Phosphating treatment of high-pressure bolts

Phosphating is a common surface treatment, primarily aimed at preventing rust.

I think phosphating treatment can increase the surface hardness of bolts

I’m giving you the materials downloaded from the Internet; it’s like offering flowers to another deity. For reference only: Phosphating is a process of chemical and electrochemical reaction that results in the formation of a phosphate-based chemical conversion coating; the coating formed in this way is known as a phosphating coating. The main purpose of phosphating is to provide protection for the base metal, thereby preventing corrosion of the metal to a certain extent ； Used as a primer before painting to improve the adhesion and corrosion resistance of the paint coating ； It is used as an anti-friction lubricant in metal cold working processes. The phosphating treatment process has been in use in industry for over 90 years, and can be roughly divided into three periods: the period when the foundations of phosphating technology were laid, the period of rapid development of this technology, and the period of its widespread application. 　　Yun Qing provides technical support. Phosphating films are used as corrosion-resistant protective coatings for steel; the earliest reliable record of their use is a patent granted to Charles Ross in the UK in 1869 (B.P.No.3119). From then on, the phosphating process was applied in industrial production. Over the course of nearly a century, phosphating treatment technology has accumulated extensive experience and seen many significant discoveries. During World War I, the center of development for phosphating technology shifted from Britain to the United States. In 1909, T.W. Coslet in the United States created the first zinc-based phosphating solution by dissolving zinc, zinc oxide, or zinc phosphate salts in phosphoric acid. This research achievement **has promoted the development of phosphating processes and broadened their future prospects.** The phosphating solution Parco Power, developed by Parker Anti-Rust Company, overcomes many of the disadvantages of traditional phosphating processes and reduces the phosphating time to 1 hour. In 1929, the Bonderizing phosphating process reduced the phosphating time to 10 minutes. In 1934, a revolutionary development took place in phosphating technology: the method of spraying the phosphating solution onto the workpieces was adopted. After the end of World War II, there were few breakthroughs in phosphating technology; it merely developed and improved steadily. Phosphating is widely used in corrosion prevention technology and the metal cold deformation processing industry. The major improvements in phosphating technology during this period included: low-temperature phosphating, various methods for controlling the thickness of the phosphating film, and high-speed phosphating of continuous steel strips. Currently, research in the field of phosphating technology focuses on improving quality, reducing environmental pollution, and saving energy. 　　(II) Phosphating is a commonly used pre-treatment technique; in principle, it falls under chemical conversion coating treatment. It is primarily applied to the phosphating of steel surfaces, and it can also be used for non-ferrous metal parts such as aluminum and zinc. 　　(III) Basic Knowledge of Phosphating Principles of Phosphating 1. Phosphating The process in which a workpiece (made of steel, aluminum, or zinc) is immersed in a phosphating solution – a solution primarily composed of certain acid phosphates – to form an insoluble crystalline phosphate conversion coating on its surface is known as phosphating. 　　2. Principle of phosphating Steel parts are immersed in a phosphating solution (an acidic aqueous solution composed of Fe(H2PO4)2, Mn(H2PO4)2, and Zn(H2PO4)2, with a pH value of 1–3 and a relative density of 1.05–1.10). The reaction for the formation of the phosphating film is as follows: Endothermic 3Zn(H2PO4)2 → Zn3(PO4)2↓ + 4H3PO4 or Endothermic 3Mn(H2PO4)2 → Mn3(PO4)2↓ + 4H3PO4 Steel workpieces are steel alloys; under the action of phosphoric acid, Fe and FeC3 form numerous galvanic cells. In the anodic region, iron begins to dissolve into Fe2+, while electrons are released. 　　Fe + 2H3PO4 → Fe(H2PO4)2 + H2↑
Fe → Fe2+ + 2e-
In the solution near the surface of steel parts, the concentration of Fe2+ increases continuously. When the concentrations of Fe2+ and HPO42-, PO43- exceed the solubility product of phosphates, precipitation occurs, forming a phosphating film on the surface of the parts:
Fe(H2PO4)2 → FeHPO4↓ + H3PO4
Fe + Fe(H2PO4)2 → 2FeHPO4↓ + H2↑
3FeHPO4 → Fe3(PO4)2↓ + H3PO4
Fe + 2FeHPO4 → Fe3(PO4)2↓ + H2↑
A large amount of hydrogen is released in the cathodic area:
2H+ + 2e- → H2↑
O2 + 2H2O + 4e- → 4OH-
Overall reaction equation: Endothermic
3Zn(H2PO4)2 → Zn3(PO4)2↓ + 4H3PO4 (Endothermic)
Fe + 3Zn(H2PO4)2 → Zn3(PO4)2↓ + 2FeHPO4↓ + 3H3PO4 + 2H2↑ (Exothermic)
Classification of phosphating processes:
1. Classification by phosphating temperature:
(1) High-temperature type: Treatment at 80–90°C for 10–20 minutes; the thickness of the resulting phosphating film is 10–30 g/m2. The ratio of free acid to total acid in the solution is 1:(7–8).
Advantages: The film has strong corrosion resistance and good adhesion. 　　Disadvantages: long heating time, high solution evaporation, high energy consumption, excessive phosphating deposition, unstable free acidity, uneven crystal size, and it is now used less frequently. 　　(2) Medium-temperature type: 50–75°C, treatment time of 5–15 minutes; the thickness of the phosphating film is 1–7 g/m2. The ratio of free acid to total acid in the solution is 1:(10–15). Advantages: stable free acid level, easy to control; short phosphating time, high production efficiency; corrosion resistance similar to that of high-temperature phosphating films. This type is widely used at present. 　　(3) Low-temperature type: 30-50°C – energy-efficient and easy to use. 　　(4) Room temperature type: Phosphating at normal (low) temperatures of 10–40°C (an accelerator is added in addition to the oxidizing agent); the processing time is 10–40 minutes. The ratio of free acid to total acid in the solution is 1:(20–30), and the film thickness is 0.2–7 g/m2. 　　Advantages: No heating required, low drug consumption, and stable solution. 　　Disadvantages: Long processing time, and the preparation of the solution is relatively complicated. 　　2. Classification by the composition of the phosphating solution: (1) Zinc-based phosphating; (2) Zinc-calcium-based phosphating; (3) Iron-based phosphating; (4) Manganese-based phosphating; (5) Composite phosphating. The phosphating solution is composed of elements such as zinc, iron, calcium, nickel, and manganese. 　　3. Classification by phosphating method 　　(1) Chemical phosphating 　　The workpiece is immersed in a phosphating solution, and phosphating is achieved through chemical reactions; this method is widely used today. 　　(2) Electrochemical phosphating: In the phosphating solution, the workpiece is connected to the positive pole while steel is connected to the negative pole for phosphating. 　　4. Classification by phosphating film quality 　　(1) Heavy-duty (thick-phosphating) with a film thickness of 7.5 g/m2 or more. 　　(2) Light heavyweight (medium-membrane phosphating) with a film weight of 4.6–7.5 g/m2. 　　(3) Lightweight (thin film phosphating) film weight: 1.1–4.5 g/m2. 　　(4) Sub-lightweight (ultra-thin phosphating) film with a weight of 0.2–1.0 g/m2. 　　5. Classification by processing method 　　(1) Immersion phosphating 　　Suitable for high, medium, and low-temperature phosphating. Characteristics: Simple equipment – only a heating tank and corresponding heating devices are required; it is best to use tanks lined with stainless steel or rubber, with stainless steel heating pipes placed on both sides of the tank. 　　(2) Sprayed phosphating Applicable to medium and low-temperature phosphating processes; it can be used to treat large-area workpieces such as automobile, refrigerator, and washing machine casings. Features: short processing time, fast film-forming reaction, high production efficiency. The phosphating film obtained by this method is dense and uniform in crystal structure, thin in thickness, and possesses good corrosion resistance. 　　(3) Brush-on phosphating: When the above two methods cannot be used, this method is employed. It can be applied at room temperature, is easy to apply, helps remove rust, and allows the workpiece to dry naturally after phosphating; it provides good rust prevention properties, but the phosphating effect is not as good as that of the first two methods. Phosphating and Its Applications 1. Phosphating (1) Role of phosphating before coating ① Enhance the adhesion between the coating layer (such as paint coating) and the workpiece. 　　②Improve the corrosion resistance of the coating on the surface of the workpiece after painting. 　　③Improve decorativeness. 　　(2) Functions of uncoated phosphating 　① Improve the wear resistance of the workpiece. 　　②Make the workpiece lubricious during machining. 　　③Improve the corrosion resistance of the workpiece. 　　2. Uses of phosphating Steel phosphating is mainly used for corrosion protection and as a base coat for paint. 　　(1) Phosphating film for corrosion protection 　　① Protection phosphating film used for corrosion protection treatment of steel parts. Phosphating film types can be zinc-based or manganese-based. The mass per unit area of the membrane is 10–40 g/m2. After phosphating, apply rust preventive oil, rust preventive grease, rust preventive wax, etc. 　　②A phosphating film is used as the base coat, enhancing the adhesion between the paint layer and the steel substrate as well as its protective properties. The type of phosphating film can be zinc-based or zinc-calcium-based. The mass per unit area of the phosphating film is 0.2–1.0 g/m2 (used as a primer for steel parts subject to significant deformation) ； 1-5 g/m2 (for the primer coat on ordinary steel parts) ； 5-10 g/m2 (for the primer coat on steel parts that are not subject to deformation). 　　(2) Phosphating film for cold working lubrication – used in the drawing of steel wires and welded steel pipes; film thickness: 1–10 g/m2 per unit area ； Drawing of precision steel tubes: film weight per unit area is 4–10 g/m2 ； Cold extrusion of steel parts: the film weight per unit area is greater than 10 g/m2. 　　(3) Anti-friction phosphating film The phosphating film can serve an anti-friction function. Manganese-based phosphating is commonly used, but zinc-based phosphating can also be used. For workpieces with a small clearance for motion, the quality of the phosphating film is 1–3 g/m2 ； For workpieces with large dynamic fit clearances (gearboxes), the quality of the phosphating coating is 5–20 g/m2. 　　(4) Phosphating film for electrical insulation Zinc-based phosphating is generally used. Phosphating treatment of silicon wafers used in motors and transformers. 　　IV. Composition and Properties of the Phosphating Film Classification, Main Components of the Phosphating Solution, Film Composition, Appearance of the Film, Film Weight per Unit Area/g/m2 Zinc-based: Zn(H2PO4)2; zinc phosphate and zinc iron phosphate; light gray to dark gray; 1-60 Zinc-calcium based: Zn(H2PO4)2 and Ca(H2PO4)2; zinc calcium phosphate and zinc iron phosphate; light gray to dark gray; 1-15 Manganese-based: Mn(H2PO4)2 and Fe(H2PO4)2; zinc iron manganese phosphate; gray to dark gray; 1-60 Manganese-zinc based: Mn(H2PO4)2 and Zn(H2PO4)2; mixture of zinc phosphate, manganese phosphate, and iron phosphate; gray to dark gray; 1-60 Iron-based: Fe(H2PO4)2; iron phosphate; dark gray; 5-10 2. Composition of the Phosphating Film The phosphating film consists of shiny, uniform, fine crystals that are gray in color and porous, with strong adhesion. Most of these crystals are zinc phosphate, while a smaller portion is iron hydrogen phosphate. The zinc-iron ratio depends on the solution composition, phosphating time, and temperature. 　　3. Properties 　　(1) Corrosion resistance 　　It exhibits excellent corrosion resistance in the atmosphere, mineral oils, vegetable oils, benzene, and toluene, but its corrosion resistance is poor in the presence of alkalis, acids, and water vapor. It retains a certain degree of corrosion resistance at 200–300°C, but the corrosion resistance of the film layer decreases significantly when the temperature reaches 450°C. 　　(2) Special properties Such as improved adhesion, lubricity, and anti-friction/wear properties. Phosphating process: Pre-degreasing → Degreasing → Rust removal → Washing → (Surface treatment) → Phosphating → Washing → Post-phosphating treatment (such as electrophoresis or powder coating). Influencing factors: 1. Temperature – The higher the temperature, the thicker the phosphating layer and the coarser its crystals. 　　The lower the temperature, the thinner the phosphating layer and the finer the crystals. 　　However, the temperature should not be too high, otherwise Fe2+ is easily oxidized to Fe3+, increasing the amount of precipitate and making the solution unstable. 　　2. Free acidity Free acidity refers to free phosphoric acid. Its function is to promote the dissolution of iron; as a result, more nuclei are formed, leading to denser crystallization of the film. 　　If the free acidity is too high, it accelerates the reaction with iron, resulting in the release of large amounts of hydrogen. This makes it difficult for the phosphate layer at the interface to become saturated, hindering the formation of crystal nuclei. As a consequence, the membrane structure becomes loose and porous, its corrosion resistance decreases, and the phosphating process takes longer. 　　If the free acidity is too low, the phosphating film becomes thin or even disappears entirely. 　　3. Total acidity Total acidity refers to the sum of phosphates, nitrates, and acids. It is generally better to keep the total acidity within the upper limit of the specified range, as this facilitates the acceleration of the phosphating reaction and results in finer grains in the film layer. During the phosphating process, the total acidity gradually decreases, indicating a slow progression of the reaction. 　　If the total acidity is too high, the membrane layer becomes thinner; it can be diluted by adding water. 　　The total acidity is too low, resulting in a loose and rough membrane layer. 　　4. pH Value The pH value of manganese-based phosphating solutions is generally kept between 2 and 3; when pH > 3, powder tends to form on the surface of the parts. Film formation is difficult when PH‹1.5. The iron level is generally kept between 3 and 5.5. 　　5. Ion concentration in the solution 　　①Fe2+ in the solution is highly prone to oxidation to Fe3+, which makes it difficult to form a film. However, the concentration of Fe2+ in the solution should not be too high; otherwise, the film grains formed will be coarse, there will be white residue on the film surface, and its corrosion resistance and heat resistance will decrease. 　　②The effect of Zn2+: when the concentration of Zn2+ is too high, the grains of the phosphating film become larger, its brittleness increases, and the surface takes on a white, dusty appearance ； When the Zn2+ concentration is too low, the film layer becomes loose and darker. Post-phosphating treatment Purpose: To improve the corrosion resistance and rust prevention properties of the phosphated film. 　　Phosphating slag 1. Effects of phosphating slag ① The phosphating slag generated during the phosphating process not only wastes chemicals but also increases the workload associated with removing this slag; if not handled properly, it can affect the quality of phosphating, which is considered a negative aspect. 　　②During phosphating, phosphoric acid is also volatilized alongside the formation of phosphating slag, which helps to maintain the free acidity of the phosphating solution and keep it in balance, which is considered advantageous. 　　2. Control of phosphating slag formation 　① Lower the phosphating temperature. 　　②Reduce the free acidity of the phosphating solution. 　　③Increase the phosphating speed and reduce the phosphating time. 　　④Increase the ratio of NO-3 to PO3-4. Quality inspection of phosphating film 　　① Visual inspection 　　Under normal viewing conditions, the phosphating film should have a uniform, continuous, and dense crystal structure. There should be no residual bare areas or rust on the surface that have not been phosphated. Due to the differences in pretreatment methods and effects, phosphating films of varying colors are permitted, but a brown color is not allowed. 　　②Corrosion resistance test  (1) Immersion method  Immerse the phosphated sample in a 3% sodium chloride solution; after two hours, remove it. If there is no rust on the surface, it is considered qualified. The longer it takes for rust to appear, the better the corrosion resistance of the phosphating film. 　　②Drip method: At room temperature, drop the blue dot reagent onto the phosphating film and observe the time it takes for a color change to occur. The color change time varies depending on the thickness of the phosphating film. Thick film > 5 minutes, medium film > 2 minutes, thin film > 1 minute. 　　X. Determination of free acidity and total acidity. 　　1. Determination of free acidity: Using a pipette, transfer 10 ml of the sample solution into a 250 ml conical flask, add 50 ml of distilled water, and add 2–3 drops of methyl orange indicator (or bromophenol blue indicator). Titrate with 0.1 mol/L sodium hydroxide standard solution until the solution turns orange (or use bromophenol blue as an indicator until it changes from yellow to blue-violet); the volume of sodium hydroxide standard solution used at this point is the value corresponding to the free acidity of the sample. 　　2. Determination of total acidity: Use a pipette to transfer 10 ml of the sample solution into a 250 ml conical flask, add 50 ml of distilled water, and add 2–3 drops of phenolphthalein indicator. Titrate with 0.1 mol/L sodium hydroxide standard solution until a pink color appears, which marks the endpoint; the volume of sodium hydroxide standard solution used is then recorded as the total acidity value. Phosphating of non-ferrous metals 　　 Mainly involves the phosphating of aluminum and zinc components. 　　Phosphating blackening solution – for use at room temperature; phosphating protection achieved in just one step! Also known as steel colorant! Use after diluting 1:4-5, soak at room temperature for about 30 minutes, and then seal for protection! 　　Processing procedure: Oil and rust removal – Immersion in rust preventive solution – Phosphating and blackening – Drying – Sealing for protection