The corrosion of metal products is mainly caused by electrochemical corrosion. Electrochemical corrosion is mainly due to the electrochemical non-uniformity of the metal surface. When it comes in contact with the medium, a rusted battery is formed. Based on this, we can study effective packaging technologies and methods for preventing metal corrosion. Obviously, in order to prevent metal 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)……. This method can achieve the purpose of rust prevention, but they are "permanent" and the rust-proof layer cannot be removed. Therefore, these methods cannot be widely used in the rust-prevention packaging of metal products; and "temporary" prevention Rust 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 “temporary†nature of the rust-proof layer. The rust-proof period of "temporary" rust-proof materials can reach several months, years or even more than ten years. "Temporary" rust-proof packaging technology is the main research object of this chapter. 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 product packaging For various reasons, the production of metal products often produce and adhere to the surface of a variety of substances, such as grease, rust products and a variety of dust. 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. Cleaning of metal products The common cleaning methods for metal products are mainly lye method, surfactant method and organic solvent method.
(1) Alkali solution Aqueous solution of alkali can wash off the oil stain on the metal surface, 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 class of organic substances having two parts, a hydrophilic group and a hydrophobic group, on the molecular structure. This special structure makes them have special dispersibility in aqueous solution-- It absorbs on the surface or interface of the solution (such as oil-water interface) in a concentrated and orientation manner, and can reduce the 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, 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 with an organic solvent having a relatively 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 products need not be heated during cleaning, and can be washed with immersion or scrubbing, and have no rusting property to the metal. However, cleaning of large quantities of metal products 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 may be significantly reduced due to the absorption of a large amount of heat when the metal surface is volatilized. In a high-humidity environment, condensation may occur on the surface of the product and cause rust.
(4) other cleaning fluid complex cleaning fluid. 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 metal surface.
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 products In the actual rust-proof packaging, the derusting process and the cleaning oil pollution work are often combined, that is, the derusting agent is added to the cleaning fluid. The rust removal methods of metal products can be divided into physical and mechanical rust removal methods and chemical rust removal methods.
(1) Physical and mechanical derusting method a. Manual derusting method: Derusting with 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 the derusting of large-scale products or metal materials and requires spraying 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 removing method Chemical rust removing method includes pickling and alkaline rinsing (alkali liquid electrolysis, alkali reduction, lye boiling, etc.), among which the acid washing method is 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.
Comparing acid pickling method with physical mechanical method, the main advantage is 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 all corners of the metal product can be removed, which is suitable for derusting of a large number of small products, and Without special equipment, the cost is low. Therefore, it is a commonly used chemical rust removal method, but it has a rusting effect on metals 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, dry metal surface is often attached to water or solvent cleaning, it should be removed as soon as possible to avoid further rust, and then coated anti-rust agent.
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 metal surface is cooled to a certain temperature, otherwise it will cause the rust inhibitor to decompose.
The metal surface treatment process is the basis of rust-proof packaging. Only when the metal surface treatment is very clean and completely dry, can the full play of the role of rust-proof materials, otherwise even if the rust-proof material with excellent performance can not be satisfied with rust effect.
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