Anti-corrosion technology for metal packaging containers

The loss caused by the corrosion of metal packaging in the world every year is incalculable, especially for the packaging of important military and warfare supplies. In a relatively hostile war environment, the corrosion of metal packaging often results in the failure of materials. Metal corrosion has always been a subject that people pay much attention to. Therefore, the continuous innovation of metal packaging materials and the continuous improvement of packaging technology have greatly improved the anti-corrosion performance of metal packaging containers, but due to the characteristics of the metal itself. Corrosion has never stopped. The resulting economic losses have continued to increase with the increase in the output of metal packaging.

For the metal packaging industry, for the corrosion of metal packaging, the selection of corrosion-resistant metal materials is mainly adopted, and advanced scientific technology is applied to prevent premature corrosion. However, the principles and methods of anti-corrosion for metal packaging are far more than this. The scientific community of metal anti-corrosion at home and abroad has also made a major contribution to this, laying the foundation for the development of metal packaging.

Therefore, foreign advanced metal packaging industry, for metal packaging anti-corrosion technology, are constantly conducting research, in the production process of metal packaging, through scientific techniques and methods to reduce the corrosion of metal packaging, in order to effectively improve the metal packaging The corrosion resistance of the container.

The corrosion of metal packaging products is mainly caused by electrochemical corrosion. Electrochemical corrosion is mainly due to the electrochemical non-uniformity of the surface of the metal packaging material. When it comes in contact with the medium, a rusted battery is formed. Based on this principle, we can study effective techniques and methods for preventing corrosion of metal packaging. Obviously, in order to prevent metal packaging products from rust, the most effective anti-corrosion packaging technology is to try to eliminate the various conditions that produce rusted batteries.

There are many ways to prevent rust. According to the length of the rust-proof period, it can be divided into "permanent" rust-proof and "temporary" rust-proof. “Permanent” rust prevention methods, such as changing the internal structure of the metal; alloying the metal surface; coating the metal surface (electroplating, spray plating, electroless plating); applying non-metallic coating on the metal surface (enamel, rubber, plastic, paint, etc. Floor)……. These methods can all achieve the purpose of rust prevention. However, they are "permanent". In use, the rust-proof layer cannot be removed. Therefore, these methods can be widely used in metal packaging products; but due to technical and methodological reasons These permanent anti-corrosion are not permanent in many special situations. The general "temporary" rust-proofing does not mean that the rust-proof period is short, but refers to the "temporary" nature of the process of transporting, storing, and selling metal products to the hands of consumers and the rust-proof layer. Temporary." The rust-proof period of "temporary" rust-proof materials can reach several months, years or even more than ten years. Practice has proved that only the combination of “permanent” anti-rust and “temporary” anti-rust can achieve the best anti-corrosion effect.

“Permanent” rust prevention is currently the most widely used method in the metal packaging industry, and “temporary” rust prevention is often overlooked. Therefore, here we focus on the “temporary” rust prevention methods commonly used in the domestic and international metal packaging industry. These methods are most practical when using non-coated metal packaging materials to produce packaging containers, and have been applied more and more widely in recent years. The process of “temporary” rust-proof packaging has three aspects: pretreatment technology of anti-rust packaging (including cleaning, rust removal, drying, etc.), anti-rust treatment technology of various anti-rust materials, and anti-rust packaging. Processing technology.

First, the pretreatment technology of metal packaging containers

For various reasons, various substances such as grease, corrosion products, and various dusts are often generated and adhered to the surfaces of the produced metal containers. These are the factors that cause electrochemical corrosion. Therefore, when metal products are rust-proofed, they must be cleaned, rusted, dried, and other pretreatments.

1, metal container cleaning

Commonly used cleaning methods for metal containers are alkali liquor, surfactants, and organic solvents.

(1) Alkali method

Aqueous solutions of bases can wash off oil stains on metal surfaces, which is one of the commonly used cleaning methods. Bases that can be used for metal cleaning include sodium hydroxide, sodium carbonate, trisodium phosphate, sodium pyrophosphate, sodium hexametaphosphate, water glass, and the like.

The mineral oil that cannot afford the saponification reaction does not have good cleaning effect with caustic alkali, and most of them use a weak base such as sodium silicate, sodium phosphate, and sodium carbonate as a main component and a base cleaning liquid with a surfactant.

Alkaline cleaning, the type of base used, generally based on the metal material and the type of grease attached to the selected.

The main advantage of the lye cleaning method is that the oil washing effect is good, even if the oil-heavy products can be washed, the non-oil-based dirt can also be washed off, and the lye can be used repeatedly, which is more economical. The disadvantage is that poor control may cause rust or discoloration of the metal product.

(2) Surfactant method

Surfactant refers to a group of organic substances that have two parts, a hydrophilic group and a hydrophobic group, on the molecular structure. This special structure makes them have special dispersibility in the aqueous solution - more concentrated orientation and sorption in the solution. Surface or interface (such as oil-water interface), and can reduce surface tension and interfacial tension. Therefore, they have functions such as wetting, penetration and emulsification, and washing. There are many types of surfactants, such as soaps and other fatty acid salts, synthetic detergent alkyl sulfonates such as sodium sulfonate, alkyl trimethyl ammonium chloride, 6501 cleaning agent, flat and cleaning agent, TX-10 cleaning Agent, 6503 cleaning agent, 105 (R-5) cleaning agent, 664 cleaning agent, etc.

Surfactant cleaning is characterized by safe operation, good oil wash effect, and non-greasy dirt cleaning, and no obvious rust effect on metal, which is more suitable for metal packaging products.

(3) Organic solvent method

The organic solvent method is a method of washing the surface of a metal package with an organic solvent that has a strong dissolving power against oil stains. Commonly used petroleum series solvents, such as gasoline (mainly 200 # industrial gasoline or 160 #, 120 # gasoline), kerosene, followed by chlorinated hydrocarbon solvents, such as trichloroethylene, tetrachloroethylene, and so on.

The advantage of solvent cleaning is that the effect is good, a small amount of metal packaging does not need heating when cleaning, and can be washed by immersion or scrubbing, and has no rustiness to metal. However, cleaning of large quantities of metal packaging containers requires the necessary equipment (such as washing machines, etc.). The disadvantage is that the petroleum series solvent is easy to ignite and catch fire. At the same time, the metal temperature can be significantly reduced due to the absorption of a large amount of heat when the surface of the metal container is volatilized. In a high-humidity environment, condensation occurs on the surface of the container and condensation occurs.

(4) Other cleaning fluids

Complex cleaning solution. A complex cleaning solution can be used for the dirt that is difficult to remove with the above three methods. Common amino-complexing agent: ethylenediaminetetraacetic acid or disodium ethylenediaminetetraacetate. All of them can form soluble chelates with the dirt, leaving the dirt off the surface of the metal container.

Human sweat cleaning fluid. Human sweat stains can be cleaned with hot methanol or human sweat replacement rust inhibitors.

There are ultrasonic purification, steam purification, electrolytic purification, etc. These are more advanced purification methods.

Cleaning is the basic process of the entire rust-proof packaging. The cleaning must be thorough. If necessary, it can be cleaned with two or more cleaning fluids.

2. Derusting of metal containers

In the actual rust-proof packaging, the rust removal process and the cleaning oil work are often combined, that is, the rust removal agent is added to the cleaning solution. Metal container rust removal methods can be divided into physical and mechanical rust removal methods and chemical rust removal methods.

(1) Physical and mechanical derusting method

a. Manual rust removal method: use steel brush, shovel, shovel (scraper), gauze, sandpaper, etc. This method is simple, but it is not suitable for rust removal of small and large quantities of products.

b. Mechanical rust removal method: There are spray method and grinding wheel, wheel rust removal method.

Jetting is a method of spraying sand on a metal surface with strong force to remove rust by impact and friction. The spray method can be divided into spray blasting methods (using sea sand, river sand, and rock particles as injection materials), steel shot blasting (using small steel bullets or broken steel particles as injection materials), and soft particle spraying ( Use plant seeds or plastic particles as spray material). The injection system is divided into a power injection method (a method of spraying dry injection material with high-pressure air), a wet injection method (a method of mixing fine sand and water into a slurry and spraying with high-pressure air), a vacuum injection method, and the like.

The spray method is suitable for rust removal of large metal containers and requires jetting machinery. When using the wet method, a water-soluble corrosion inhibitor must be added to the water. Its advantages are high rust removal efficiency and low cost.

The wheel rust removal method can only be used on non-machined surfaces. Cloth wheel rust removal method is the surface smoothness of the surface coating or surface finish on the higher requirements of copper and iron or non-ferrous metals and other products.

(2) Chemical rust removal method

Chemical rust removal methods include pickling and alkaline rust removal (lye electrolysis, alkali reduction, lye boiling, etc.), of which the acid pickling method is the most widely used.

The pickling method is a method in which a metal product is immersed in a solution of various acids, and the acid chemically reacts with the metal corrosion product to turn the insoluble corrosion product into a soluble substance and to dissociate the metal surface into the aqueous solution.

Acids used in pickling include sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, and hydrofluoric acid. Among them, the rust removing package of hydrochloric acid solution is the strongest product; the mechanical function of sulfuric acid generating hydrogen is large and the price is low, and it is widely used for derusting of steel. Nitric acid and hydrofluoric acid can be used for rust removal of non-ferrous metals such as aluminum products. Phosphoric acid and hydrochloric acid sulfuric acid, compared with poor rust removal, but weak corrosion, can react with the surface of copper and iron to produce insoluble iron phosphate film, after washing in the air have a temporary anti-rust effect.

The main advantages of the pickling method compared with the physical mechanical method are that it does not cause deformation of the metal material, the treated surface is not rough, the operation is simple, and the efficiency is high. The rust of each corner of the metal container can be removed, and it is suitable for derusting of a large number of small containers. Without special equipment, the cost is low. Therefore, it is a commonly used chemical rust removal method, but it has a rusting effect on metal containers and is prone to “hydrogen embrittlement” and affects the surface finish. Alkali washing has been developed in recent years. Alkali washing is carried out in a solution containing caustic alkali, carboxyacetic acid, a complexing agent, and a foaming agent. Alkali washing method does not rust metal matrix, does not occur "hydrogen embrittlement", smooth metal surface, suitable for iron and steel and copper and magnesium and other non-ferrous metals.

3, drying

After the metal container is cleaned, it often adheres to water or solvents. It should be removed as soon as possible to prevent further rusting before applying the rust inhibitor.

Commonly used drying methods include heating method, oil bath dehydration method, compressed air drying method, gasoline drainage method containing surfactant, infrared drying and the like. No matter what kind of drying method is used, the rust inhibitor should be coated when the surface of the metal container is cooled to a certain temperature, otherwise it will cause the rust inhibitor to decompose.

The surface treatment process of the metal container is the basis of rust prevention. Only when the surface of the metal container is treated cleanly and completely dried can the rust prevention function be fully exerted. Otherwise, even a rust-proof material with excellent properties will not be satisfactory. Anti-rust effect.

Second, anti-rust oil treatment technology

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 corrosion of the metal atmosphere can be achieved. Anti-rust oil packaging technology is based on this principle to prevent corrosion.

Anti-rust oil is a temporary anti-rust paint consisting of oil-soluble corrosion inhibitor and other additive components. Anti-rust oil in the oil or resin material as a film-forming material applied to the surface of the metal container, the corrosion factor has a certain degree of isolation. However, in general, oils and fats can dissolve a small amount of oxygen in the air, and can also dissolve a small amount of water. Simply using grease can not obtain a satisfactory anti-rust effect, so it is necessary to add corrosion inhibitors, which have very good anti-rust oil anti-rust effect. Great influence.

1, the role of anti-rust oil

(1) Adsorption on the surface of metal containers

Due to the fact that the surface active substances have a common molecular structure, they have two components, hydrophilic polar groups and lipophilic non-polar groups. When the anti-rust oil is coated on the surface of the metal container, the oil is dispersed slowly. The etched molecules will be adsorbed on the interface between the metal and the oil (polar base and metal adsorption, non-polar base and oil adsorption), and can form multi-layer interface films. Tests have shown that if only one or two molecular layers can be formed, the rust resistance is poor. When the adsorption layer is more than six molecules, it has good corrosion resistance.

This kind of adsorption has a shielding effect on the corrosion factor. The second is to improve the adhesion of the oil film to the metal surface. Since the surface of the adsorption film is hydrophobic, it has better water resistance and at the same time can increase the resistance of the surface of the metal container.

Many studies have shown that the chemisorption of corrosion inhibitors on the surface of metal containers (formation of coordination bonds) reduces the activity of the metal, which is one of the main reasons for rust prevention.

(2) The interfacial tension between the water droplets and the oil layer falling on the oil film can be reduced.

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 makes the water droplets spherical, so that it can easily penetrate into the oil film and reach the metal surface by gravity. Corrosion inhibitors such surface active substances can reduce the surface tension of water, so that the water droplets can not exist in the form of a spherical oil film, and tend to spread out. This reduces the pressure of the water droplet on the oil film, making it difficult to penetrate the oil film to the surface of the metal container. The flatter the water droplets are spread on the surface of the oil film, the better the oil's anti-rust effect is. 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 anti-rust oil.

The relationship between the contact angle of water droplets on the oil surface and the interfacial tension is as follows:

So-Suo=Sucosθ

Cosθ=(So-Suo)/Su

Where: So - the surface tension of the oil

Su——surface tension of water

Suo - Interfacial tension between oil and water

(3) Displacement of water

Surface active corrosion inhibitors, by virtue of their interfacial adsorption, can displace the adsorbed water on the metal surface. In addition, the water contained in the oil can be stabilized in the oil by the colloidal particles or the interfacial film of the corrosion inhibitor, so that it cannot be in direct contact with the metal.

The above-mentioned effects all have an inhibitory effect on the formation of a rusted battery on the metal surface, thereby achieving the purpose of preventing corrosion of the metal container.

2. Type of rust preventive oil

There are many types of rust preventive oils. The rust preventive oils that are applicable to metal packaging containers include anti-rust grease, solvent dilution anti-rust oil, thin-layer oil, and instrument rust preventive oil.

(1) Anti-rust grease

Anti-rust grease is a type of rust preventive oil based on petrolatum at room temperature. It is composed of film-forming substance (or base oil) and corrosion inhibitor.

Film-forming substances (or base oils): Vaseline and lubricating oils are the main products. Vaseline is generally used in industrial petrolatum. Its chemical composition is paraffin 15%, petroleum jelly 45%, cylinder oil 25%, and mechanical oil 15%. Vaseline for rust preventive oils requires no tarnish, no moisture, and a certain degree of water resistance, dropping point temperature is 45 °C. Anti-rust oil commonly used lubricants are mechanical oil, spindle oil and cylinder oil. Its chemical composition is mainly alkane, naphthenic and aromatic hydrocarbons, as well as a small amount of oxides and sulfides.

Corrosion Inhibitors: Oil-soluble corrosion inhibitors commonly used in anti-rust greases include petroleum sulfonates, aluminum stearate, zinc naphthenate, oxidized petroleum grease, lanolin and its derivatives, etc. Added benzotriazole and so on. The type of corrosion inhibitor used in anti-rust grease can affect the performance of anti-rust grease. For example, anti-rust grease added with aluminum stearate has good salt water resistance, but its ability to adhere to metal is weak; anti-rust grease with antimony oil sulphonic acid added is more resistant to salt water and can be used for rust prevention in the marine atmosphere; The anti-rust grease with lanolin and its soap added has strong adhesion to metal, and it has a certain emulsifying ability for water, and strong rust resistance; zinc naphthenate anti-rust grease has strong adhesion to metal and has certain Salt water resistance, but poor rust resistance to cast iron; anti-rust grease with oxidized petroleum jelly and its soaps is superior to fatty acid soaps, but its salt water resistance is poor; anti-salt-80- synthetic lipids The rust grease has poor thermal stability and is rusting to copper. Benzotriazole, on the other hand, has excellent anti-rust effects on copper and its alloys. In order to obtain satisfactory results in practice, several corrosion inhibitors are often used in combination.

Anti-rust grease is in an ointment at room temperature, so the film is generally thick (up to 0.5mm), not easy to lose, not volatile, after sealing the package, the general rust-proof period is longer, up to two years or more.

The method for applying the anti-rust grease is mainly a hot-dip method, ie, the anti-rust grease is heated and melted to a fluid state before coating, the cleaned, rust-removed, and dried metal container is immersed for a while, and is taken out and cooled to solidify the oil film. During hot dip coating, the thickness of the oil film formed on the surface of the metal container is different due to the dip coating temperature of the same kind of oil. The lower the temperature, the greater the thickness of the oil film and the stronger its oil resistance. Large-scale parts can be hot brushed, that is, heating and melting rust-proof grease applied to the surface of metal products with a soft brush. After the metal container is oiled, it should be timely sealed with paraffin paper or plastic bag to prevent the oil layer from drying out. And pollution packaging.

(2) Solvent dilution rust preventive oil

Solvent-diluted rust preventive oil is composed of a rust-inhibiting oil with mineral oil or resin as a film former, and other additives such as antioxidants and stabilizers. This type of anti-rust oil can be divided into three types according to the types of solvents: petroleum series solvents, organic solvents, and water dilution. Among them, organic solvents are toxic and therefore have less application. According to the nature of the oil film, such anti-rust oil can be divided into hard film oil and soft film oil.

Hard film oils - Resins used as hard film oils should first have greater solubility in gasoline and kerosene, followed by rusting of various metals. The currently used resins include t-butyl phenol formaldehyde resin (ie 2402), long oil alkyd resin, melamine formaldehyde resin, terpene resin, petroleum resin and alkyl acid resin. Of these, 2402 resins and alkyl acid resins are preferred. Hard oils commonly used are hard-1, hard-3,1,2,74A-2. Hard -1 and -3 are 2402 resin film material, but hard 1 in a larger amount of 2402, so the oil film toughness is not suitable for edge sharp metal packaging products, while in long-term sealed packaging may powder While the content of 2402 in Hard-3 is less, the addition of 389-9 alkyd resin increases the toughness of the oil film. However, hard-3 is less adaptable to non-ferrous metals such as copper alloy than hard-1. For ease of identification, there is a small amount of 3902 oil-soluble red pigments in the hard-3. 74A-2 is a hard alkyd oil based on alkyl acid amino resin and oxidized petroleum film soap. 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 winter does not crack in winter, and convenient construction, cheap, but its hard film is not easy to remove. The natural drying time of film formation is about 1 h.

Soft film oil - commonly used soft film oil has 204-1, Shanghai stone -201, F-35,112-5,704,3 anti-rust oil, 33-612 and soft -1 and so on. 204-1 oil is a sulphonated lanolin calcium soap as a film former and is also a corrosion inhibitor. Shanghai Shi-201 and F-35 are Vaseline as a film former. 112-5, 704, 3 anti-corrosion oil, 33-6, and soft-1 are all anti-corrosion oils that use oxidized petroleum jelly soap (T743 soap) as a film former and corrosion inhibitor. The main characteristic of solvent-based soft film oil is that the surface of the metal container can form a film, so the loss is small but it is not easily removed, and the rust-proof period is short.

As rust-proof oil has many deficiencies as a rust-proof seal coating, such as environmental pollution during construction, affecting the appearance of metal products, and in addition to the use of membrane. Therefore, the anti-rust coating for storage has gradually been replaced by thin films and ultra-thin films.

The film rust-preventive coating can form a complete film with a thickness of 1 22 μm or less. The film and the metal container surface have strong adhesion. The film has good rust resistance and does not affect the performance of the lubricant. And when used, do not have to remove rust. Currently, thin-film and ultra-thin-film anti-rust paints mainly include polyvinyl fluoride and polyvinyl fluoride types, and other synthetic resin types, organosilicone ammonia, and silicone resin types.

Solvent dilution type rust preventive oil is mainly used for the sealing of metal container products. It can be sprayed or dip coated (the amount of solvent can be less); then sealed with plastic bags. The main coating method of hard coating oil is sprayed with compressed air, but this spray coating method may not be able to be sprayed locally. The surface of a metal container without a rust-preventive coating will continue to rust in the air, and even rust will be faster.

(3) Thin oil

This is a rust-inhibiting oil that uses a resin as a film-forming material. It is characterized by a long rust-proof period, a thin and transparent oil film, and a good appearance. Thin oils are solvent-based and solvent-free.

Thin oils are usually dip or sprayed.