Anti-corrosion packaging technology (3)

Anti-corrosion packaging technology (3)

1. Gas phase rust prevention mechanism
(1) Common characteristics of vapor phase corrosion inhibitors
a. In the molecular structure of the compound, a group having a corrosion inhibition effect can be separated from the action of water. Such groups are, for example, NO2-, CrO42-, OH-, PO43-, C6H5COO-, and organic compounds having -NO2, -COOH, -NH2, compounds capable of ionizing organic cations, and the like.
b. It has a certain volatility at normal temperature. The so-called vapor phase inhibitor is that its anti-rust effect can only be manifested when it can fill the inner space of the package in a short time. It has a corrosion-inhibiting group and cannot be volatilized at normal temperature. This requires that the material as a vapor phase inhibitor must have a certain vapor pressure, and the vapor pressure is required to be moderate. The vapor pressure of the vapor phase inhibitor is too large. Although it can quickly fill the inside of the package, it is absolutely sealed by the packaging container. It is difficult to do in practice, so it will consume faster and affect the effective period of rust prevention. If the vapor pressure is too low, the corrosion inhibitor vapor will not reach the effective concentration in the internal space of the container in a short time after packaging, so the metal products are very It may be rusted due to lack of protection. It has been proved that the vapor pressure of the vapor phase inhibitor is suitable at 0.0133 to 0.1333 Pa (Table 6-1).
Table 6-1 Vapor pressure of some vapor phase inhibitors at 21 ° C

Corrosion inhibitor

Vapor pressure (Pa)

Corrosion inhibitor

Vapor pressure (Pa)

Cyclohexylamine carbonate
Benzotriazole (30 ° C)
Dicyclohexylamine carbonate
Fabric acid carbonate
Diisopropylamine nitrite
Cyclohexylamine nitrite
Phenylamine nitrite

53.3288
5.3328
51.9955
1.0265
0.8665
0.3653
0.2533

Dibenzylamine nitrite
Dioctaethyl octoate
Butyl chromate
Hexylamine benzoate
Dicyclohexylamine nitrite
O-nitrophenol (35 ° C)
Nitrophenol dicyclohexylamine (35 ° C)

0.1159
0.0733
0.0239
0.0159
0.0159
0.0039
0.0026

The vapor pressure of a compound depends mainly on its molecular structure. Generally, it is between compounds with similar molecular weights. The stronger the polarity, the lower the vapor pressure. Between the molecules with the same polarity and the same molecular weight, the larger the molecular weight, the smaller the vapor pressure. The gas phase corrosion inhibitors with too high or too low vapor pressure are adjusted by a mixed use method.
In addition, the gas phase corrosion inhibitor is required to have a certain solubility in water (generally should be >1%), so as to be practically applied, and also has good chemical stability, and does not decompose and fail under the action of general light and heat, etc. Generate harmful substances.
(2) Mechanism of action of vapor phase corrosion inhibitor
At present, the basic understanding of the mechanism of action of vapor phase inhibitors is as follows:
a. Anodization is performed on the metal surface to block the electrochemical process of the cathode, and the group having a passivation effect is NO2-OH-.
b. Oriented adsorption of organic cations with larger non-polar groups forms a hydrophobic film on the metal surface, which not only shields the same effect of the rust medium, but also reduces the electrochemical reaction ability of the metal. The cation of hexylamine [(C6H11)2NH2+].
c. It is combined with a metal surface to form a stable complex film to increase the surface resistance of the metal, thereby protecting the metal, such as benzotriazole.
The above points are the main effects of the gas phase corrosion inhibitor. In fact, the rust prevention effect is often the result of a combination of various factors.
The anti-rust effect of the vapor phase inhibitor does not occur until it reaches the metal surface. There are two main forms of anti-corrosion groups reaching the metal surface: one is that the vapor phase corrosion inhibitor hydrolyzes or dissociates under the action of humid air to form a volatile protective group. This group reaches the metal by its own volatilization. Surface; another is that after the entire molecule of the corrosion inhibitor volatilizes to reach the metal surface, the protective layer is hydrolyzed or ionized on the metal surface under the influence of humid air.
Many experiments have shown that the protection of metal by gas phase corrosion inhibition has a great relationship with the adsorption characteristics on the metal surface. The stronger the adsorption force of the metal surface on the vapor phase inhibitor, the higher the concentration of the vapor phase inhibitor, and the slower the metal phase. The longer the storage time in the etchant, the more the amount of corrosion inhibitor adsorbed on the metal surface. At the same time, the chemical adsorption layer of the metal surface vapor phase corrosion inhibitor is also often a reliable protective film, which will desorb due to the influence of external factors (the chemical adsorption layer of the metal surface vapor phase inhibitor), due to external factors The change of the potential to the negative direction is called the desorption phenomenon, so the metal can be kept passivated only in the atmosphere of the corrosion inhibitor which maintains a certain concentration.
2, gas phase corrosion inhibitor types
There are many compounds with vapor phase inhibition, and there are two or three kinds of compounds that have been found so far. These substances are mainly amine salts, esters, nitro compounds, base amine salts and heterocyclic compounds of inorganic or organic acids. In use, gas phase corrosion inhibitors should be correctly selected according to different protection objects. Several commonly used vapor phase inhibitors are described below.
(1) ferrous metal vapor phase inhibitor
A common feature of ferrous corrosion inhibitors is the anti-rust effect of cations of ammonia or organic amines.
a. Dicyclohexylamine nitrite (VPI-260)
Dicyclohexylamine nitrite is a white to pale yellow crystalline material at room temperature, melting at 175 ° C, and decomposes at the melting point. Soluble in water and some organic solvents. The solubility in water increases with increasing temperature. The pH of its aqueous solution is approximately equal to 7.
The vapor pressure of dicyclohexylamine nitrite is relatively low, and is 0.0666644Pa at 25 ° C. Its volatilization rate is affected by wind speed and packaging conditions at the same temperature.
Dicyclohexylamine nitrite is the most widely used vapor phase inhibitor and has good rust resistance. The experiment proves that 35g of powder is added to the packaged 1m3 cargo, and the rust prevention period can reach 10-15 years. The metal product packaged in a rust-proof paper coated with dicyclohexylamine nitrite (content of 0.2 g/dm2) is placed in a louver box and does not rust for 4 to 5 years.
Dicyclohexylamine nitrite has excellent rust resistance to steel, cast iron and steel products with blue or phosphated surface. It has a certain protective effect on chromium, nickel, aluminum, tin, silver, etc. under normal storage temperature conditions. Copper and its alloys can cause "darkness". It has a rusting effect on magnesium, zinc, aluminum and its alloys.
Dicyclohexylamine nitrite has no significant effect on most non-metallic materials (such as most plastics, natural rubber, various packaging woods, as well as paint coatings, binders, desiccants, paper and various textiles, leather, etc.). It only has a certain influence on a few substances such as nitrile rubber.
b. Cyclohexyl carbonate (CHC)
Cyclohexylamine carbonate is a white powder with a melting point of 110.5 to 111.5 ° C. It has an ammonia smell and is non-toxic. However, it is slightly irritating to the skin, soluble in water and common organic solvents, and its aqueous solution is alkaline. It has a large vapor pressure (hundreds of times more than dicyclohexylamine nitrite at the same temperature) and increases rapidly with increasing temperature, so it can quickly fill the packaging space when it is used alone, but only in a better seal. Long-term rust prevention can be obtained under conditions, and this disadvantage can be overcome by mixing with dicyclohexylamine nitrite.
Cyclohexylamine carbonate has a large effective radius of action (46 cm), and has a good anti-oxidation effect against sulfur dioxide, and even good protection for rusted metal products. It has a protective effect on ferrous metals and aluminum, chromium, tin, zinc, etc., but can accelerate the corrosion of copper and magnesium.
The effective amount of cyclohexyl carbonate is generally 5 to 10 g/m 2 in a sealed package, and the coating amount on paper is about 20 to 40 g/m 2 .
c. Diisopropylamine nitrite (VPI-220)
Diisopropylamine nitrite is a colorless crystalline substance with a melting point of 136 ° C, which is easily soluble in an organic solvent, and its aqueous solution is slightly alkaline. Its vapor pressure is 6.6661Pa at 25 ° C, between dicyclohexylamine nitrite and cyclohexylamine carbonate, the volatilization speed is moderate, and good rust prevention effect can be obtained by using alone. Many experiments have proved that the rust-proof property of diisopropylamine nitrite is very good, has strong protective ability to steel, and also has certain protective effect on chromium, tin and nickel, but on copper, magnesium, zinc, aluminum and lead. Etc.
d. Benzyl carbonate
The benzylamine carbonate is a colorless phosphorus flake-like crystalline material having a melting point of 95.7 to 96.7 ° C, a special ammonia smell, and is easily soluble in water. The solubility in water at 20 ° C is 13 g / 100 ml. Its aqueous solution is slightly alkaline. It is soluble in organic solvents such as methanol, ethanol and benzyl. Its chemical stability is poor, it is easy to decompose into yellow or brown under the action of light and heat, and is easily affected by acid and alkali. Therefore, it must pay attention to the effects of light prevention, heat and acid and alkali.
Benzyl carbonate is an excellent rust inhibitor for steel. It has strong resistance to sulfur dioxide and can prevent the rust of steel. It also protects tin, chromium and silver, and aluminum, zinc, magnesium and copper. Cadmium and the like have different degrees of corrosion. When it is in direct contact with non-metal and packaging materials, it has no obvious effect on asphalt laminated paper, kraft paper and polyethylene, polyvinyl chloride; it can cause deterioration in gauze, wood and paint, but steam contact has no obvious effect.
(2) Non-ferrous metal vapor phase corrosion inhibitor
Non-ferrous metal vapor phase corrosion inhibitors are mainly classified into the following categories:
a. The chromate of organic amines. Such as dicyclohexyl chromate, cyclohexyl chromate, etc., these substances have a protective effect on steel, brass, zinc, nickel, etc., and are often used as rust-proof wrapping paper.
b. Phosphates of organic amines. Such as dicyclohexyl phosphate, cyclohexyl phosphate, etc., it is mainly used for the protection of brass.
c. Organic acid esters. For example, dibutyl oxalate has a protective effect on ferrous metals, copper, brass and aluminum. The amount of gas-phase rust-proof wrapping paper is 79g/m2, and it can also be dissolved in oil to form gas phase anti-rust oil. The dosage is less than 10%.
d. Organic phenols and their derivatives. For example, o-nitrophenol diethylene triamine, o-nitrophenol octadecylamine and the like are mixed with benzotriazole, and have protective effects on steel, copper and aluminum products.
e. Heterocyclic compound. The most important of these compounds is benzotriazole, which is an excellent corrosion inhibitor for copper and its alloys.

(3) Mixed vapor phase corrosion inhibitor
The mixed vapor phase corrosion inhibitor is a mixture of several substances and chemically reacts to form ammonia, thereby achieving the purpose of preventing rust of ferrous metals. The commonly used mixed vapor phase corrosion inhibitors are mainly as follows:
a. Sodium nitrite and urea, which are used for the rust prevention of ferrous metals, the domestic 1#, 2# and 11# rust-proof wrapping papers use this corrosion inhibitor.
b. Sodium nitrite and urethane, which are practically valuable ferrous metal vapor phase inhibitors in a one-to-one mixture. The anti-rust with this kind of mixed corrosion inhibitor has no obvious influence on the non-metallic materials such as nickel-chromium plating and paint. It is mostly used to manufacture anti-rust packaging paper. The domestic 651# anti-rust packaging paper is added with this mixed vapor phase corrosion inhibitor. Made of sodium benzoate.
c. Sodium nitrite and ammonium benzoate, their anti-rust effect is the formation of ammonium nitrite. It can be used in powder method or rust-proof wrapping paper method. The powder method is formulated to be 66% ammonium benzoate and 34% sodium nitrite in an amount of 50 to 100 g/m3. Domestic 652# rust-proof wrapping paper is this type.
d. Sodium nitrite and diammonium hydrogen phosphate and sodium hydrogencarbonate. They can be used for the protection of steel, cast iron, silicon steel sheets and nickel, chromium, zinc and tin. The formula for using powder is:
Sodium nitrite: dihydrogen phosphate: ammonium bicarbonate = 54:35:11
The dosage is 300-400 g/m3 and the working radius is 30 cm. If the 15% aqueous solution of the packaging paper is sealed with steel parts, the rust prevention period can be up to three years.
e. Sodium benzoate and triethanolamine carbonate. This corrosion inhibitor is an effective rust inhibitor for ferrous metals. Domestic 15# rust-proof wrapping paper is this type, but this paper is easy to absorb moisture, so the packaging is easy to break, and the paint film can cause paint film. Sticky. The amount of paper used should be 60-80 g/m2.

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