Background and overview[1][2]
Brush phosphating is a special pre-treatment method, that is, the brush phosphating agent is in contact with the metal surface by brushing (wiping) or soaking. After a complex chemical reaction and electrochemical reaction, the phosphating agent is applied on the metal surface. An insoluble dense covering film with a certain thickness (film weight 2 to 6 g/m2) with phosphate as the main component is formed. Its particularity is that the brush-coating phosphating agent can be carried out on the surface of workpieces with oil rust, so the brush-coating phosphating agent must have excellent oil and rust removal functions.
The quality of brush-coated phosphating film depends on its oil removal, rust removal and phosphating effects. In order to obtain a high-quality brush-coated phosphating film, the degreasing agent, rust remover and phosphating agent must be scientifically optimized and combined. This is also one of the difficulties in developing brush-coated phosphating agents. Because brush phosphating has unique advantages (simplifying the process, reducing equipment and working area, shortening working hours, improving labor productivity, on-site operations, etc.), it is still widely used in a certain range (such as large steel components or local Phosphating, etc.).
Category[3]
1. Low-temperature zinc phosphating agent for priming before various types of painting
Liquid agent and powder products can be immersed or sprayed for phosphating treatment. They are suitable for automobile and electrical appliance coating. They have the characteristics of fast phosphating speed, less sediment, wide application range and easy use: ultra-low temperature rapid phosphating, It does not require heating, can be soaked or sprayed, is simple to manage, and is suitable for all types of workpieces and processes; low-temperature powder type, containing accelerator, the powder is easy to transport and store, the bath liquid has a long service life, the number of additions is small, and it is simple and convenient to use. Suitable for all types of iron, zinc and aluminum workpieces and processes.
2. Zinc-manganese-nickel ternary system phosphating agent for high-end coating primer
Film, the phosphating film is dense and has good alkali resistance. It is suitable for low temperature, medium temperature and wide temperature conditions, and can be used for spraying and soaking workpieces. Suitable for all types of iron, zinc and aluminum workpieces and processes, especially high-end coating processes.
3. Painting, anti-rust, wear-resistant, friction-reducing
The phosphating agent used for priming before painting has fast phosphating speed, good corrosion resistance, and good compatibility with paint; anti-rust phosphating has good corrosion resistance, and copper sulfate dripping can last 5-30 minutes; used Anti-friction, lubrication and phosphating of cold drawn and cold extruded parts; used for wear-resistant, anti-friction and phosphating of precision parts, loaded and moving bodies.
Composition[1][2]
1. Basic film-forming substances of phosphating solution
The phosphating film is deposited from metal phosphate, and its main component is metal phosphate. Different types of phosphating solutions have different compositions. For zinc-based phosphating of iron and steel, the film-forming components of the phosphating solution mainly include free phosphoric acid and zinc dihydrogen phosphate; for manganese-based phosphating, the basic film-forming components of this type of phosphating solution include phosphoric acid and acid manganese phosphate; zinc calcium phosphorus The phosphating liquid contains free phosphoric acid, zinc dihydrogen phosphate and calcium dihydrogen phosphate.
2. Phosphating accelerator
1) Oxidation accelerator: Commonly used oxidation accelerators include NO2- and NO3-. NO2- is often used in conjunction with NO3-. Nitro compounds such as trinitrophenol, nitro aromatic compounds, etc. can be used as accelerators. Sodium nitrobenzene sulfonate (SNBS) is commonly used, but its cost is high and must be combined with other accelerators such as NO2-, NO3- , ClO3- used together. When ClO3- is used as a phosphating accelerator, if the concentration is too high, the phosphating solution becomes unstable and is prone to excessive precipitation, causing passivation of the steel surface and yellowing of the film, thus reducing the corrosion resistance of the phosphating film. Generally only used for zinc-based phosphating.
H2O2 is the strongest accelerator in industry. The reduction product is water, but it is unstable and not commonly used. MoO4- is an ideal phosphating accelerator. It has the advantages of less sediment, stable solution, long service life, and easy adjustment[. It also has the function of accelerating the film formation speed, reducing the film weight, and improving the corrosion resistance of the phosphating film. At the same time, it can form complexes with metal ions to reduce the formation of thick phosphating films caused by the increase of metal salts. Used alone, it can quickly form a thin and dense film layer. It can directly participate in film formation and become a component of the film without adjusting the surface of the metal workpiece before phosphating, thereby reducing the consumption of active ingredients in the phosphating solution.
2) Metal accelerator: Metal ions in the phosphating solution are conducive to the generation of crystal nuclei and grain refinement, and are conducive to accelerating the phosphating process at room temperature. Common metal accelerators include Cu2+, Ni2+, Mn2+, Ca2+, etc. A very small amount of Cu2+ can greatly increase the phosphating speed, but its dosage must be controlled, otherwise a CuO film will be formed and affect the formation of the phosphating film; Ni2+ is the most effective and commonly used phosphating accelerator, and its advantage is that it can accelerate phosphating , refine the crystal and improve the corrosion resistance of the phosphating film.
Mn2+ can reduce the phosphating treatment temperature, increase the reaction speed, reduce the film thickness, reduce the amount of metallic nickel, and improve the wear resistance of the substrate surface. Ca2+ can refine grains, but requires higher temperatures and is not suitable for room temperature phosphating. Metal ion accelerators can be used alone or in combination to achieve good synergistic effects.
3) Film-forming aid: The buffer mainly plays a role in stabilizing the pH value of the system during the phosphating process, and also partially complexes zinc and iron ions. F- can be used as a buffer and can complex with Al3+ to refine crystallization. Tartaric acid, sodium citrate, and sodium pyrophosphate can also play a complexing role. Acid and free iron ions are easy to remain in the phosphating film, which reduces the corrosion resistance of the film and is prone to yellowing.
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Medium-temperature phosphating has faster phosphating speed, finer phosphating crystals, and good corrosion resistance. However, dipropylene glycol butyl ether consumes a lot of energy and the phosphating film is still thick. The gloss of the coating film after painting is not good. Generally suitable for automobile corrosion-resistant protective layer and the bottom layer of spray and paint.
3. Low temperature phosphating
The film formation rate of calcium-based phosphating liquid is slow and usually requires medium and high-temperature treatment. Zinc-calcium low-temperature black phosphating is achieved by adding Mn2+, Ni2+, oxidants, surfactants, blackening agents, etc. to achieve formation at low temperatures. It is a black phosphating film with excellent performance, dense film layer and strong corrosion resistance. Its common process specifications are:
The power of low-temperature phosphating film formation mainly relies on accelerators and other substances in the formula. The formed phosphating film is thin and dense, flat and smooth, has stable bath liquid, low energy consumption, less sediment, easy maintenance, and low overall cost of use. It is currently the main technology for coating bottom layer treatment at home and abroad. However, the quality of the phosphating film formed by low-temperature phosphating is unstable, and bath management is complicated.
Medium- and high-temperature phosphating has the disadvantages of high energy consumption, large corrosion to equipment, large amounts of sediment, simple process, high cost, low efficiency, and unfavorable for automated production. However, the phosphating film has good corrosion resistance and fast phosphating speed. . Low-temperature phosphating saves energy and has the characteristics of low cost, stable solution, less sediment, etc. However, the phosphating film formation speed of low-temperature phosphating is slow, the composition of the phosphating solution is complex, and the quality of the phosphating film is poor, so it is not suitable for large-volume products.
Application examples[1][2]
1) Anti-rust film: The design of various hot-rolled steel accessories for forklifts requires anti-rust phosphating. In particular, the oil-free phosphating film is required to have excellent anti-rust properties, so the forklift parts factory chooses phosphating agents. Extremely strict. A factory once used a commercially available corrosion-resistant phosphating agent, but the phosphating film had poor rust resistance and could not be used. In recent years, 05-02-1 multifunctional brush-coating phosphating agent has been screened. It adopts the three-step phosphating process described in the article. It is easy to construct, has reliable quality of phosphating film, and can meet the rust prevention requirements (≥6 months).
2) Coating base layer: Most of the parts of an air compressor factory use room temperature zinc phosphating before painting, but a small number of hot-rolled steel parts choose brush phosphating. It is recommended to use 05-02-1 multifunctional brushing phosphating agent, and then paint again 24 hours after brushing. The manufacturer has tested that the coating adhesion is good and the overall corrosion resistance of the coating is high. The base of a large-scale embroidery machine produced by an embroidery machine manufacturer is made of thick hot-rolled steel plates welded together. Since the base is heavy, on-site pre-treatment is carried out before painting, and the 05-02-1 function is used to brush the phosphating agent.
The brushing process is: brushing to remove rust and cleaning the surface (manual brushing and cleaning with a wire brush) → brushing with multi-functional phosphating agent (repeatedly brushing 2 times) → natural drying (surface drying) → Brush off loose objects (same as above) → paint. After use, the paint adhesion is greatly enhanced than before use.
Main reference materials
[1] Lin Mei, Yan Guiyang. Progress in the development of special phosphating agents for automobile panels [J]. Chemical Engineering and Equipment, 2012 (7): 142-145.
[2] Tang Chunhua, Tang Bin. Development and application of multifunctional brush phosphating agent[J]. Electroplating and Finishing, 2006, 25(5): 24-25.
[3] Phosphating agent series
