Anti-corrosion packaging technology (1)

Anti-corrosion packaging technology (1)

Corrosion of metal products is mainly caused by electrochemical corrosion. Electrochemical corrosion is mainly caused by electrochemical non-uniformity of the metal surface. When it comes into contact with the medium, it forms a rusted battery. Based on this, we can study effective packaging techniques and methods to prevent metal corrosion. Obviously, in order to prevent metal products from being rusted, the most effective anti-corrosion packaging technology is to try to eliminate the various conditions that produce rusted batteries.
There are many methods for rust prevention, which can be classified into "permanent" rust prevention and "temporary" rust prevention according to the length of the rust prevention period. "Permanent" rust prevention methods, such as changing the internal structure of the metal; alloying the metal surface; coating the metal surface (electroplating, spraying, electroless plating); applying a non-metallic coating on the metal surface (enamel, rubber, plastic, paint, etc.) Floor)……. This method can better achieve the purpose of rust prevention, but they are "permanent", the rustproof layer can not be removed, and therefore these methods are not universally used in the rustproof packaging of metal products; and "temporary" prevention Rust does not mean that the anti-rust period is short, but refers to the "temporary" of the process of metal products being transported, stored, sold, etc. to the consumer, and the "temporary" of the anti-rust layer. The "temporary" rust-preventing material can be rust-proof for months, years or even decades. The “temporary” rust-proof packaging technology is the main research object of this chapter. The process of “temporary” anti-rust packaging has three aspects: pre-treatment technology for anti-rust packaging (including cleaning, descaling, drying, etc.), anti-rust treatment technology for various anti-rust materials, and anti-rust packaging. Processing technology.
First, the pretreatment technology of metal product packaging
For various reasons, various materials such as grease, rust products, and various dusts are often formed and adhered on the surface of the produced metal products. These are all factors that cause electrochemical corrosion. Therefore, when metal products are rust-proof packaged, they must be cleaned, derusted, dried, etc.
1. Cleaning of metal products
Common cleaning methods for metal products are mainly alkali solution, surfactant method and organic solvent method.
(1) lye method
An aqueous solution of the alkali can wash away the oil on the metal surface, which is one of the commonly used cleaning methods. The bases which can be used for metal cleaning are sodium hydroxide, sodium carbonate, trisodium phosphate, sodium pyrophosphate, sodium hexametaphosphate, water glass and the like.
Sorry for the saponification reaction of mineral oil, such as the use of caustic cleaning effect is not good, the use of sodium silicate, sodium phosphate and sodium carbonate and other weak base as the main component with surfactant alkaline cleaning solution.
Alkaline cleaning, the type of alkali used is generally selected according to the metal material and the type of grease to be attached.
The main advantage of lye cleaning is that the oil washing effect is good. Even products with heavy oil can be washed, non-oily dirt can be washed off at the same time, and the lye can be used repeatedly, which is economical. The disadvantage is that if the control is not good, it may cause rust or discoloration of the metal product.
(2) Surfactant method
Surfactant refers to a kind of organic substance having two parts of a hydrophilic structure and a hydrophobic group in a molecular structure, and this special structure makes them have a special dispersibility in an aqueous solution--concentrating in a concentrated manner in a solution. Surface or interface (such as the interface between oil and water), and can reduce surface tension and interfacial tension. Therefore, they have the functions of wetting, infiltration and emulsification, washing and the like. There are many kinds of surfactants, such as fatty acid salts such as soap, sulfonates such as synthetic detergent alkyl sulfonate, alkyl trimethylamine chloride, 6501 cleaning agent, flat cleaning agent, TX-10 cleaning. Agent, 6503 cleaning agent, 105 (R-5) cleaning agent, 664 cleaning agent, and the like.
The surfactant cleaning method is characterized by safe operation, good washing effect, and can also clean non-greasy dirt, and has no obvious rusting effect on metal, so it is more suitable for metal precision products.
(3) Organic solvent method
The organic solvent method is a method of washing the surface of a metal product by using an organic solvent having a strong dissolving power to oil. 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 that the effect is good. A small amount of metal products need no heating when cleaning, and can be washed by soaking or scrubbing, and has no rust to metal. However, it is necessary to clean the bulk of metal products (such as washing machines, etc.). The disadvantage is that the petroleum series solvent is easy to burn and fire. At the same time, due to the large amount of heat absorbed during the evaporation of the metal surface, the temperature of the metal may be significantly lowered. In the high-humidity environment, the surface of the product may be dew condensation, which may cause rust.
(4) Other cleaning fluids
Complex cleaning solution. A complex cleaning solution can be used for the dirt which is difficult to remove by the above three methods. Commonly used aminocarboxylate complexing agent: ethylenediaminetetraacetic acid or ethylenediaminetetraacetic acid disodium salt. They all form soluble chelates with the soil and leave the soil off the metal surface.
Human sweat cleaning solution. Human sweat stains can be cleaned with hot methanol or human sweat replacement rust inhibitor.
There are also ultrasonic purification, steam purification, electrolytic purification, etc. These are relatively 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 by two or more cleaning solutions.
2. Derusting of metal products
In the actual rustproof packaging, the rust removing process and the cleaning oil staining work are often combined, that is, a rust removing agent is added to the cleaning liquid. The rust removal method of metal products can be divided into physical mechanical rust removal method and chemical rust removal method.
(1) Physical mechanical derusting method
a. Manual descaling method: rust removal with steel brush, iron shovel, shovel (scraper), gauze, sandpaper, etc. This method is simple, but not suitable for rust removal of small and large products.
b. Mechanical descaling method: There are spraying method and grinding wheel, cloth wheel descaling method.
The spraying method is a method in which the sand is sprayed on the metal surface with strong force, and the rust is removed by the action of impact and friction. According to the spray material, the spray method can be divided into sandblasting method (using sea sand, river sand, and stone particles as spraying materials), steel particle spraying method (using small steel bullets or crushed steel particles as spraying materials), and soft particle spraying method ( Plant seeds or plastic pellets are used as the spray material). According to the injection mode hall, it is divided into a power injection method (a method of spraying a dry spray material with high-pressure air), a wet spray method (a method of mixing fine sand particles with water into a slurry form by high-pressure air injection), a vacuum injection method, and the like.
The spray method is suitable for the descaling of large products or metal materials, and requires a jetting machine. In the wet process, a water-soluble corrosion inhibitor is also added to the water. The advantages are high rust removal efficiency and low cost.
The grinding wheel descaling method can only be used on non-machined surfaces. The cloth wheel descaling method is a product whose surface plating or surface finish is high in requirements such as copper iron or non-ferrous metal.
(2) Chemical descaling method
The chemical rust removal method includes pickling and alkali washing and rust removal (alkali electrolysis, alkali reduction, lye boiling, etc.), and the most widely used one is the pickling method.
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 rust product to make the insoluble rust product into a soluble substance and is dissolved in the aqueous solution from the metal surface.
The acids used for pickling mainly include sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, hydrofluoric acid and the like. Among them, the hydrochloric acid solution rust-removing package is the strongest product; the sulfuric acid produces hydrogen gas with large mechanical effect and low price, and is widely used for steel rust removal. Nitric acid and hydrofluoric acid can be used for derusting non-ferrous metals such as aluminum products. Compared with hydrochloric acid sulfuric acid, phosphoric acid has poor rust removal ability, but has weak rust property, and can react with copper-iron surface to form an insoluble film of iron phosphate salt. After washing, it has a temporary anti-rust effect in the air.
Compared with the physical mechanical method, the main advantage of the pickling method is that it does not cause deformation of the metal material, the surface to be treated is not rough, the operation is simple, the efficiency is high, the rust of all corners of the metal product can be removed, and it is suitable for the rust removal of a large number of small products, and No special equipment is required and the cost is low. Therefore, it is a commonly used chemical descaling method, but it has a rusting effect on metals, which is prone to "hydrogen embrittlement" and affects surface finish. In recent years, alkali washing and rust removal have been developed, and alkali washing and rust removal are carried out in a solution containing caustic alkali, carboxyacetic acid, a complexing agent, and a foaming agent. Alkaline washing method does not rust the base metal, does not occur "hydrogen embrittlement", the metal surface is smooth, suitable for non-ferrous metals such as steel and copper and magnesium.
3, dry
The metal surface is often adhered to moisture or solvent after cleaning, and should be removed as soon as possible to avoid rusting before the rust inhibitor can be applied.
Common drying methods include heating, oil bath dehydration, compressed air drying, gasoline drainage with surfactants, infrared drying, and the like. No matter what kind of drying method is used, the rust inhibitor should be applied when the metal surface is cooled to a certain temperature, otherwise the rust inhibitor will be decomposed.
The metal surface treatment process is the basis of rust-proof packaging. Only when the metal surface is treated very cleanly and completely dry, can the rust-preventing material be fully utilized. Otherwise, even a rust-proof material with excellent performance cannot be satisfactorily rust-proofed. effect.