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Corrosion inhibitor knowledge
        Corrosion inhibitor can prevent or slow down the corrosion of metal by corrosive medium, which is mainly used in water treatment, oil field, oil refining, lubricant, boiler water supply and so on.
There are three types of water treatment corrosion inhibitors.
Oxide film type corrosion inhibitor, which oxidizes on metal surface to form a passivation film with corrosion resistance, which can diffuse in adjacent areas to achieve corrosion inhibition purpose. This kind of corrosion inhibitor takes the inhibitor itself as oxidant or dissolved oxygen in the medium as oxidant, which makes the metal surface form a passive oxide film to slow down the corrosion rate of metal, so the oxide film corrosion inhibitor is also called passivation film corrosion inhibitor.
Oxide film type corrosion inhibitors can also be divided into anodic inhibitor and cathodic depolarization type. These corrosion inhibitors are:
a) chromate and dichromate. It can form a stable passivation film with iron and aluminum;
b) Nitrite. The effect is similar to the corrosion inhibition performance of dichromate, especially suitable for aluminum and aluminum alloy;
c) Molybdate. The toxicity is small, the price is low, but the passivation effect is poor;
d) Tungstate and heteropoly tungstate. This kind of salt has a bright future and its performance is superior to that of molybdenum series.
Precipitated film type corrosion inhibitor, forming a precipitated film on the metal surface. Precipitation film formed by the interaction between corrosion inhibitor and some ions in water can also form a kind of insoluble precipitate or complex by the reaction between corrosion inhibitor and metal ions existing in corrosive medium, which can be divided into cathode inhibition type and mixed inhibition type according to different inhibition electrode processes. These corrosion inhibitors are:
a) polyphosphate. It is the most widely used corrosion inhibitor in the world at present, and it is generally used in combination with other corrosion inhibitors. When polyphosphate coexists with calcium, zinc, manganese and other divalent metal ions, it can improve the corrosion inhibition performance, but it is easy to hydrolyze and pitting corrosion at high temperature;
b) Silicate. Most of them are used as corrosion inhibitors for drinking water treatment, which have good corrosion inhibitors such as copper and nickel, but poor corrosion inhibitors such as aluminum, zinc and iron;
c) Zinc salt. In cooling water treatment, cathode corrosion inhibitor is commonly used;
d) Borate. It is a new corrosion inhibitor with low toxicity, good chemical stability and promising development.
e) Organophosphates. Its main advantages are low toxicity, good chemical stability, difficult hydrolysis, good corrosion inhibition and scale inhibition.
f) sarcosine. It reacts with metals to form pentacyclic or hexacyclic complexes, which has good corrosion inhibition effect.
Adsorption film corrosion inhibitor: The molecular structure has hydrophilic groups that can adsorb on the metal surface and hydrophobic groups that shield the metal surface. The hydrophilic groups are directionally adsorbed on the metal surface, while the hydrophobic groups hinder the diffusion of water and dissolved oxygen to the metal surface, thus playing a corrosion inhibition role. These inhibitors include:
a) organic amines, the adsorption group is amine group, and the hydrophobic group is alkyl group, such as hexadecylamine, octadecylamine, morpholine, ethyl piperazine, triethyl diamine, quaternary ammonium salt, etc.;
b) Mercaptans, used mostly in copper and copper alloys, forming a protective film by chemical adsorption of sulfhydryl groups and metals. There are mercaptobenzothiazole, β-mercaptopropionic acid, mercaptomaleic acid, mercaptosuccinic acid and so on;
c) Lignin, a natural cellulose, is adsorbed on metal surfaces to inhibit corrosion. Sodium lignin has good solubility and dispersion, is cheap and can be mixed with other organic compounds;
d) Gluconate. Sodium gluconate has good complexing effect on anions such as calcium and magnesium, and its price is cheap. It is often mixed with zinc molybdate, salicylic acid and polyacrylic acid to improve corrosion inhibition performance;
e) Sulfonate. Sulfonated petroleum is made from petroleum by-products, and then potassium, calcium, barium and ammonium salts are made as corrosion inhibitors;
f) sulfonamide compound, used in high concentration chloride ion cooling water treatment, the effect is better;
g) hydroxyacid groups have obvious inhibition effect on iron corrosion. If dimer acid and fatty acid ester are used, dimer cyclic fatty acid compound can be made under the action of boron fluoride catalyst, which can be used as iron corrosion inhibitor;
h) polyaminohydroxyl compound, polyaminohydroxyl compound prepared by polymerization of C5 ~ C18 aliphatic aldehyde and ethylamine, can be used as pickling corrosion inhibitor, and the corrosion inhibition rate reaches 98%.
Corrosion inhibitors used in other applications include sulfonates, primary amines, phosphates, propionates, diethanolamines, etc.

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