In recent years, due to the continuous innovation of metal packaging materials and the continuous improvement of packaging technology, the anti-corrosion performance of metal packaging containers has been greatly improved. However, due to the characteristics of the metal itself, corrosion has never stopped. The economic losses caused by this have also continued to increase with the increase in metal packaging production.
For the metal packaging production industry, the main approach to the corrosion of metal packaging is to select corrosion-resistant metal materials and use advanced scientific and technological coating to prevent it from rusting prematurely. However, the principles and methods of anti-corrosion of metal packaging are far more than this. The metal anti-corrosion scientific community at home and abroad has also made significant contributions to this, laying the foundation for the development of metal packaging.
Therefore, the advanced metal packaging industry abroad is constantly conducting research on the anti-corrosion technology of metal packaging. In the production process of metal packaging, scientific technologies and methods are used to reduce the corrosion of metal packaging to effectively improve the corrosion resistance of metal packaging containers.
The corrosion of metal packaging products is mainly caused by electrochemical corrosion, which is mainly due to the electrochemical inhomogeneity of the surface of the metal packaging material. When it comes into contact with the medium, a corrosion cell is formed. Based on this principle, we can study effective technologies and methods to prevent metal packaging corrosion. Obviously, in order to prevent metal packaging products from rusting, the most effective anti-corrosion packaging technology is to try to eliminate the various conditions that produce corrosion cells.
There are many methods of rust prevention, which can be divided into "permanent" rust prevention and "temporary" rust prevention according to the length of the anti-rust period. "Permanent" rust prevention methods, such as changing the internal structure of the metal; alloying the metal surface; coating the metal surface (electroplating, spraying, chemical plating); applying non-metallic coatings on the metal surface (enamel, rubber, plastic, paint, etc.)... These methods can achieve the purpose of rust prevention well, but they are "permanent" and the anti-rust layer cannot be removed during use. Therefore, these methods can be widely used in metal packaging products; but due to technical and methodological reasons, these permanent anti-rust methods are not permanent in many special cases. The general "temporary" rust prevention does not mean that the rust prevention period is short, but refers to the "temporary" nature of the process from the transportation, storage, sales and other circulation links of metal products to the hands of consumers, as well as the "temporary" nature of the rust prevention layer. The rust prevention period of "temporary" rust prevention materials can reach several months, several years or even more than ten years. Practice has proved that only by organically combining "permanent" rust prevention and "temporary" rust prevention can the most ideal anti-corrosion effect be achieved.
"Permanent" rust prevention is currently the most widely used method in the metal packaging industry, while "temporary" rust prevention is often overlooked. So here we focus on the "temporary" rust prevention methods commonly used in the metal packaging industry at home and abroad. These methods are most practical when using non-coated metal packaging materials to produce packaging containers, and have been increasingly widely used in recent years. The process of "temporary" rust prevention packaging has three aspects: the pretreatment technology of rust prevention packaging (including cleaning, rust removal, drying, etc.), the rust prevention treatment technology of various rust prevention materials, and the post-treatment technology of rust prevention packaging.
For various reasons, various substances such as grease, rust products and various dust are often generated and attached on the surface of produced metal containers. These are factors that cause electrochemical corrosion. Therefore, when metal products are packaged for rust prevention, they must be pretreated by cleaning, rust removal and drying.
1. Cleaning of metal containers
The commonly used cleaning methods for metal containers are mainly alkali method, surfactant method and organic solvent method.
The aqueous solution of alkali can wash away the oil on the metal surface, which is one of the commonly used cleaning methods. Alkali that can be used for metal cleaning includes sodium hydroxide, sodium carbonate, trisodium phosphate, sodium pyrophosphate, sodium hexametaphosphate, water glass and the like.
Mineral oil that is sorry for saponification reaction is not effective when cleaned with caustic soda. Sodium silicate, sodium phosphate and sodium carbonate are often used as weak alkalis with surfactants as the main ingredients of alkaline cleaning liquid.
For alkali cleaning, the type of alkali used should generally be selected according to the metal material and the type of grease attached. The main advantage of the alkaline solution cleaning method is that it has a good oil removal effect. Even products with heavy oil stains can be cleaned. Non-greasy dirt can also be washed away at the same time. The alkaline solution can be reused, which is relatively economical. The disadvantage is that if it is not well controlled, it may cause rust or discoloration of metal products.
Surfactants refer to a class of organic substances with two parts, hydrophilic and hydrophobic, in their molecular structure. This special structure gives them special dispersibility in aqueous solution - they are more concentrated and directed on the surface or interface of the solution (such as the interface between oil and water), and can reduce surface tension and interfacial tension. Therefore, they have the functions of wetting, penetration, emulsification, and cleaning. There are many varieties of surfactants, such as soap and other fatty acid salts, synthetic detergents such as alkyl sodium sulfonate and other sulfonates, alkyl trimethyl ammonium chloride, 6501 cleaning agent, Pingpingjia cleaning agent, TX-10 cleaning agent, 6503 cleaning agent, 105 (R-5) cleaning agent, 664 cleaning agent, etc. The characteristics of surfactant cleaning are safe operation, good oil removal effect, and can also clean non-greasy dirt. It has no obvious corrosion effect on metal, so it is more suitable for metal packaging products.
The organic solvent method is a method of using organic solvents with strong dissolving ability for oil to clean the surface of metal packaging. Commonly used petroleum series solvents, such as gasoline (mainly 200# industrial gasoline or 160#, 120# gasoline), kerosene, etc., followed by chlorinated hydrocarbon solvents, such as trichloroethylene, tetrachloroethylene, etc.
The advantage of solvent cleaning is good effect. When cleaning a small amount of metal packaging, it does not need to be heated. It can be cleaned by soaking or scrubbing, and it has no corrosion to metal. However, necessary equipment (such as cleaning machines, etc.) is required to clean a large number of metal packaging containers. The disadvantage is that petroleum series solvents are easy to burn and catch fire. At the same time, because they absorb a lot of heat when volatilizing on the surface of metal containers, the metal temperature may drop significantly. In a high humidity environment, condensation will appear on the surface of the washed container, causing rust.
Complex cleaning fluids. The dirt that is difficult to remove by the above three methods can be cleaned with complex cleaning liquid. Commonly used aminocarboxylic complexing agents: ethylenediaminetetraacetic acid or disodium ethylenediaminetetraacetic acid. They can form soluble chelates with dirt and remove the dirt from the surface of the metal container.
Human sweat cleaning liquid. Human sweat stains can be cleaned with hot methanol or human sweat replacement type rust preventive.
There are also ultrasonic cleaning, steam cleaning, electrolytic cleaning, etc., which are relatively advanced cleaning methods.
Cleaning is the basic process of the entire anti-rust packaging. Cleaning must be thorough. If necessary, two or more cleaning liquids can be used for combined cleaning.
2. Rust removal of metal containers
In actual rust-proof packaging, the rust removal process is often combined with the cleaning of oil stains, that is, rust removers are added to the cleaning liquid. The rust removal methods of metal containers can be divided into two categories: physical and mechanical rust removal and chemical rust removal.
a. Manual rust removal: rust removal with steel brushes, iron files, shovels (scrapers), gauze, sandpaper, etc. This method is simple, but not suitable for rust removal of small and large-scale products.
b. Mechanical rust removal method: There are jetting method and grinding wheel and cloth wheel rust removal method.
The jetting method is a method of using strong force to spray sand particles on the metal surface to remove rust by impact and friction. According to the spraying material, the jetting method can be divided into sandblasting method (using sea sand, river sand, stone particles as spraying materials), steel particle spraying method (using small steel bullets or broken steel particles as spraying materials), and soft particle spraying method (using plant seeds or plastic particles as spraying materials). According to the spraying method, it can be divided into power spraying method (a method of spraying dry spraying materials with high-pressure air), wet spraying method (a method of mixing fine sand particles with water into a muddy state and spraying with high-pressure air) and vacuum spraying method.
The jetting method is suitable for rust removal of large metal containers and requires a jetting machine. When using the wet method, a water-soluble corrosion inhibitor must also be added to the water. Its advantages are high rust removal efficiency and low cost.
The grinding wheel rust removal method can only be used on non-processed surfaces. The cloth wheel rust removal method is for products with smooth surfaces such as copper, iron or non-ferrous metals that require high surface coating or surface finish.
Chemical rust removal methods include pickling and alkali washing (alkali electrolysis, alkali reduction, alkali boiling, etc.), among which the most widely used is the pickling method.
The pickling method is to immerse metal products in various acid solutions, and the acid reacts with the metal rust products to make the insoluble rust products become soluble substances, which are separated from the metal surface and dissolved in the aqueous solution.
The acids used for pickling are mainly sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, hydrofluoric acid, etc. Among them, hydrochloric acid solution has the strongest ability to remove rust; sulfuric acid has a large mechanical effect of generating hydrogen and is cheap, so it is widely used for rust removal of steel. Nitric acid and hydrofluoric acid can be used for rust removal of non-ferrous metals such as aluminum products. Compared with hydrochloric acid and sulfuric acid, phosphoric acid has poorer rust removal ability, but weaker rust resistance. It can react with the surface of copper and iron to form an insoluble film of iron phosphate, which has a temporary rust-proof effect in the air after washing. Compared with the physical and mechanical method, the main advantages of the pickling method are that it does not cause deformation of metal materials, the treated surface is not rough, the operation is simple, the efficiency is high, the rust in every corner of the metal container can be removed, and it is suitable for the rust removal of a large number of small containers, and no special equipment is required, and the cost is low. Therefore, it is a commonly used chemical rust removal method, but it has a rusting effect on metal containers, is prone to "hydrogen embrittlement" and affects the surface finish. In recent years, alkaline washing and rust removal have been developed. Alkaline washing and rust removal are carried out in solutions containing caustic soda, carboxylic acetic acid, complexing agents and foaming agents. The alkaline washing method does not corrode the base metal, does not cause "hydrogen embrittlement", and the metal surface is smooth. It is suitable for non-ferrous metals such as steel and copper and magnesium.
3. Drying
After cleaning, water or solvents often adhere to the surface of metal containers. They should be removed as soon as possible to avoid rusting again, and then rust inhibitors can be applied.
Common drying methods include heating, oil bath dehydration, compressed air drying, gasoline drainage with surfactant, infrared drying, etc. No matter what drying method is used, the rust inhibitor should be applied only when the surface of the metal container cools to a certain temperature, otherwise it will cause the rust inhibitor to decompose.
The surface treatment process of metal containers is the basis of rust prevention. Only when the surface of the metal container is treated very cleanly and completely dry can the rust prevention effect be fully exerted. Otherwise, even if the rust prevention material with excellent performance is used for coating, it is impossible to obtain a satisfactory rust prevention effect.
If the surface of the metal container is isolated from various factors that cause atmospheric corrosion (that is, the metal surface is protected), the purpose of preventing atmospheric corrosion of the metal can be achieved. The packaging technology of rust-proof oil is based on this principle to prevent rust.
Rust-proof oil is a temporary rust-proof coating composed of grease or resin substances as the main body, oil-soluble corrosion inhibitors and other additives. After the grease or resin substances in the rust-proof oil are applied to the surface of the metal container as a film-forming substance, they have a certain isolation effect on the rust factors. However, general grease can dissolve a small amount of oxygen in the air, and can also dissolve a small amount of water. Using grease alone cannot achieve a satisfactory rust-proof effect, so corrosion inhibitors must be added. Such substances have a great influence on the rust-proof effect of rust-proof oil.
1. Principle of Anti-Rust Oil
Since surfactants have a common feature in molecular structure—two components, hydrophilic polar groups and lipophilic non-polar groups, when anti-rust oil is applied to the surface of metal containers, the corrosion inhibitor molecules dispersed in the oil will be directional adsorbed on the interface between metal and oil (polar groups adsorbed on metal, non-polar groups adsorbed on oil), and can form a multi-molecular interfacial film. Experiments show that if only 1 to 2 molecular layers of film can be formed, the anti-rust ability is very poor. When the adsorption layer is more than 6 molecular layers, it has a good anti-corrosion ability.
This adsorption has a shielding effect on rust factors. Second, it can improve the adhesion between the oil film and the metal surface. Since the surface of the adsorption film is hydrophobic, it has better water resistance and can increase the resistance of the metal container surface.
Many studies have shown that the chemical adsorption of corrosion inhibitors on the surface of metal containers (forming coordination bonds) reduces the activity of metals, which is one of the main reasons for the anti-rust effect.
If water droplets are formed on the surface of the oil film (such as condensation, etc.), the interfacial tension between the water droplets and the oil will make the water droplets spherical, and gravity will make it easier for them to penetrate the oil film and reach the metal surface. Surfactants such as corrosion inhibitors can reduce the surface tension of water, so that water droplets cannot exist in a spherical state on the oil film, but tend to spread out. This reduces the pressure of water droplets on the oil film, making it difficult for them to penetrate the oil film and reach the surface of the metal container. The flatter the water droplets are spread on the surface of the oil film, the better the anti-rust effect of the oil is generally. Therefore, measuring the contact angle of water droplets on the oil film is the most commonly used method to identify the anti-rust effect of rust-proof oil.
The relationship between the contact angle of a water droplet on the oil surface and the interfacial tension is as follows: So-Suo=Sucosθ cosθ=(So-Suo)/Su Where: So——Surface tension of oil Su——Surface tension of water Suo——Interfacial tension between oil and water
The surface-active corrosion inhibitor can replace the water adsorbed on the metal surface by its interfacial adsorption. In addition, the water contained in the oil can be stabilized in the oil by the colloid particles or interfacial film of the corrosion inhibitor, so that it cannot come into direct contact with the metal.
The above-mentioned effects all inhibit the formation of rust cells on the metal surface, thereby achieving the purpose of preventing the metal container from rusting.
2. Types of anti-rust oil
There are many types of anti-rust oil. The anti-rust oil suitable for metal packaging containers mainly includes anti-rust grease, solvent-diluted anti-rust oil, thin-layer oil, instrument anti-rust oil, etc.
Anti-rust grease is a type of anti-rust oil based on vaseline that is grease-like at room temperature. It is composed of film-forming substances (or base oils) and corrosion inhibitors.
Film-forming substances (or base oils): mainly vaseline and lubricating oil. Vaseline is generally industrial vaseline, and its chemical composition is 15% paraffin, 45% petroleum grease, 25% cylinder oil, and 15% machine oil. Vaseline used in anti-rust oil is required to be non-corrosive, free of moisture, have a certain degree of water resistance, and a dropping point temperature of 45°C. Commonly used lubricating oils in anti-rust oils include machine oil, spindle oil, and cylinder oil. Its chemical composition is mainly alkanes, cycloalkanes and aromatic hydrocarbons, as well as a small amount of oxides and sulfides.
Corrosion inhibitors: The commonly used oil-soluble corrosion inhibitors in anti-rust greases are mainly petroleum sulfonates, aluminum stearate, zinc cycloalkanes, oxidized petroleum grease, lanolin and its derivatives, etc. Benzotriazole is often added to prevent rust on non-ferrous metals. The type of corrosion inhibitor used in the anti-rust grease can affect the performance of the anti-rust grease. For example, the anti-rust grease with aluminum stearate has good salt water resistance, but weak adhesion to metal; the anti-rust grease with barium petroleum sulfonate has better salt water resistance and can be used for rust prevention in the marine atmosphere; the anti-rust grease with lanolin and its soaps has strong adhesion to metal, and has a certain emulsification ability for water, and has strong anti-rust ability; cyclohexane zinc anti-rust grease has strong adhesion to metal and has a certain resistance to salt water, but poor anti-rust ability for cast iron; the anti-rust grease with oxidized petroleum grease and its soaps has better performance than fatty acid soaps, but its resistance to salt water is poor; the anti-rust grease with Siben-80-type synthetic lipids has poor thermal stability and is corrosive to copper. Benzotriazole has excellent anti-rust effect on copper and its alloys. In practice, in order to achieve satisfactory results, a formula of several corrosion inhibitors is often used in combination.
Anti-rust grease is in the form of ointment at room temperature, so the film layer is generally thicker (up to 0.5mm), not easy to lose, not easy to volatilize, and after sealed packaging, the anti-rust period is generally longer, up to more than two years. The main coating method of anti-rust grease is hot dipping, that is, the anti-rust grease is heated and melted to a fluid state before coating, and the cleaned, rust-removed, and dried metal container is immersed for a while, and then taken out and cooled to solidify the oil film. During hot dipping, the thickness of the oil film formed on the surface of the metal container is different due to different dipping temperatures for the same oil. The lower the temperature, the thicker the oil film, and the stronger its oil-proof ability. Large parts can be coated with hot brushing, that is, the heated and melted anti-rust grease is applied to the surface of the metal product with a soft brush. After the metal container is oiled, it should be sealed with paraffin paper or plastic bags in time to prevent the oil layer from drying up and becoming ineffective and contaminating the packaging.
Solvent-diluted anti-rust oil is made by adding solvent to anti-rust oil with mineral oil or resin as film-forming agent. In addition, other additives such as antioxidants and stabilizers are also added. This type of anti-rust oil can be divided into three types according to the type of solvent: petroleum series solvents, organic solvents and water dilution. Among them, organic solvents are toxic, so they are less used. According to the properties of the oil film, this type of anti-rust oil can be divided into hard film oil and soft film oil.
Hard film oil-The resin used as hard film oil should first have a large solubility in gasoline and kerosene, and secondly, it should not corrode various metals. The resins currently used include tert-butylphenol formaldehyde resin (i.e. 2402), long oil alkyd resin, melamine formaldehyde resin, terpene resin, petroleum resin and alkyl acid resin. Among them, 2402 resin and alkyl acid resin are better. Commonly used hard film oils include hard-1, hard-3, 1, 2, and 74A-2. Both hard-1 and -3 use 2402 resin as film-forming material, but the amount of 2402 in hard-1 is large, so the oil film toughness is poor, and it is not suitable for metal packaging products with sharp edges. At the same time, it may powderize in long-term sealed packaging. The content of 2402 in hard-3 is relatively low, and the addition of 389-9 alkyd resin improves the toughness of the oil film. However, the adaptability of hard-3 to non-ferrous metals such as copper alloys is not as good as that of hard-1. For easy identification, a small amount of 3902 oil-soluble red pigment is added to hard-3. 74A-2 is a hard alkane oil with alkyl acid amino resin and oxidized petroleum film barium soap as film-forming materials. It is suitable for a variety of metals (including magnesium alloys) with good stability and strong adhesion. Hard film anti-rust oil has good film-forming protection, smooth and non-sticky film surface, no flow in summer and no cracking in winter, and convenient construction and low price, but its hard film is not easy to remove. The natural drying time of the film is about 1h. Soft film oil - Commonly used soft film oils include 204-1, Shanghai-201, F-35, 112-5, 704, No. 3 anti-rust oil, 33-612 and Soft-1. 204-1 oil uses sulfonated lanolin calcium soap as a film-forming agent and is also a corrosion inhibitor. Shanghai-201 and F-35 use vaseline as a film-forming agent. 112-5, 704, No. 3 anti-corrosion oil, 33-6 and Soft-1 are all anti-corrosion oils that use oxidized petroleum fat barium soap (T743 barium soap) as a film-forming agent and a corrosion inhibitor. The main feature of solvent-based soft film oil is that it can form a film on the surface of metal containers, so it loses less but is not easy to remove, and the anti-rust period is short.
Because anti-rust oil has many shortcomings as an anti-rust sealing coating, such as polluting the environment during construction, affecting the appearance of metal products, and removing the film when in use. Therefore, the anti-rust coating for sealing has gradually been replaced by thin films and ultra-thin films.
Thin film anti-rust coating can form a complete film with a thickness of less than 1 to 2 μm. The film has strong adhesion to the surface of the metal container and has good anti-rust properties. It does not affect the performance of the lubricant and does not require rust removal during assembly and use. At present, thin film and ultra-thin film anti-rust coatings mainly include polyvinyl fluoride and polyvinyl fluoride types, other synthetic resin types, organic silicone amine and organic silicone resin types.
Soft film oil in solvent-diluted anti-rust oil is mainly suitable for sealing metal container products. It can be sprayed or dipped (the amount of solvent can be less); then sealed and packaged in plastic bags. The main coating method of hard film oil is to spray with compressed air, but this spraying method will have the phenomenon of partial spraying. The surface of the metal container without anti-rust coating will continue to rust in the air, and even rust faster.
This is an anti-rust oil with resin as the film-forming material. Its characteristics are long anti-rust period, thin and transparent oil film, low dosage and good appearance. Thin layer oil is divided into solvent type and solvent-free type. Thin layers of oil are usually applied by dipping or spraying.
