There are many classification methods for corrosion inhibitors, which can be classified from different angles.
a chemical substance or composite substance that exists in a medium in proper concentration and form and can prevent or slow down metal corrosion.
also known as corrosion inhibitor or corrosion inhibitor.
its dosage is very small (.1% ~ 1%), but the effect is remarkable.
this method of protecting metals is called corrosion inhibitor protection.
corrosion inhibitors are used in neutral media (boiler water and circulating cooling water), acidic media (hydrochloric acid for removing boiler scale, acid leaching solution for rust removal of plated parts before electroplating) and gas media (gas phase corrosion inhibitors).
The protective effect of corrosion inhibitor is usually expressed by inhibition efficiency or inhibition efficiency I:
where V is the corrosion rate of metal without corrosion inhibitor; V is the corrosion rate of metal after adding corrosion inhibitor.
the greater the corrosion inhibition efficiency, the better the corrosion inhibition effect.
sometimes several different kinds of corrosion inhibitors with lower dosage can be used together to obtain better corrosion inhibition effect, which is called synergistic effect; On the contrary, if different types of corrosion inhibitors * * * are used together, they will reduce their respective corrosion inhibition efficiency, which is called antagonistic effect.
corrosion inhibitors can be classified according to the mechanism of action or the characteristics of protective film.
(1) According to the chemical composition of the product, it can be divided into inorganic corrosion inhibitors, organic corrosion inhibitors and polymer corrosion inhibitors.
① inorganic corrosion inhibitor
inorganic corrosion inhibitors mainly include chromate, sulfite, silicate, molybdate, tungstate, polyphosphate and zinc salt.
② Organic corrosion inhibitors
Organic corrosion inhibitors mainly include heterocyclic compounds containing nitrogen and oxygen compounds such as phosphonic acid (salt), phosphocarboxylic acid, sulfobenzothiazole, benzotriazole and sulfonated lignin.
③ polymeric corrosion inhibitors
polymeric corrosion inhibitors should only include some polymer chemicals such as polyethylene, POCA and polyaspartic acid.
(2) According to the control positions of corrosion inhibitors on electrochemical corrosion, they can be divided into anodic corrosion inhibitors, cathodic corrosion inhibitors and mixed corrosion inhibitors.
①
Anodic corrosion inhibitors
Anodic corrosion inhibitors are mostly strong inorganic oxidants, such as chromate, molybdate, tungstate, vanadate, sulfite and borate.
their function is to react with metal ions in the anode area of the metal surface to generate oxide or hydroxide oxide film covering the anode to form a protective film.
this prevents the metal from dissolving into water.
the anode reaction is controlled and the anode is passivated.
silicate can also be classified into this category, and it also achieves the purpose of corrosion inhibition by inhibiting the anodic process of corrosion reaction.
Anode corrosion inhibitor requires a high concentration, so that all anodes are passivated. Once the dosage is insufficient, it will cause pitting corrosion in the unpassivated parts.
② Cathodic corrosion inhibitor
A chemical agent that inhibits electrochemical cathodic reaction is called cathodic corrosion inhibitor.
zinc carbonate, phosphate and hydroxide, calcium carbonate and phosphate are cathodic corrosion inhibitors.
Cathode-type corrosion inhibitor can react with water and the cathode region on the metal surface, and its reaction products are deposited on the cathode to form a film. With the thickening of the film, the reaction of releasing electrons from the cathode is blocked.
In practical application, since calcium ions, carbonate ions and hydroxide ions are naturally present in water, it is only necessary to add soluble zinc salts or soluble phosphates to the water.
③ Mixed corrosion inhibitor
Some organic corrosion inhibitors containing nitrogen, sulfur or hydroxyl groups and having surface activity have two polar groups with opposite properties in their molecules, which can be adsorbed on clean metal surfaces to form a monolayer, and they can form films on both anodes and cathodes.
It prevents the diffusion of water and dissolved oxygen in water to the metal surface, and plays a role in corrosion inhibition. Thiobenzothiazole, benzotriazole, hexadecylamine, etc. belong to this kind of corrosion inhibitor.
(3) According to the type of protective film, except for water treatment agents with neutralization performance, the corrosion inhibition mechanism of most corrosion inhibitors used for water treatment is to form a metal protective film on the metal surface in contact with water, so as to achieve the purpose of corrosion inhibition.
according to the type of protective film formed by corrosion inhibitors, corrosion inhibitors can be divided into oxide film type, deposition film type and adsorption film type.
① oxide film corrosion inhibitor
chromate, sulfite, molybdate, tungstate, vanadate, orthophosphate and borate are all regarded as oxide film corrosion inhibitors.
chromate and sulfite are both strong oxidants, which can react with metals without the help of dissolved oxygen in water, forming a dense oxide film on the anode region of the metal surface.
the others, either because of their weak oxidation ability or because they are not oxidants, all need the help of oxygen to form oxide films on metal surfaces.
Because these oxide film corrosion inhibitors achieve corrosion inhibition by inhibiting the anodic process of corrosion reaction, these anodic corrosion inhibitors can react with metal ions at the anode to form oxides or oxychlorides.
deposition covers the anode to form a protective film. For example, chromate reacts at the anode to form Cr(OH)3 and Fe(OH)3, and after dehydration, it becomes a mixture of CrO3 and Fe2O3 (mainly γ-Fe2O3) to form a protective film at the anode.
Therefore, it is sometimes called anode type corrosion inhibitor or dangerous type corrosion inhibitor, because once their dosage is insufficient (when they are used alone to inhibit corrosion, the dosage required to treat 1L water is often as high as several hundred or even over a thousand milligrams), pitting corrosion will be caused, which will make the corrosion problem which was not too serious become more serious.
Chloride ion, high temperature and high water flow rate will destroy the oxide film, so the concentration of corrosion inhibitor should be appropriately changed according to the process conditions.
silicate can also be roughly classified into this category, because it mainly achieves corrosion inhibition by inhibiting the anodic process of corrosion reaction.
However, it is not through the interaction with metallic iron itself, but may be formed by the corrosion products of silicon dioxide and iron by adsorption mechanism.
② precipitated film corrosion inhibitor
Zinc carbonate, phosphate and hydroxide, calcium carbonate and phosphate are the most common precipitated film corrosion inhibitors.
They are also called cathodic corrosion inhibitors because they are formed by the reaction of zinc and calcium cations with carbonate, phosphate and hydroxide anions in water and in the cathode region of metal surface.
Cathode corrosion inhibitor can react with related 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, which acts as a protective film.
when zinc salt is used in combination with other corrosion inhibitors, it can play a synergistic role. In the presence of orthophosphate, Zn3(PO4)2 or (Zn,Fe)3(PO4)2 precipitates and adheres to the metal surface tightly, which has a better corrosion inhibition effect.
in practical application, because calcium ions, carbonate and hydroxide exist naturally in water, it is generally only necessary to add soluble zinc salts (such as zinc, zinc sulfate or zinc chloride, zinc ions) or soluble phosphates (such as sodium orthophosphate or sodium polyphosphate which can be hydrolyzed into sodium orthophosphate, phosphate radical) to water, so these soluble zinc salts and soluble phosphates are usually called deposition film corrosion inhibitors or cathode corrosion inhibitors.
In this way, soluble phosphate (including polyphosphate) is both an oxide film type corrosion inhibitor and a deposition film type corrosion inhibitor.
In addition, some phosphorus-containing organic compounds, such as organic phosphoric acid (salt), organic phosphate ester and organophosphorus carboxylic acid, can also be classified as this kind of corrosion inhibitor, which is probably related to its ultimate hydrolysis to orthophosphate.
because the precipitation type corrosion inhibition film is not directly combined with the metal surface, and it is porous, the phenomenon of poor adhesion to the metal surface often occurs, and the corrosion inhibition effect is not as good as that of the oxidation type film.
③ Adsorption film type corrosion inhibitors
Adsorption film type corrosion inhibitors are mostly organic corrosion inhibitors, which have polar genes and can be adsorbed by surface charges of metals to form a monolayer on the whole anode and cathode regions, thus preventing or slowing down the corresponding electrochemical reactions.
For example, some organic compounds with nitrogen, sulfur or hydroxyl groups and surface activity have two groups with opposite properties in their molecules; Hydrophilic group and lipophilic group.
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, hexadecylamine and octadecylamine, are common adsorption membrane corrosion inhibitors in water treatment.
mercaptobenzothiazole, benzotriazole and methylbenzotriazole are ideal corrosion inhibitors for nonferrous metals, especially copper.
although they react with copper itself to form a film, they are different from the above-mentioned typical oxide film corrosion inhibitors in that they form a film by chemical adsorption instead of oxidation.
when the metal surface is clean or active, this kind 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 is difficult to form a good corrosion inhibitor film. At this time, a small amount of surfactant can be added appropriately to help this kind of corrosion inhibitor form a film.
because the inhibition mechanism of corrosion inhibitor lies in film formation, it is the key to obtain successful corrosion inhibition to quickly form a dense and solid film on the metal surface.
In order to be quick, the concentration of corrosion inhibitor in water should be high enough, and after the film is formed, it will be reduced to the concentration that can only repair the damage of the film; In order to be compact, the metal surface should be very clean. Therefore, it is an essential step to chemically clean the metal surface to remove oil, dirt and scale before film formation.
Except neutralizing amine and membrane amine, which are mainly used for boiler condensate water treatment and silicate for drinking water treatment, other kinds of corrosion inhibitors are commonly used for cooling water treatment.
in terms of corrosion inhibition effect on carbon steel alone, chromate, especially chromate mixed with polyphosphate and zinc salt, is still the most ideal corrosion inhibitor for circulating cooling water treatment.
the United States still uses it to a considerable extent.
in application, the pH value of water is generally controlled to be slightly acidic, so as to inhibit the scaling of scaling salts.
However, chromate (hexavalent) is toxic. Although it can kill 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) phosphate in the world.
this marks the beginning of the era of alkaline treatment of circulating cooling water.
This concept is to let the pH value of water be natural instead of deliberately controlling it.
The scaling problem of scaling salts in water is solved by efficient scale inhibitors and dispersants such as organic phosphoric acid (salt) and polyacrylic acid (salt).
However, phosphate is the nutrient source of microorganisms in water, and its discharge will lead to eutrophication of water, and as a result, it will pollute the environment on the other hand.
Therefore, where chromate and (poly) phosphate are not allowed to be used, several other types of corrosion inhibitors have been applied.
However, the application cost of molybdate is high; Sulfite should not be used as a corrosion inhibitor for open circulating cooling water system unless biocides are effectively controlling microorganisms that can make it decompose and fail; Silicate has poor corrosion inhibition effect (sometimes it takes 2 ~ 3 weeks to form a complete film on the metal surface due to the long film forming time), and once the scale is produced, it is difficult to remove it; Zinc in zinc salt, like chromium, is also a heavy metal, which also poses a threat to organisms in water.
Therefore, people show great interest in the development and application of organic corrosion inhibitors with less phosphorus content, which leads to the listing of "all-organic" water treatment agents.
however, so far, there has been no breakthrough in the development and application of corrosion inhibitors, such as changing from polyphosphate to chromate, or changing from chromate to polyphosphate.
With "all-organic" corrosion inhibitor, the corrosion condition of water should not be too harsh, otherwise, it must be remedied with inorganic corrosion inhibitor.
(4) Common corrosion inhibitors ① Copper-silver corrosion inhibitor benzotriazole
BTA,
1,2,3bezotriazol. Copper-silver corrosion inhibitor BTA can be adsorbed on metal surface to form a thin film, which can protect copper and other metals from atmospheric and harmful media; BTA, a copper corrosion inhibitor, can be used together with various scale inhibitors and algaecides in circulating cooling water system, and has a good corrosion inhibition effect on circulating cooling water system, with the dosage of 2-4mg/L in circulating water.
BTA can also be used as an anti-tarnishing agent for copper and silver, automobile cooling liquid and lubricating oil agent.
copper corrosion inhibitor mercaptobenzothiazole
MBT②
CAS
No.149-3-3 alias: water-soluble mercaptobenzothiazole copper corrosion inhibitor MBT can be used as a copper corrosion inhibitor in circulating cooling water system.
The corrosion inhibition effect of copper corrosion inhibitor MBT mainly depends on a chemical adsorption with active copper atoms or copper ions on the surface of metal copper; Or chelation occurs to form a dense and firm protective film, so that copper equipment can be well protected, and the usage amount is generally 4 mg/L. MBT can also be used as plasticizer, acidic copper plating photometer and so on.
the copper corrosion inhibitor MBT is packed in plastic barrels, 25kg each, or determined according to user's requirements.
stored in a cool and dry place for six months.
③ copper corrosion inhibitor methyl benzotriazole
(TTA)
CAS
No.29385-43-1
molecular formula: C7H7N3
relative molecular weight: 133.16 copper corrosion inhibitor TTA can be used as a corrosion inhibitor for nonferrous metals copper and copper alloys.
TTA, a copper corrosion inhibitor, is adsorbed on the metal surface to form a thin film, which protects copper and other metals from the corrosion of harmful media in the atmosphere and water.
TTA, a copper corrosion inhibitor, is more uniform in film formation, and it is more effective when combined with mercaptobenzothiazole
(MBT).
the copper corrosion inhibitor TTA is dissolved in alcohol or alkali and then added to the circulating water. The concentration of this product in the water is 2-1 mg/L. If the non-ferrous metals in the water system have been seriously corroded, this product can be added at 5-1 times the normal concentration to make the system passivated quickly.
④ corrosion inhibitor for hydrochloric acid pickling
inhibitor
for
hydrochloric acid
cleaning corrosion inhibitor is a series of products and belongs to imidazoline.
When cleaning metal with hydrochloric acid, adding hydrochloric acid pickling inhibitor can inhibit the corrosion of hydrochloric acid to steel.
the premise of application of hydrochloric acid pickling inhibitor is that the cleaning medium is hydrochloric acid, sulfuric acid and sulfamic acid, and the substrate to be cleaned is ferrous metal.
hydrochloric acid pickling inhibitor is suitable for pickling of various types of high, medium and low pressure boilers, as well as pickling of large equipment and pipelines.
corrosion performance in acid solution (dosage is 1-3‰)
corrosion rate ≤ 1g/m2 _ h.
add the pickling corrosion inhibitor to the diluted acid solution in proportion, start the circulating pump for circular cleaning, and add the pickling corrosion inhibitor in proportion when adding acid solution during cleaning.
(5) articles on corrosion inhibitor pickling corrosion inhibitor technology
copper alloy pickling corrosion inhibitor technology
hydrochloric acid pickling corrosion inhibitor technology
closed system corrosion inhibitor technology
citric acid pickling corrosion inhibitor technology
carbon steel pickling corrosion inhibitor technology
stainless steel pickling corrosion inhibitor technology oilfield acidification corrosion inhibitor technology
phosphate imidazoline derivative technology.