Your metal parts are still rusting? Surface passivation might be the best solution!

What is mechanical processing surface passivation?

Mechanical processing surface passivation is a technical process that forms a dense and stable passivation film on the metal surface through chemical or electrochemical methods. Although this film is extremely thin (usually only a few nanometers to tens of nanometers), it can significantly improve the corrosion resistance, wear resistance and service life of metal parts.

Analysis of Passivation Principle

Passivation, in essence, is the formation of a dense oxide film on the surface of a metal through chemical or electrochemical methods. This oxide film is like the “armor” of the metal, effectively preventing direct contact between the metal and the external environment, thereby significantly enhancing the metal’s corrosion resistance. Take stainless steel as an example. The main alloy element chromium (Cr) plays a crucial role in the passivation process. When stainless steel is exposed to a specific passivation environment, the chromium atoms on the surface react with oxygen to form a passivation film mainly composed of chromium dioxide (Cr₂O₃). Although this passivation film is extremely thin, typically ranging from 1 to 5 nanometers, its structure is dense and can block corrosive media, such as chloride ions (Cl⁻), from eroding the metal, significantly reducing the possibility of metal corrosion.

Passivation: The “immune system” of stainless steel

The microscopic world of the passivation film The corrosion resistance of stainless steel stems from the Cr₂O₃ passivation film on its surface, which has a thickness of only 5-20 nanometers. This film isolates the metal substrate from the corrosive medium, and its protective efficacy can be directly demonstrated by the following data.

Corrosion rate comparison: The corrosion rate of passivated stainless steel in a 3.5% NaCl solution decreased from 0.5 mm/a to 0.001 mm/a.

Point corrosion potential increase: Passivation treatment raised the point corrosion critical potential from -0.2 V (vs SCE) to 0.8 V.

Diversified passivation processes

(1) Chemical passivation

Chemical passivation is a common process. In actual operation, metal workpieces are immersed in a passivation solution containing specific chemicals. Taking the nitric acid passivation of stainless steel as an example, nitric acid (HNO₃) is the main component of the passivation solution, which can react chemically with impurities on the metal surface and some metal atoms. During this process, nitric acid not only dissolves the free iron and other impurities on the metal surface but also promotes the oxidation of chromium elements on the metal surface, thereby forming a passivation film. The advantage of chemical passivation is that the operation is relatively simple, the cost is low, and it is suitable for large-scale production and processing. However, this process has strict requirements for the concentration, temperature, and processing time of the passivation solution, and precise control is needed; otherwise, it may affect the quality of the passivation film（.

（2) Electrochemical Passivation

Electrochemical passivation is achieved by utilizing the effect of an external electric field on the metal surface. In an electrochemical passivation device, the metal workpiece acts as the anode and is placed in an electrolyte containing passivating agent. After the power is connected, the anode undergoes an oxidation reaction, causing the atoms on the metal surface to lose electrons and become ions that enter the solution. Simultaneously, an oxide film forms on the metal surface. Compared with chemical passivation, electrochemical passivation can more precisely control the passivation process. By adjusting parameters such as current density and voltage, the thickness and performance of the passivation film can be flexibly controlled. This process is often used in fields where the quality of the passivation film is extremely demanding, such as the processing of aerospace components. However, electrochemical passivation equipment is relatively complex, requires a large initial investment, and has high technical requirements for operators.

(3) Mechanical Passivation

Mechanical passivation mainly achieves this through mechanical processing methods, such as shot peening and grinding, which cause plastic deformation on the metal surface, thereby enabling the formation of a dense deformation layer on the metal surface. Taking shot peening treatment as an example, the high-speed ejected projectiles impact the metal surface, causing plastic flow in the surface layer and distortion of the crystal lattice structure, thereby forming a layer with higher hardness and dense structure. This deformation layer can to some extent enhance the wear resistance and corrosion resistance of the metal. The advantage of mechanical passivation lies in its environmental friendliness, as it does not require the use of a large amount of chemical reagents, and can improve the surface properties while enhancing the surface finish of the metal. However, this process causes significant wear to the equipment and has relatively low processing efficiency, and is suitable for processing some workpieces with high requirements for surface quality and small batches.

The process flow of surface passivation in mechanical processing

Pre-treatment: This includes steps such as degreasing, cleaning, and pickling to ensure a clean and pollution-free surface.
Passivation treatment: Select the appropriate passivation method and parameters based on the material.
Neutralization cleaning: Remove the remaining chemical reagents.
Post-treatment: May include steps such as sealing treatment or drying.
Quality inspection: The passivation effect is verified through methods such as salt spray test and blue spot test.

The significance of surface passivation: Why do your parts require it?

(1) Enhanced corrosion resistance

After passivation treatment, the corrosion resistance of metals is significantly improved. For instance, in marine environments, unpassivated ordinary carbon steel components may suffer severe corrosion within a short period of time, while passivated stainless steel components can have their corrosion resistance enhanced by several times or even tens of times. Relevant studies show that stainless steel treated with strict passivation processes can have a corrosion resistance duration in salt spray tests that can be extended from the original several dozen hours to several hundred hours or even thousands of hours, enabling metal components to operate stably in harsh environments for a long time and significantly reducing the maintenance costs and replacement frequency of equipment.

(2) Excellent surface decoration effect

Passivation treatment not only enhances the internal properties of metals but also improves their appearance. The passivated metal surface usually presents a more uniform, fine, and smooth finish, making the product more aesthetically pleasing. In the field of architectural decoration, passivated stainless steel pipes and plates have a long-lasting surface gloss that can add a unique aesthetic appeal to buildings. At the same time, this excellent surface quality also enhances the overall quality and market competitiveness of the product, meeting consumers’ demands for products that are both aesthetically pleasing and practical.

(3) Improved mechanical properties

Passivation treatment has a positive impact on the mechanical properties of metals. On one hand, the presence of the passivation film can increase the hardness of the metal surface, making it more resistant to wear and scratches. During the operation of mechanical components, the passivation film can effectively resist friction and wear, extending the service life of the components. On the other hand, certain passivation processes can form the passivation film while improving the lubricity of the metal surface, reducing the friction coefficient, and minimizing energy loss, thereby enhancing the operational efficiency of mechanical equipment. For example, applying passivation technology to some key components of automotive engines can improve the performance and reliability of the engines.

(4) Environmentally friendly and reliable

Compared to some traditional surface treatment methods, the passivation process has environmental advantages. Many passivation processes do not require the use of large amounts of toxic and harmful chemicals, reducing environmental pollution. For example, citric acid passivation is widely used in the passivation treatment of stainless steel due to its relatively environmentally friendly nature. Moreover, the passivation film has a good adhesion to the metal substrate, does not peel off easily, and provides long-term reliable protection for the metal, ensuring the stability of product quality.

Widely applied fields

Aerospace field

In the aerospace field, the performance requirements for components are extremely strict. As the core component of an aircraft, the internal high temperature, high pressure, and complex chemical environment of an aircraft engine pose extremely high challenges to the corrosion resistance and mechanical properties of the components. Through surface passivation treatment, the key components such as the blades and turbine discs of the engine can maintain good performance under harsh working conditions, ensuring the safe and reliable operation of the engine. Moreover, after the surface passivation treatment, the fuselage structural components and landing gear of the aircraft can effectively resist the corrosive media in the high-altitude atmospheric environment, improving the overall safety and service life of the aircraft.

Medical device industry

Medical devices come into direct contact with the human body, and their safety and reliability are of utmost importance. Stainless steel, as one of the commonly used materials for medical devices, can effectively prevent surface corrosion after being passivated, avoid the release of harmful substances, and ensure the safety of medical devices during use. For example, surgical instruments, implantable medical devices, etc., after undergoing strict passivation treatment, not only meet hygiene standards but also extend the service life of the devices and reduce medical costs. At the same time, the surface of passivated medical devices is highly smooth, making them easy to clean and disinfect, further ensuring the safety and hygiene of the medical process.

Marine engineering equipment

The marine environment has the characteristics of high salinity, high humidity and strong corrosiveness, which poses a severe challenge to the corrosion resistance of marine engineering equipment. The metal structural components of marine engineering equipment such as offshore drilling platforms and ships, after undergoing passivation treatment, can effectively resist the erosion of seawater in harsh marine environments and extend the service life of the equipment. For example, the hulls of ships, propellers and other components can significantly improve their corrosion resistance in seawater by adopting appropriate passivation processes, reducing the workload of maintenance and repair, and ensuring the safe and stable operation of marine engineering equipment.

Conclusion

The surface passivation treatment process of stainless steel is an important method to enhance the corrosion resistance and aesthetic appearance of stainless steel. Through steps such as surface cleaning, acid washing, and passivation treatment, a uniform and dense oxide film can be formed on the surface of stainless steel, significantly improving its corrosion resistance and wear resistance. In practical applications, it is necessary to strictly control the process parameters, wear protective equipment, properly handle waste liquid and avoid secondary pollution to ensure the effectiveness and safety of the passivation treatment. With the advancement of technology and the continuous improvement of the process.

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