What is passivation? The basics of passivation

What is passivation? The basics of passivation

Ⅰ Introduction to passivation

Passivation is an important surface treatment technology, which is widely used in industrial products to improve the corrosion resistance, wear resistance and high temperature resistance of metal materials.

Passivation refers to the process of metal being oxidized by strong oxidants or electrochemical methods to make the surface become inactive, that is, passivation. This process converts the metal surface to a state that is not easily oxidized, thus delaying the corrosion rate of the metal. In industry, passivating agents (mainly oxidants) are usually used to passivate metals to form a protective film. This film is closely covered on the surface of the metal, the metal and the corrosive medium are completely separated, and the metal is prevented from contacting the corrosive medium, thus playing a role in preventing corrosion.

Ⅱ Passivation classification

1. The following are classified by passivation film properties:

Oxidative passivation

Features: The passivation film formed on the metal surface is mainly composed of oxides. The passivation film has high chemical stability and corrosion resistance, and can effectively prevent corrosive media such as oxygen and water from contacting the metal matrix.

Application: Widely used in steel, stainless steel and other metal passivation treatment. For example, the oxide film naturally formed in the air of steel and the passivation film of iron oxide formed by chemical treatment are both oxidized passivation.

Precipitation passivation

Features: The passivation film is formed by the precipitation of insoluble compounds generated by the reaction of the metal surface with ions in the solution. The corrosion resistance of precipitation-type passivation film depends on the stability and densification of precipitation.

Application: It is often used for passivation treatment of copper, zinc and other metals. For example, when copper is passivated in a solution containing chromates, a precipitation-type passivating film composed of compounds such as copper chromate is formed on the surface.

Adsorption passivation

Features: The passivation film is formed by organic or inorganic molecules adsorbed on the surface of the metal. The thickness of the adsorption passivation film is thin, but it can effectively prevent the erosion of corrosive media.

Application: In some special environments, such as high temperature, high pressure, strong corrosive media, adsorption passivation can be used as an auxiliary anti-corrosion means. For example, in the petrochemical industry, some metal equipment will adsorb a layer of organic corrosion inhibitor molecules on the surface to form an adsorption-type passivation film.

2. Classification by passivation method:

Chemical passivation

Definition: Method by which a metal workpiece is immersed in a specific chemical solution to form a passivation film on the metal surface through a chemical reaction.

Common chemical solutions: nitric acid, potassium dichromate, chromic acid, etc. For example, for the passivation of stainless steel, the common chemical solution is a mixture of nitric acid and potassium dichromate.

Process flow: degreasing → washing → pickling → washing → chemical passivation → washing → drying. First remove the oil and impurities on the metal surface, then pickling to remove the oxide and rust, and then immerse the metal workpiece in a chemical passivation solution for passivation treatment, and finally clean and dry.

Electrochemical passivation

Definition: Method of forming a passivation film on the surface of a metal by anodizing it in a specific electrolyte.

Principle: In the process of anode polarization, the potential of the metal surface rises, when the potential reaches a certain value, the oxidation reaction will occur on the metal surface, forming a dense oxide film. For example, in the electrochemical passivation of aluminum, sulfuric acid or oxalic acid is usually used as the electrolyte to obtain a good passivation effect by controlling the anode current density and polarization time.

Process flow: degreasing → washing → pickling → washing → electrochemical passivation → washing → drying. Similar to chemical passivation, pre-treatment is performed first, followed by electrochemical passivation in a specific electrolyte.

Ⅲ Passivation process

1.Pre-treatment

(1) Clean

Degreasing: The use of alkaline cleaning agents or solvents to remove grease, dust, and other contaminants from metal surfaces. Common alkaline cleaning agents include sodium hydroxide (NaOH) solutions.

Ultrasonic cleaning: For dirt that is difficult to remove, ultrasonic cleaning equipment can be used to help remove contaminants through high-frequency vibration.

(2) Rust removal

Mechanical rust removal: Use tools such as wire brushes, sandpaper or polishing wheels to remove rust from metal surfaces.

Chemical rust removal: The use of an acidic solution (such as hydrochloric or sulfuric acid) to remove surface rust. It should be noted that this method may damage the metal itself, so it needs to be done with caution.

2.Passivation treatment

(1) Preparation of passivation solution

Choose the right passivation agent: Common passivation agents include nitric acid (HNO₃), phosphoric acid (H₃PO₄), chromic acid (CrO₃), or a mixture of them. For stainless steel, nitric acid or a mixture of nitric acid and phosphoric acid is usually used.

Ratio and concentration: Adjust the concentration of passivation solution according to the type and requirements of metal. For example, for stainless steel, the commonly used concentration of nitric acid is about 20%.

(2) Passivation treatment

Immersion method: The clean metal is immersed in passivation solution. Depending on the thickness and surface state of the metal, the impregnation time is determined, generally ranging from a few minutes to tens of minutes.

Spray method: For large or complex metal parts, spray equipment can be used to evenly spray the passivation liquid to the metal surface.

Treatment temperature: Passivation treatment is usually carried out at room temperature, but the passivation solution can also be properly heated as needed to speed up the reaction.

(3) Processing time

Control time: The treatment time needs to be determined according to the thickness of the metal and the concentration of the passivation solution. Too short a time will lead to incomplete passivation film, too long may cause corrosion of the metal surface.

3.Post-processing

(1) Neutralization

Neutralize residual acids: Use an alkaline solution (such as sodium carbonate Na₂CO₃ or sodium hydroxide NaOH) to neutralize residual acids to prevent continued corrosion of the metal.

(2) Clean thoroughly

Rinse: Rinse the metal surface thoroughly with clean water to remove all residual passivation liquid and neutralizer.

Ultrapure water rinsing: For applications requiring high precision, ultrapure water can be used for final rinsing to ensure that there is no residue on the surface.

(3) Drying

Air dry: dry naturally in the air.

Compressed air drying: Use compressed air to dry metal surfaces to speed up drying.

Drying: For some applications, it may be necessary to dry the metal in an oven to ensure that the surface is completely dry.

4.Check

Appearance inspection: Visually check whether the metal surface is clean, bright and without residue.

Corrosion test: Perform corrosion tests, such as salt spray tests, to verify the integrity and corrosion resistance of the passivation film.

Electrochemical testing: The passivation effect of metal surfaces is assessed by electrochemical testing, such as polarization curve testing.

Ⅳ Factors affecting passivation

Composition and concentration of passivation solution

The quality and properties of passivation film are affected by different composition and concentration of passivation solution. In general, a more oxidizing solution is conducive to the formation of a dense passivation film, but too high a concentration may lead to excessive corrosion. For example, in the passivation of stainless steel, the concentration of nitric acid is too high may cause the corrosion of the stainless steel surface.

It is necessary to select the appropriate passivation solution composition and concentration according to the type of metal material, surface condition and passivation requirements.

Passivation temperature and time

Passivation temperature and time also affect the quality of passivation film. Generally speaking, rising temperature can speed up the chemical reaction speed and shorten the passivation time, but too high temperature may destroy the structure of the passivation film. For example, in the passivation of aluminum, too high a temperature may make the passivation film loose and reduce its corrosion resistance.

Too short passivation time may lead to incomplete passivation film and poor corrosion resistance. Too long a time may waste energy and time, and may even have an adverse effect on the metal surface. The appropriate passivation temperature and time need to be determined according to the specific situation.

The surface condition of metal materials

Factors such as surface roughness and cleanliness of metal materials will affect the formation and quality of passivation film. Metal materials with rough surfaces require a longer passivation time to form a complete passivation film. For example, for metal workpieces with scratches or oil on the surface, surface treatment is required before passivation.

The composition and organizational structure of metal materials also have an impact on passivation. Different metal materials may require different passivation processes and conditions.

Ⅴ Passivation advantage

Improve corrosion resistance

Forming a protective film: Passivation treatment forms a dense oxide film or other type of protective film on the surface of the metal. This film can effectively prevent oxygen, moisture and other corrosive media from contacting the metal matrix, thus slowing down the occurrence of corrosion reactions. For example, after passivation treatment of stainless steel, the passivation film on the surface can resist a variety of corrosive environments, such as atmosphere, seawater, acid, alkali, etc., which greatly improves its corrosion resistance in different environments.

Reduced corrosion rate: The passivation film has high chemical stability and can significantly reduce the corrosion rate of the metal. Even under harsh corrosion conditions, the passivated metal can maintain a relatively low corrosion rate and extend its service life. For example, in the chemical industry, some metal equipment can operate stably in corrosive media for a long time after passivation treatment, reducing equipment maintenance and replacement costs.

Enhances surface hardness and wear resistance

Increase the surface hardness: During the passivation process, some microstructure changes may occur on the metal surface, so that the surface hardness is improved. This makes the passivated metal more resistant to wear and scratches during use. For example, after some tool steels are passivated, the surface hardness increases, which can better maintain the sharpness of the cutting edge and improve the service life of the tool.

Improved wear resistance: Passivation film can reduce the coefficient of friction between the metal surface and other objects, reducing the degree of wear. At the same time, the passivation film itself also has a certain wear resistance, which can protect the metal matrix from wear to a certain extent. For example, the piston rings of automotive engines are usually made of materials that have been passivated to improve their wear resistance and reduce engine wear and fuel consumption.

Improve appearance quality

Uniform surface color: Passivation treatment can make the color of the metal surface more uniform and beautiful. For some metal products with high appearance requirements, such as decorations, furniture hardware, etc., passivation treatment can improve the overall appearance quality. For example, stainless steel jewelry after passivation treatment, the surface shows a bright, uniform color, increasing the attractiveness of the product.

Removal of surface defects: In the passivation process, some minor surface defects, such as scratches, oxidation spots, etc., can be repaired to a certain extent. The formation of passivation film can cover these defects, making the metal surface more smooth and flat. For example, some aluminum alloy profiles after passivation treatment, the surface scratches and defects are significantly reduced, improving the quality of the product.

Improve cleanliness and hygiene

Easy to clean: The passivated metal surface is smoother, and it is not easy to absorb dust, dirt and other impurities. This makes the passivated metal products easier to clean and maintain good appearance and hygiene. For example, after passivation treatment, kitchen utensils, medical equipment, etc., are easy to clean and disinfect, and meet the requirements of health standards.

Prevent bacterial growth: Some passivation membranes have antibacterial properties that can inhibit the growth and reproduction of bacteria on metal surfaces. This is very important for some metal products used in health care, food processing and other fields. For example, stainless steel medical devices are passivated to reduce the risk of bacterial infection and improve medical safety.

Enhance the adhesion of paints and coatings

Provide a good base: The metal surface after passivation treatment is cleaner and more uniform, providing a good basis for the adhesion of paint and coating. The paint and coating can better bond with the passivated metal surface, improving the adhesion and durability of the coating. For example, in automobile manufacturing, the body of the car is usually passivated before painting is applied to ensure that the paint can firmly adhere to the body surface, improving the appearance quality of the car and corrosion resistance.

Prevent the coating from falling off: Because the passivation film is closely combined with the metal matrix, it can effectively prevent the paint and coating from falling off and foaming during use. This extends the service life of the coating and reduces maintenance costs. For example, passivation treatment of steel structure buildings before anti-corrosion coating can improve the adhesion and anti-corrosion performance of the coating and extend the service life of the building.

Ⅵ Passivation application

1. Stainless steel products

(1) Kitchen utensils

Cookware: such as POTS, POTS, etc., passivation can improve the corrosion resistance of the surface and extend the service life.

Tableware: such as knives, forks, spoons, etc., passivation can improve its surface finish and corrosion resistance.

(2) Medical devices

Surgical instruments: such as scalpels, forceps, etc., passivation can improve their biocompatibility and corrosion resistance to ensure the long-term reliability of medical devices.

Implants: such as orthopedic implants, heart stents, etc., passivation can improve the stability of their surface and reduce the risk of corrosion in the body.

(3) Architectural decoration

Stainless steel guardrail: Passivation can improve its resistance to atmospheric corrosion and extend its service life.

Stainless steel handrail: Passivation can improve its surface finish, increase aesthetics and durability.

2. Chemical industry

(1) Storage tank

Chemical storage tank: Passivation can protect the storage tank from the erosion of internal corrosive media and extend the service life of the storage tank.

Reactor: Passivation can improve the corrosion resistance of the reactor to ensure the safe storage of chemicals and the reaction process.

(2) Pipes and valves

Transport pipeline: Passivation can protect the transport of corrosive media pipeline, improve its service life.

Valve: Passivation can improve the corrosion resistance of the valve to ensure its normal operation in a corrosive environment.

3. Electronics industry

(1) Electronic components

Printed circuit board: Passivation can improve the corrosion resistance of metal lines and extend the service life of the circuit board.

Connector: Passivation can improve the contact reliability and corrosion resistance of the connector to ensure stable signal transmission.

(2) Battery components

Battery case: Passivation can improve the corrosion resistance of the battery case to prevent electrolyte leakage and corrosion.

4. Aerospace industry

(1) Aircraft parts

Fuselage: Passivation can improve the corrosion resistance of the aircraft fuselage and ensure the long-term use of the aircraft in harsh environments.

Engine components: Passivation can improve the corrosion resistance of the internal components of the engine and extend the life of the engine.

(2) Satellite components

Solar panel support: Passivation can improve the corrosion resistance of the support and ensure the long-term stable operation of the solar panel in space.

Communication equipment housing: Passivation can improve the corrosion resistance of the communication equipment housing and extend the service life of the equipment.

5. Ocean engineering

(1) Ocean platform

Structural parts: Passivation can improve the corrosion resistance of offshore platform structural parts and ensure the long-term stability of the platform in seawater.

Piping systems: Passivation can improve the corrosion resistance of piping systems that transport seawater and corrosive liquids.

(2) Ships

Hull: Passivation can improve the corrosion resistance of the hull and extend the service life of the ship.

Propeller: Passivation can improve the corrosion resistance of the propeller and reduce the frequency of maintenance.

6. Food processing

(1) Food containers

Storage tank: Passivation can improve the corrosion resistance of food storage tanks to ensure food safety.

Transport drums: Passivation can improve the corrosion resistance of food transport drums and prevent food contamination.

(2) Processing equipment

Mixer: Passivation can improve the corrosion resistance of the mixer and extend the service life of the equipment.

Ducting system: Passivation can improve the corrosion resistance of food processing ducting system to ensure food hygiene.

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