Phosphorization of seamless steel pipe
Phosphorization is a process of forming a phosphate chemical conversion film by chemical and electrochemical reactions. The phosphate conversion film formed is called a phosphate film. The main purpose of Phosphorization is: to provide protection to the base metal and prevent the metal from being corroded to a certain extent; for primer before painting to improve the adhesion and anti-corrosion ability of the paint film; to reduce friction in the metal cold working process Use for lubrication. Phosphorization treatment technology has been applied to industry for more than 90 years. It can be roughly divided into three periods: the period of laying the foundation of Phosphorization technology, the period of rapid development of Phosphorization technology and the period of wide application.
 

Brief introduction of Phosphorization seamless steel pipe
Phosphorization film is used as an anti-corrosion protective film for steel. The earliest reliable record is the patent (B.P. No. 3119) obtained by Charles Ross in England in 1869. Since then, the Phosphorization process has been applied to industrial production. In the long years of nearly a century, Phosphorization technology has accumulated rich experience and made many important discoveries. During World War I, the development center of Phosphorization technology was transferred from Britain to the United States. In 1909, American T.W.Coslet made the first zinc-based Phosphorization solution by dissolving zinc, zinc oxide or zinc phosphate in phosphoric acid. This research result has greatly promoted the development of Phosphorization process and broadened the development prospects of Phosphorization process. The Parco Power formulated Phosphorization solution researched and developed by Parker Antirust Company overcomes many shortcomings and increased the Phosphorization treatment time to 1h (1 hour). In 1929, the Bonderizing Phosphorization process shortened the Phosphorization time to 10 minutes. In 1934, the Phosphorization treatment technology made a revolutionary development in the industry, that is, the method of spraying the Phosphorization solution onto the workpiece was adopted. After the end of World War II, there were few breakthroughs in Phosphorization technology, but steady development and improvement. Phosphorization is widely used in anti-corrosion technology, metal cold deformation processing industry. The important improvements of Phosphorization treatment technology during this period mainly include: low-temperature Phosphorization, various methods of controlling the film weight of Phosphorization film, and high-speed Phosphorization of continuous steel strip. Currently, the research direction in the field of Phosphorization technology is mainly focused on improving quality, reducing environmental pollution, and saving energy.
 
 
Principle and application
Phosphorization is a commonly used pretreatment technology. In principle, it should be a chemical conversion film treatment. It is mainly used for Phosphorization on the surface of steel. Non-ferrous metals (such as aluminum and zinc) can also be Phosphorization.
 

Basic knowledge of Phosphorization
Phosphate classification
1. Classified by Phosphorization temperature
(1) High temperature type
The treatment time at 80-98℃ is 10-20 minutes, the thickness of the Phosphorization film is 10-30g/m2, and the ratio of the free acidity of the solution to the total acidity is 1: (7-8)
Advantages: the film has strong corrosion resistance and good bonding force.
Disadvantages: long heating time, large amount of solution volatilization, high energy consumption, more Phosphorization deposition, unstable free acidity, uneven crystal thickness, and has been seldom used.
(2) Medium temperature type
50-75℃, treatment time 5-15 minutes, Phosphorization film thickness is 1-7 g/m2, the ratio of solution free acidity to total acidity is 1: (10-15)
Advantages: stable free acidity, easy to master, short Phosphorization time, high production efficiency, basically the same corrosion resistance as high temperature Phosphorization film, and more applications.
(3) Low temperature type
30-50℃ saves energy and is easy to use.
(4) Normal temperature type
10-40℃ Normal (low) temperature Phosphorization (in addition to the oxidizer, but also the accelerator), the time is 10-40 minutes, the ratio of the free acidity of the solution to the total acidity is 1: (20-30), and the film thickness is 0.2- 7g/m2.
Advantages: no need to heat, less medicine consumption, stable solution.
Disadvantages: long processing time and complicated solution preparation.
 
2. Classified according to the composition of Phosphorization solution
(1) Zinc-based Phosphorization
(2) Phosphorization of zinc-calcium series
(3) Iron-based Phosphorization
(4) Manganese Phosphorization
(5) Compound Phosphorization The Phosphorization solution is composed of zinc, iron, calcium, nickel, manganese and other elements.
 
3. Classified by Phosphorization treatment method
(1) Chemical Phosphorization
The workpiece is immersed in the Phosphorization solution, and Phosphorization is realized by chemical reaction, which is widely used.
(2) Electrochemical Phosphorization
In the Phosphorization solution, the workpiece is connected to the positive electrode, and the steel is connected to the negative electrode for Phosphorization.
 
4. According to the quality of phosphate coating
(1) Heavyweight (thick film Phosphorization) The film weight is above 7.5 g/m2.
(2) The weight of the heavyweight (medium Phosphorization) film is 4.6-7.5 g/m2.
(3) Lightweight (thin film phosphated) film weight 1.1-4.5 g/m2.
(4) Sub-lightweight (special thin film Phosphorization) film weight 0.2-1.0 g/m2.
 
5. Classified by construction method
(1) Impregnation Phosphorization
Suitable for high, medium and low temperature Phosphorization. Features: Simple equipment, only heating tank and corresponding heating equipment are required. It is best to use stainless steel or rubber lined tanks. Stainless steel heating pipes should be placed on both sides of the tank.
(2) Spray Phosphorization
It is suitable for medium and low temperature Phosphorization process, and can process large area workpieces, such as automobile, refrigerator, washing machine shell. Features: Short processing time, fast film-forming reaction speed, high production efficiency, and the Phosphorization film obtained by this method has dense, uniform crystals, thin film, and good corrosion resistance.
(3) Brush coating and Phosphorization
When the above two methods cannot be implemented, use this method, operate at room temperature, easy to paint, and can remove rust. After Phosphorization, the workpiece is naturally dried and has good rust resistance, but the Phosphorization effect is not as good as the previous two [1]
 
 
Phosphorization use
1. Phosphorization
(1) The effect of Phosphorization before painting
①Enhance the bonding force between the coating film (such as paint coating) and the workpiece.
② Improve the corrosion resistance of the surface coating of the workpiece after painting.
③ Improve decoration.
(2) The effect of non-coating Phosphorization
① Improve the wear resistance of the workpiece.
② Make the workpiece lubricating during machining.
③Improve the corrosion resistance of the workpiece.
 
2. Use of Phosphorization seamless steel pipe
Steel Phosphorization is mainly used for corrosion protection and base film for paint.
(1) Phosphorization film for corrosion protection
① Phosphorization film for protection Used for corrosion protection treatment of steel parts. The type of Phosphorization film can be zinc or manganese. The mass per unit area of ​​the membrane is 10-40g/m2. After Phosphorization, apply anti-rust oil, anti-rust grease, anti-rust wax, etc.
②Phosphorization film for paint bottom layer
Increase the adhesion and protection of the paint film and steel workpieces. The type of Phosphorization film can be zinc or zinc calcium. The mass per unit area of ​​the Phosphorization film is 0.2-1.0g/m2 (used for the paint bottom layer of larger deformed steel parts); 1-5g/m2 (used for the paint bottom layer of general steel parts); 5-10g/m2 (used for no occurrence Deformed steel parts paint bottom layer).
(2) Phosphate film for cold working lubrication
Drawing of steel wires and welded steel pipes, the film weight per unit area is 1-10g/m2; the drawing of precision steel pipes, the film weight per unit area is 4-10g/m2; the cold extrusion of steel parts, the film weight per unit area is more than 10g/m2.
(3) Phosphate film for anti-friction
Phosphorization film can reduce friction. Generally, manganese series Phosphorization is used, but zinc series Phosphorization can also be used. For the workpiece with a small dynamic fit clearance, the quality of the phosphate film is 1-3g/m2; for the workpiece with a large dynamic fit (reduction gear), the quality of the phosphate film is 5-20g/m2.
(4) Phosphorization film for electrical insulation
 Generally use zinc Phosphorization. Used in the Phosphorization treatment of silicon wafers in motors and transformers [1].
 
 
Phosphate film properties
1. Phosphate film composition
The Phosphorization film is a bright, uniform, fine, gray, porous and strong adhesion crystal. Most of the crystal is zinc phosphate, and a small part is iron hydrogen phosphate. The ratio of zinc to iron depends on the solution composition, Phosphorization time and temperature.
 
2. Nature
(1) Corrosion resistance
It has good corrosion resistance in the atmosphere, mineral oil, vegetable oil, benzene, toluene, but poor corrosion resistance in alkali, acid, and water vapor. It still has a certain degree of corrosion resistance at 200-300°C, and the corrosion resistance of the film decreases significantly when the temperature reaches 450°C.
(2) Special nature
Such as increasing adhesion, lubricity, anti-friction and wear resistance.
Phosphorization seamless steel pipe Phosphorization process
Degreasing and rust removal → washing → Phosphorization → washing → Phosphorization post-treatment.
 
 
Influencing factors
1. Temperature
The higher the temperature, the thicker the Phosphorization layer and the coarser the crystals.
The lower the temperature, the thinner the Phosphorization 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 the solution is unstable.
2. Free acidity
Free acidity refers to free phosphoric acid. Its function is to promote the dissolution of iron, and more crystal nuclei have been formed, making the film crystal dense.
If the free acid is too high, the interaction with iron will accelerate, and a large amount of hydrogen will be precipitated, making the interface layer phosphate difficult to saturate, resulting in difficulty in the formation of crystal nuclei, loosening of the film structure, porous, reduced corrosion resistance, and prolonging the Phosphorization time.
If the free acid is too low, the Phosphorization film becomes thinner or even no film.
3. Total acidity
Total acidity refers to the sum of phosphate, nitrate and acid. The total acidity is generally controlled at the upper limit of the specified range, which is beneficial to accelerate the Phosphorization reaction and make the film grain fine. During the Phosphorization process, the total acidity continues to decrease and the reaction is slow.
The total acidity is too high and the film becomes thinner, which can be diluted with water.
The total acidity is too low and the film is loose and rough.
4. PH value
The manganese series Phosphorization solution is generally controlled between 2-3. When the pH is 3, the surface of the workpiece is easy to form powder. It is difficult to form a film when PH‹1.5. The iron series is generally controlled between 3-5.5.
5. Ion concentration in solution
①Fe2+ in the solution is easily oxidized 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 formed film will have coarse grains, white floating dust on the surface of the film, and reduced corrosion resistance and heat resistance.
②The influence of Zn2+. When the Zn2+ concentration is too high, the phosphate film has coarse grains, increased brittleness, and white floating gray on the surface; when the Zn2+ concentration is too low, the film becomes loose and dark.
 
 
After Phosphorization
Purpose: To increase the corrosion resistance and rust resistance of the phosphate coating.
Methods: spraying, powder spraying, painting, electrophoresis, applying anti-rust oil, etc.
 
Phosphorization Slag
1. The influence of Phosphorization slag
① Phosphorization slag generated in Phosphorization not only wastes medicines but also increases the workload of slag cleaning. If it is not handled well, it also affects the quality of Phosphorization, which is regarded as disadvantageous.
② Phosphoric acid is also volatilized when Phosphorization slag is generated during Phosphorization, which helps to maintain the free acidity of the Phosphorization solution and maintain the balance of the Phosphorization solution.
2. Control of Phosphorization slag generation
①Reduce the Phosphorization temperature.
②Reduce the free acidity of Phosphorization solution.
③Increase the Phosphorization speed and shorten the Phosphorization time.
④Improve the ratio of NO-3 to PO3-4.
 
Phosphate film inspection
①Appearance inspection
The Phosphorization film should have a uniform, continuous and dense crystal structure when observed by naked eyes. There should be no unphosphatized residual blanks or rust stains on the surface. Due to the different pre-treatment methods and effects, phosphate films with different colors are allowed, but brown is not allowed.
②Corrosion resistance inspection
 ⑴ Immersion method
Dip the phosphated sample into 3% sodium chloride solution and take it out after two hours. The surface is qualified without rust stains. The longer the rust stains appear, the better the corrosion resistance of the phosphate coating.
②Dip method
At room temperature, drop the blue point reagent on the phosphate film and observe the color change time. The thickness of the Phosphorization film is different, and the discoloration time is different. Thick film ﹥5 minutes, medium film ﹥2 minutes, thin film ﹥1 minute.
 

Acidity
1. Determination of free acidity
Use a pipette to suck 10 ml of the test 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 0.1mol/l sodium hydroxide standard solution until the solution is orange (or titrate with bromophenol blue indicator to turn from yellow to blue-purple), which is the end point. The number of milliliters of sodium hydroxide standard solution that is written down is the titration Free acidity points.
2. Determination of total acidity
Use a pipette to suck 10 ml of the test solution into a 250 ml conical flask, add 50 ml of distilled water, and add 2-3 drops of phenolphthalein indicator. Titration with 0.1mol/l sodium hydroxide standard solution to pink is the end point, and the number of milliliters of sodium hydroxide standard solution noted down is the total acidity point of the titration.