ideal corrosion inhibitors for non-ferrous metals, especially copper. Although they interact with the copper metal itself to form a film, they are different from the typical oxide film-type corrosion inhibitors mentioned above. Instead of oxidizing, they form a complex with copper ions on the metal surface and form a film by chemical adsorption. When the metal surface is clean or active, this type of corrosion inhibitor can form an adsorption film with satisfactory corrosion inhibition effect. However, if there are corrosion products or scale deposits on the metal surface, it will be difficult to form a good corrosion inhibitor film. At this time, a small amount of surfactant can be added appropriately to help such corrosion inhibitors form a film. 　　Since the corrosion inhibition mechanism of corrosion inhibitors is film formation, quickly forming a dense and solid film on the metal surface is the key to successful corrosion inhibition. In order to be rapid, the concentration of the corrosion inhibitor in the water should be high enough. After the film is formed, it can then be reduced to a concentration that only repairs the damage to the film. ； In order to be dense, the metal surface should be very clean. For this reason, chemical cleaning of the metal surface to remove oil, dirt and scale before film formation is an essential step. 　　The above types of corrosion inhibitors, except neutralizing amines and membrane amines are mainly used for boiler condensate water treatment and silicates for drinking water treatment, other types are commonly used for cooling water treatment. In terms of corrosion inhibition effect on carbon steel, chromate, especially chromate combined with polyphosphate and zinc salt, is still the most ideal corrosion inhibitor for circulating cooling water treatment. The United States is still using it to a considerable extent. When used, the pH value of the water is generally controlled to be slightly acidic to inhibit scale-causing salts from scaling. However, chromate (hexavalent) is toxic. Although it has a killing effect on harmful microorganisms such as bacteria and algae in circulating cooling water, it causes pollution to the environment. Therefore, it has been gradually replaced by (poly)phosphates around the world. This marked the beginning of the era of alkaline treatment of circulating cooling water. The concept is to no longer control the pH of the water, but to let nature take its course. The scaling problem of scale-causing salts in water is solved by highly efficient scale inhibitors and dispersants such as organic phosphoric acid (salt) and polyacrylic acid (salt). However, phosphate is a nutrient source for microorganisms in water, and its discharge will cause eutrophication of water bodies. As a result, it will pollute the environment on the other hand. Therefore, where the use of chromates and (poly)phosphates is not allowed, several other types of corrosion inhibitors have found application opportunities. However, applications such as molybdate are costly ； Nitrite is not suitable as a corrosion inhibitor in open circulating cooling water systems unless there is a specific biocide to effectively control the microorganisms that can cause its decomposition and failure. ； The corrosion inhibition effect of silicate is poor (due to the long film formation time, sometimes it takes 2 to 3 weeks to form a relatively complete film on the metal surface), and once scale is produced, it is difficult to remove. ； Zinc in zinc salts, like chromium, is a heavy metal and also poses a threat to organisms in water bodies. Therefore, people have shown strong interest in the development and application of organic corrosion inhibitors with less phosphorus content, which has led to the launch of "all-organic formula" water treatment agents. However, so far, in the development and application of corrosion inhibitors, there has not been a breakthrough like the past transition from polyphosphate to chromate, or the conversion from chromate to polyphosphate. When using "all-organic formula" corrosion inhibitors, the corrosion conditions of water cannot be too harsh, otherwise, inorganic corrosion inhibitors must be used to remedy them. 1# mopeizhi

1 Composition and structure of corrosion inhibitor 2 Properties of medium 3 Type and surface state of metal 4 Concentration of corrosion inhibitor 5 Operating temperature 6 Movement speed of medium