Moving Forward Every Day — 3/9/2010

Answer: Phosphating the steel surface can affect the weight of the resulting phosphate film due to differences in solution composition, processing methods, and mixing intensity; the weight of this phosphate film can range from 0.1 to 45 g/m2. For most phosphate films (with thicknesses of 1–50 μm), the ratio of film weight to film thickness is 1.5–3.5, while for thin and medium-thickness films it is 1.5–2. When the phosphate film is heated, crystal dehydration occurs; for example, when heated at 155°C, Zn3(PO4)2·4H2O loses 2 molecules of crystalline water, causing the crystal to become opaque. A 10μm thick phosphate film has a resistance of about 5×107Ω, making it a very poor electrical conductor. If the phosphate film is coated with oil or varnish, its insulating properties will be higher. Fe3O4 and Fe2O3 present in the sub-transformed phosphate film are semiconductors, whereas Fe3((PO4)2·8H2O is a poor conductor. The phosphate film has a good absorption capacity for oils and soaps. This property not only improves the protective capabilities of the membrane but also enables its use in certain special applications, such as cold forming and electrical insulation. The absorption capacity should be related to the pore structure of the membrane, but generally speaking, the porosity of phosphate membranes is not high, accounting for approximately 0.5%-1.5% of the membrane area. Therefore, the good absorption performance of the membrane cannot be simply attributed to the capillary adsorption phenomenon at the membrane pores. As for the phosphate film itself, its corrosion resistance is not high; when subjected to a salt spray test using a 3% NaCl solution, corrosion and rust appear on the surface of the film after just a few hours. In acidic and alkaline solutions, phosphate films are easily dissolved; even solutions of weak acids and weak bases can cause significant changes in the film. However, composite coatings composed of a phosphate film and other coating layers (oils or paints) provide good protection for the base metal. The applications of phosphating coatings on steel parts are as follows: ① As a base layer under paint on the surface of steel parts; ② For steel parts that require electrical insulation; ③ For protecting the interior of ducts or parts with complex shapes; ④ As a lubricant during cold heading; ⑤ For parts operating in environments with lubricating oils; ⑥ For protecting assemblies where electroplating is not allowed; ⑦ For protecting high-strength steel; ⑧ For parts that have undergone local nitriding, to prevent the nitrided areas from sticking to tin.

The process of chemically treating steel parts in a phosphate solution containing manganese, iron-zinc, and calcium to form an insoluble phosphate protective film on their surface is called phosphating. Performance: 1. It remains stable under atmospheric conditions, and its corrosion resistance is higher compared to steel after oxidation treatment – about 2-10 times higher. Further improvement in corrosion resistance can be achieved through chromate filling, oil immersion, or painting. 2. It has a microporous structure, providing excellent adsorption capacity for oils and paints. 3. Has no adhesion to molten metal. 4. The phosphating film has excellent electrical insulation properties. 5. The thickness is generally 10–20 μm; since the phosphating film is formed along with the dissolution of iron, there is little change in its dimensions. Uses: 1. Corrosion prevention. 2. Apply a coating as a base layer to improve lubricity; in cold deformation processing, this helps to reduce oxidation and friction, thereby minimizing processing cracks and surface scratches. 3. It is used to prevent the adhesion of low-melting-point molten metal. 4. The silicon steel sheets used in transformers, the rotors and stators of motors, and other electromagnetic devices are treated with phosphating, while the basic mechanical properties, strength, and magnetic properties of the base metal remain unchanged.

The phosphating film is dark gray or black in color, has a porous structure, and exhibits good corrosion resistance after being filled, impregnated with oil, or coated. Due to its excellent adsorption capacity and lubricity, the phosphating coating is widely used as a base layer for coatings and as a lubricant layer during the cold heading and cold extrusion of parts, thereby reducing surface scratches and cracks. The phosphating layer can also serve as an electrical insulating layer for silicon steel sheets, preventing parts from sticking to molten metals with low melting points and avoiding adhesion between die-cast parts and the molds.

The chemical treatment of steel parts in solutions containing zinc, manganese, iron, or alkali metal phosphates, resulting in the formation of a water-insoluble phosphate film on their surface, is known as phosphating. The film that is formed is called a phosphating layer, and this layer can be divided into pseudo-transformed films and transformed films. Phosphating coatings can be classified by coating thickness into the low-density category, with a thickness of only 0.4–1 g/m2; these are generally amorphous ferrous-based coatings that are suitable as a base layer for painting or powder coating ； Lightweight, 1.1–4.5 g/m2, widely used as a coating base layer ； Light weight grade, 4.6–7.5 g/m2, can be used for corrosion protection, cold working, and anti-friction lubrication ； Heavy-duty, greater than 7.5 g/m2, used for corrosion protection, insulation, and wear reduction in cold working. It can be seen that the function of the phosphate film is to serve as a base layer for painting, to prevent corrosion, to lubricate the surface, and to improve efficiency in cold working processes. Based on the phosphating temperature, it can be divided into low temperature (15–35°C) ； Room temperature (35–45°C) ； Moderate temperature (50–70°C) ； The applications, types, and thickness selection of phosphate coatings at high temperatures (above 80°C) are shown in the table below. Depending on the solution used for the phosphating treatment of metals, two types of phosphate coatings can be formed on them, which differ in both chemical composition and structure: chemically converted phosphate coatings and pseudo-converted phosphate coatings. The former is a product of the natural transformation of the metal surface, consisting of FePO4 and Fe2O3 ； The latter mainly consists of the hydrolysis products of heavy metal dihydrogen phosphates present in solution, and is composed of MeHPO4 crystals (where Me refers to divalent heavy metals such as Mn and Zn). Naturally, these two different types of phosphate films should each have their own unique film-forming process. The application areas of the phosphating coating on steel parts are as follows: ① The underlying layer for painting on the surface of steel parts ； ② Steel parts requiring electrical insulation ； ③ Protection of the lumen of a catheter or parts with complex shapes ； ④ Lubrication during cold heading ； ⑤ Parts operating under lubricating oil conditions ； ⑥ Protection for assemblies that do not allow electroplating ； ⑦ Protection for high-strength steel ； ⑧ For parts with local nitriding, to prevent tin adhesion at the nitrided areas.

Functions of phosphating: 1. Improve corrosion resistance: Although the phosphating film is thin, it acts as a non-metallic, non-conductive insulating layer that transforms the good conductors on the surface of the metal workpiece into poor conductors. This prevents the formation of microelectrodes on the surface of the metal workpiece, thereby effectively stopping the corrosion of the coating. 2. Improve the adhesion between the substrate and the coating, or between other organic finish layers. 3. Provide a clean surface. 4. Improve the cold working properties of materials, such as drawing, tube drawing, extrusion, etc. 5. Improve surface friction properties to facilitate sliding.

Phosphating the steel surface can affect the weight of the resulting phosphate film due to differences in solution composition, processing methods, and mixing intensity; the weight of this phosphate film can range from 0.1 to 45 g/m2. For most phosphate films (with thicknesses of 1–50 μm), the ratio of film weight to film thickness is 1.5–3.5, while for thin and medium-thickness films it is 1.5–2. When the phosphate film is heated, crystal dehydration occurs; for example, when heated at 155°C, Zn3(PO4)2·4H2O loses 2 molecules of crystalline water, causing the crystal to become opaque. A 10μm thick phosphate film has a resistance of about 5×107Ω, making it a very poor electrical conductor. If the phosphate film is coated with oil or varnish, its insulating properties will be higher. Fe3O4 and Fe2O3 present in the sub-transformed phosphate film are semiconductors, whereas Fe3((PO4)2·8H2O is a poor conductor. The phosphate film has a good absorption capacity for oils and soaps. This property not only improves the protective capabilities of the membrane but also enables its use in certain special applications, such as cold forming and electrical insulation. The absorption capacity should be related to the pore structure of the membrane, but generally speaking, the porosity of phosphate membranes is not high, accounting for approximately 0.5%-1.5% of the membrane area. Therefore, the good absorption performance of the membrane cannot be simply attributed to the capillary adsorption phenomenon at the membrane pores. As for the phosphate film itself, its corrosion resistance is not high; when subjected to a salt spray test using a 3% NaCl solution, corrosion and rust appear on the surface of the film after just a few hours. In acidic and alkaline solutions, phosphate films are easily dissolved; even solutions of weak acids and weak bases can cause significant changes in the film. However, composite coatings composed of a phosphate film and other coating layers (oils or paints) provide good protection for the base metal. The applications of phosphating coatings on steel parts are as follows: ① As a base layer under paint on the surface of steel parts; ② For steel parts that require electrical insulation; ③ For protecting the interior of ducts or parts with complex shapes; ④ As a lubricant during cold heading; ⑤ For parts operating in environments with lubricating oils; ⑥ For protecting assemblies where electroplating is not allowed; ⑦ For protecting high-strength steel; ⑧ For parts that have undergone local nitriding, to prevent the nitrided areas from sticking to tin. 1# mopeizhi

Properties of the phosphating film: It has strong adhesion to the base metal, forms a firm bond, is dense and porous, and exhibits strong adsorption capacity for coatings. Therefore, it can serve as a base for common coatings, enabling a very strong bond between the phosphating layer and the coating layer as well as improved corrosion resistance. In the atmospheric environment, after post-treatment, its corrosion resistance is superior to that of the oxidized blackening coating. During the cold extrusion of ordinary steel, the temperature rise caused by deformation can reach 300°C, which is well below the thermal threshold of the phosphating layer; therefore, the phosphating layer exhibits good resistance to thermal adhesion. In the cold extrusion process, under certain temperature conditions, a chemical reaction occurs between the phosphating film and the lubricant (soap solution or emulsifier). Some fatty acid soaps react with zinc phosphate Zn3(Po4)2 in the phosphating film, resulting in the formation of zinc fatty acids Zn(RCOO)2, which possess excellent lubricating properties and thus enhance the lubrication effect. The phosphating film has no adhesion to molten metal, and can be used to prevent parts from sticking to molten metals with low melting points. It can be used to prevent parts from being coated with tin during local nitridation, and it also helps to avoid adhesion between die-cast parts and the molds. It reduces friction and prevents or minimizes surface scratches or cracks, thereby extending the lifespan of the molds. The phosphating film also possesses high electrical insulation properties, and can be used as an electrical insulating layer for silicon steel sheets. After phosphating treatment, the mechanical properties and magnetism of steel remain essentially unchanged. Application range: 1. It is widely used to form a protective film on the inner surfaces of pipes, gas cylinders, and steel parts with complex shapes, as well as on the surfaces of parts where it is difficult to apply a protective layer using electrochemical methods. It can be applied in the automotive industry, for gears, radiators, brake pads, joints, tubular components, screws, nuts, etc. Petroleum industry, pipelines, various valves, pump casings, shaft sleeves, blades, etc. 2. The hardware, mold, and machinery manufacturing industries: various molds, hardware decorative parts, washing machine casings, flat panels, measuring tools, electric fan blades, bicycle parts, sewing machine parts, etc. 3. Arms industry, bearings, guns and cannons, pump components for ships, metal parts for ships, etc. 4. Aerospace industry, bearing journals, pistons, landing gear components, brackets, struts, etc. 5. Coating industry: to enhance the adhesion of organic coatings. 1# mopeizhi

1. Phosphating film for corrosion protection 2. Phosphating film for paint primers 3. Used for lubrication during cold working 4. For reducing friction 5. Phosphating film for electrical insulation

After degreasing and derusting, the surface of the metal is usually subjected to chemical treatment in order to prevent rust from forming again; this treatment creates a protective layer on the metal surface. This layer is typically only a few micrometers thick and serves mainly to enhance the adhesion between the coating and the substrate. A thicker layer can also improve the rust-resistant properties. The common surface chemical conversion methods are oxidation, phosphating, and passivation. Among them, phosphating is the key step in chemical treatment; it is a simple, reliable, cost-effective, and easy-to-operate process method that significantly enhances the corrosion resistance of metal workpieces, and it is widely used in industry. 　　1. Standards related to the phosphating process Metal (mainly steel) is treated with a solution containing phosphates such as zinc (Zn), manganese (Mn), chromium (Cr), and iron (Fe), resulting in the formation of an insoluble phosphate film on the surface of the base metal; this process is known as phosphating. Phosphating forms a well-adhered protective film on the metal surface; taking zinc phosphate as an example, the phosphating film formed in the presence of an oxidizing agent is a crystal of Zn3(PO4)2·4H2O and Zn2Fe(PO4)2·4H2O. This phosphating film is shiny and gray, porous (with a porosity of 0.5%–1.5% of the surface area), and its thickness is usually between 0.1 and 50 μm. 　　Regarding the phosphating process, there are corresponding standard systems both in China and internationally that can be referred to: GB/T11376—1997 Phosphate conversion coatings on metals; GB/T6807—2001 Technical requirements for phosphating treatment of steel workpieces prior to painting; GB/T12612—1990 General technical requirements for multi-functional surface treatment fluids for steel; ISO 9717—1990 (E) Phosphate conversion coatings on metals – Methods for determining requirements; ISO 10546—1993 (E) Chemical conversion coatings – Chromium oxide conversion coatings with and without rinsing on aluminum and aluminum alloys; DIN 50942—1973 Phosphating treatment of metals – Principles, abbreviations, and testing methods; ANSI/ASTM/AMS 2480C Phosphating treatment of substrates for painting. 2. Functions of phosphating: Phosphate conversion coatings are applied to iron, aluminum, zinc, cadmium, and their alloys. They can serve as the final finishing layer or as an intermediate layer under other coating layers. Their main functions include the following. 　　2.1 Improving corrosion resistance Although the phosphating film is thin, it acts as a non-metallic, non-conductive insulating layer that transforms the good conductors on the surface of the metal workpiece into poor conductors. This prevents the formation of microelectrodes on the surface of the metal workpiece, thereby effectively stopping the corrosion of the coating. Table 1 lists the effect of the phosphating film on the corrosion resistance of metals. 　　2.2 Improving adhesion between the substrate and the coating, or between other organic finish layers. The phosphating layer forms a tightly bonded integral structure with the metal workpiece. There is no clear boundary in between. The porosity of the phosphating film allows sealants, coatings, etc. to penetrate into these pores and bond tightly with the phosphating film, thereby improving adhesion. 　　2.3 Providing a clean surface The phosphating film can only form on metal surfaces that are free from oil and rust. Therefore, metal parts that have been phosphated have a clean, uniform surface free from oils and rust. 　　2.4 Improving the cold working properties of materials, such as drawing, tube drawing, extrusion, etc. 　　2.5 Improve surface friction properties to facilitate sliding.