The commonly used cleaning agents can be roughly divided into acidic, alkaline, and neutral systems. Acidic and alkaline cleaning agents are used more frequently in the market. Due to the presence of acid and alkaline substances in the system, the cleaning effect of cleaning agents tends to be more significant. Improved, but at the same time, acid-base substances can also cause metal corrosion, which adds incredible difficulty to the formulation design of the cleaning agent.
In cleaning agent formulation design, solving the corrosion problem caused by acid and alkaline substances is usually achieved by adding corrosion inhibitors. Still, there are many corrosion inhibitors and the corrosion inhibition performance of different corrosion inhibitors for other metals. There are significant differences, so the selection of corrosion inhibitors is a vital link in the design of cleaning agent formulations.
Definition of corrosion inhibitor
A corrosion inhibitor is a chemical substance or a mixture of several chemical substances that can prevent or slow down corrosion when it exists in the environment (medium) in an appropriate concentration and form. Generally speaking, corrosion inhibitors refer to those substances that are used to protect the surface of metals. Adding a small amount or a small amount of such chemical substances can significantly reduce the corrosion rate of metal materials in the medium.
Classification of corrosion inhibitors
Classification according to the mechanism of action
1. anodic corrosion inhibitor: oxidizing can make the metal surface passivation and inhibit metal corrosion, such as chromate, nitrate, phosphate, molybdate, and ketoxime, etc.
2 . Cathodic corrosion inhibitors: substances that can eliminate or reduce depolarizing agents or increase the polarizability of the cathodic process (i.e., increase the cathodic reaction overpotential). For example, hydrazine, hydrazine, sodium sulfite, etc., can remove dissolved oxygen; arsenic, antimony, bismuth, mercury salts can increase the hydrogen precipitation overpotential.
3. Hybrid corrosion inhibitor: can slow down the cathodic reaction simultaneously, caused mainly by adsorption at the cathode, sometimes also known as masking type corrosion inhibitor. Can be directly adsorbed or attached to the metal surface, or the secondary reaction to form an insoluble protective film to isolate the metal from the medium, such as di-cyclohexyl ammonium nitrite hydrolysis products can be adsorbed on the metal surface; containing nitrogen, phosphorus, sulfur, and oxygen and other organic substances with lone electron pairs of elements can be formed directly on the metal surface chemisorption layer; zinc sulfate and beryllium chloride in the cathode area to generate hydroxide deposits, also belongs to the masking type corrosion inhibitor.
Classification according to composition
1. Inorganic corrosion inhibitors: nitrates, nitrites, chromates, and dichromates, etc. (anodic type); sulfites, arsenic trioxide, antimony trichloride, etc. (cathodic type); polyphosphates, silicates, aluminates, and alkalis, etc. (mixed or masked type).
2. Organic corrosion inhibitors: heterocyclic compounds with nitrogen, phosphorus, sulfur, and oxygen, polymer alcohols, aldehydes, amines, and amides; sulfonic acids, fatty acids, and their derivatives; thiourea and its derivatives; thiazoles and thiourea azoles; quaternary amine salts; phosphides, thiols, alkyl sulfoxide, thiazines, and unsaturated chain and ring compounds, etc.
Classification according to the application environment
1. Corrosion inhibitors for acidic solutions: suitable for acidic media, such as urotropine, imidazoline, aniline, thiourea, and antimony trichloride.
2. Corrosion inhibitors for alkaline solutions: applicable to alkaline media, such as sodium nitrate, sodium sulfide, calcium superphosphate.
3. Corrosion inhibitors for neutral solutions: applicable to natural water and brine, such as sodium hexametaphosphate, zinc gluconate, zinc sulfate.
4. Vapor-phase corrosion inhibitor: applicable to warehouses and bags, such as cyclohexylamine carbonate, amyl amine benzoate.
Corrosion Inhibition Mechanisms
Electrochemical theory
When corrosion inhibitors cause anodic passivation, the corrosion of metals is strongly inhibited. Phosphate, benzoate, and other anodic inhibition type corrosion inhibitors, some corrosion inhibitors, such as nitrite and molybdate in acidic media, their corrosion inhibiting effect lies in promoting cathodic depolarization, increasing the cathodic exchange current density IR, thereby reducing the corrosion rate of passivated metals, known as cathodic depolarization type corrosion inhibitors.
Adsorption theory
Adsorption theory refers to the corrosion inhibitor itself or secondary products adsorbed on the metal surface to form a protective barrier, or eliminate the active zone, or change the structure of the double electric layer, etc., to achieve the purpose of corrosion inhibition. Adsorption can be divided into two categories: physical adsorption and chemical adsorption. Physical adsorption is by Coulomb gravitational force or Van der Waals force, long-range adsorption, and chemical adsorption by chemical bonding, which is proximity adsorption.
Film formation theory
It refers to the corrosion inhibitor and metal to generate passivation film or react with the ions in the medium to create a deposition layer and make the metal corrosion inhibition, divided into three kinds of oxide film, deposition film, and colloidal film.
1. Oxidation film
It is formed due to the oxidation of the corrosion inhibitor itself or the oxidation of dissolved oxygen.
2. Deposition film
It is generated by the corrosion inhibitor and the cathode reaction product insoluble hydroxide, or with the anode reaction product insoluble film.
3. Colloidal film
By the corrosion inhibitor itself discharge and generate insoluble material cover layers, such as polyphosphate ((Na5CaP6O18)n+) and silicate (SiO32-) can cause large colloidal cations with calcium in the water, by the cathodic reaction and the formation of insoluble colloidal protective film.
Various cleaning agents in the selection of corrosion inhibitors
Copper parts and copper alloy cleaning agents in the corrosion inhibitor screening
Benzotriazole, Methyl-benzotriazole, 2-mercaptobenzothiazole, benzimidazole, etc., are copper materials corrosion inhibitors 2-mercaptobenzothiazole are the most common. The nitrogen on the molecules of BTA and MBT molecules on the sulfur is generally believed to form a ligand bond with its unshared electron pair and Cu (Ⅰ). 20 ℃ below BTA adsorption on Cu is the largest, 25 ℃ above 90Cu -10Ni is most readily adsorbed. The increase in temperature favors the adsorption of MBT on 70Cu-30.
In addition to the above common corrosion inhibitors, you can also use some amides, carboxylic acids, phosphate esters, etc. The use of corrosion inhibitors should be selected according to different systems, and a single corrosion inhibitor is always no compound corrosion inhibitor effect is good.
Aluminum, magnesium, zinc, and other active metals and their alloys in the corrosion inhibitor selection of cleaning agents
Active metals often use some gel adsorption class substances for corrosion inhibition, such as sodium tripolyphosphate and sodium metasilicate, cheap, wide availability, in addition, will use some phosphate esters, amphoteric imidazoline class, due to the active metal in the market application is more extensive, corrosion inhibition is complex, many manufacturers for this situation launched a series of compound corrosion inhibitors, such as BASF CL5080, Huntsman 1198LA, Rhodia ASI80, etc. Rhodia ASI80, etc.
Corrosion inhibitors in steel cleaning agent screening
Steel corrosion inhibitors are commonly marketed as inorganic corrosion inhibitors, such as nitrite, chromate, thiourea, urotropine, etc., but also through the form of coating corrosion inhibition, such as phosphate film vitrified film or other coatings. In some areas, to increase the substrate placement time, rust prevention oil for painting, or spray paint drying, and thus prevent the substrate rust, to achieve corrosion inhibition effect.
In the cleaning agent formulation design, we first need to determine the corrosion inhibitor corrosion inhibition principle according to the properties of the substrate, combined with the use of working conditions, based on the corrosion inhibition principle to select the type of corrosion inhibitor, combined with the formulation system environment to choose the appropriate corrosion inhibitor, and then for the selected corrosion inhibitor, to determine the experimental program and corrosion inhibition comparison test, screening out the corrosion inhibition performance of the best corrosion inhibitor, and finally through the orthogonal test to determine the Final, the optimum amount of corrosion inhibitor was determined by orthogonal test.
