What are the differences and connections between blackening surface treatment and phosphating treatment in Dalian

In the field of metal surface treatment, Dalian blackening surface treatment and phosphating treatment are two common chemical conversion coating technologies, widely used in industries such as mechanical manufacturing, automotive parts, and military products. Both of these technologies can form a protective layer on the surface of metals, improving their corrosion resistance and wear resistance, but they have significant differences in principles, processes, and applications.

1、 Overview of Blackening Surface Treatment

Blackening, also known as bluing, is a surface treatment process that forms a dense black oxide film on the surface of steel through chemical or electrochemical methods. This oxide film is mainly composed of iron trioxide (Fe ∝ O ₄), with a thickness typically between 0.5-1.5 microns.

1. Principle of Blackening Treatment

The basic principle of blackening treatment is to generate a dense black oxide film by chemical reaction between alkaline or acidic solution and steel surface. Alkaline blackening is a commonly used method, usually carried out in a solution of sodium hydroxide and sodium nitrite at 135-155 ℃, with a reaction time of about 10-30 minutes.

2. Process flow of blackening treatment

The typical blackening process includes: degreasing → water washing → acid washing → water washing → blackening → water washing → saponification → oil immersion → drying. The saponification and oil immersion steps can significantly improve the corrosion resistance of the blackened film.

3. Characteristics of Blackening Treatment

Thin film layer, does not affect the dimensional accuracy of the parts

Appearance is blue black or black, with good decorative properties

Medium corrosion resistance, usually needs to be used in conjunction with rust proof oil

High process temperature and high energy consumption

Suitable for carbon steel and low-alloy steel

2、 Overview of phosphating treatment

Phosphating is the process of forming an insoluble phosphate conversion film on a metal surface through chemical methods. This layer of film is mainly composed of iron phosphate, zinc phosphate, or manganese phosphate, and its thickness is usually between 1-10 microns.

1. Principle of phosphating treatment

The basic principle of phosphating treatment is that the metal surface reacts with an acidic solution containing dihydrogen phosphate, dissolving some of the metal and forming insoluble phosphate deposits on the metal surface. According to the different components of the phosphating solution, it can be divided into different types such as zinc based, manganese based, and iron based.

2. Process flow of phosphating treatment

The typical phosphating process includes: degreasing → water washing → acid washing → water washing → surface conditioning → phosphating → water washing → drying → post-treatment (such as oil coating or painting). The surface adjustment step is crucial for forming a uniform and fine phosphating film.

3. Characteristics of phosphating treatment

Thick film layer can provide better corrosion resistance

Surface appears gray or dark gray, with a porous structure

Has good adhesion with paint, lubricants, etc

Wide range of process temperatures, including normal temperature, medium temperature, and high temperature processes

Suitable for various metals such as steel, zinc, aluminum, etc

3、 The difference between blackening treatment and phosphating treatment

1. Different film formation mechanisms

Blackening treatment is the process of generating an iron oxide film (mainly Fe ∝ O ₄) on the surface of steel through oxidation reaction, which belongs to an oxide film; Phosphating treatment is the process of generating a phosphate crystal film through chemical reactions, which belongs to the category of chemical conversion films.

2. Different membrane components

The main component of the blackening film is iron trioxide (Fe ∝ O ₄); The main components of the phosphating film may be zinc phosphate, manganese phosphate, or iron phosphate depending on the type of phosphating solution.

3. Different membrane structures

The black film has a dense structure and low porosity; The phosphating film has a porous crystal structure with high porosity, which is also the reason for its good adhesion with paint.

4. Appearance differences

After blackening treatment, the surface appears blue black or black with luster; After phosphating treatment, the surface appears light gray to dark gray with no metallic luster.

5. Different corrosion resistance

When compared separately, the corrosion resistance of phosphating film is usually better than that of blackening film. But after immersion in oil, the corrosion resistance of the black film can be significantly improved.

6. Differences in process temperature

Traditional blackening treatment is usually carried out at high temperatures (130-150 ℃); There are various processes available for phosphating treatment, including room temperature (20-40 ℃), medium temperature (50-70 ℃), and high temperature (80-95 ℃).

7. Different application fields

Blackening treatment is commonly used for parts that require high dimensional accuracy and a black appearance, such as firearms, precision instrument parts, etc; Phosphating treatment is widely used as a coating bottom layer or wear-resistant and friction reducing layer, such as automotive parts, fasteners, etc.

8. Different substrate adaptability

Blackening treatment is mainly suitable for steel materials; Phosphating treatment is suitable for various metal materials such as steel, zinc, aluminum, etc.

4、 The relationship between blackening treatment and phosphating treatment

1. Belonging to chemical conversion membrane technology

Blackening treatment and phosphating treatment are both techniques that form protective conversion coatings on metal surfaces through chemical methods, and both belong to the category of chemical conversion coatings.

2. Similar pre-treatment processes

Both processes require strict pre-treatment such as oil and rust removal, and the quality of the pre-treatment directly affects the quality of the final film layer.

3. Both can improve corrosion resistance

Although the mechanisms are different, both treatments can improve the corrosion resistance of metal surfaces and extend the service life of parts.

4. Can be used as final or intermediate processing

Both treatments can be used as final surface treatments or as pre-treatment for other surface treatments such as painting.

5. The post-processing method is similar

Both treatments usually require water washing, drying, and possible post-treatment such as oil immersion or coating.

6. Can be substituted for each other in specific applications

In some situations where high corrosion resistance is not required, the two treatments can be used interchangeably.

5、 Application selection suggestions

When choosing between blackening treatment or phosphating treatment, the following factors should be considered:

Material type: Both steel and non-ferrous metals are suitable, and phosphating treatment is usually chosen for non-ferrous metals.

Appearance requirements: For those that require a black appearance, choose blackening treatment, and for those without special color requirements, choose phosphating treatment.

Corrosion resistance requirements: For high corrosion resistance requirements, phosphating treatment is preferred, especially zinc based phosphating.

Subsequent processing: Phosphating treatment is preferred for those that require painting, as it has better adhesion with paint.

Dimensional accuracy: Blackening treatment is selected for sensitivity to dimensional changes, as its film layer is thinner.

Cost considerations: Phosphating treatment usually has lower costs, especially for room temperature phosphating processes.

6、 Development Trends

With the increasing demand for environmental protection, both treatment technologies are developing towards low temperature and low pollution:

Blackening treatment: Develop low-temperature blackening process (70-100 ℃) to reduce energy consumption; Replace sodium nitrite with more environmentally friendly additives.

Phosphating treatment: Developing environmentally friendly phosphating solutions without nickel and nitrite; Promote room temperature phosphating technology; Develop phosphate free conversion coatings to replace traditional phosphating.

Blackening treatment and phosphating treatment are two important metal surface treatment techniques, each with its own characteristics and advantages. Blackening treatment is irreplaceable in specific fields due to its unique black appearance and thin layer advantages; Phosphating treatment is widely used in industry due to its excellent corrosion resistance and adhesion with coatings. Understanding the differences and connections between the two can help make more reasonable choices in actual production, optimize surface treatment processes, and improve product quality and performance. In the future, with the increasingly strict environmental regulations and continuous technological progress, both processes will develop towards greater efficiency and environmental friendliness.