Phosphating for Bearing Protection

Phosphating used for bearing protection belongs to the thick film phosphating category, with a film thickness exceeding 20 μm and a film weight typically ranging from 10–30 g/m² or even higher. This ensures excellent protective performance. After phosphating, bearings are coated with oil, grease, or wax to further enhance their corrosion resistance.

Phosphating Methods

1. Slow Phosphating

The bath solution consists of zinc dihydrogen phosphate, manganese dihydrogen phosphate, iron dihydrogen phosphate, or a combination of these, along with free phosphoric acid.

• Since no oxidizing agent is included, the concentration of Fe²⁺ ions increases with the treated surface area, leading to sediment accumulation.
• The processing time ranges from 40–60 minutes.

2. Accelerated High-Temperature Phosphating

Adding oxidizing agents such as NO₃⁻ or ClO₃⁻ to the phosphating solution significantly reduces the phosphating time.

• These agents oxidize the hydrogen ions produced during the reaction between steel and acid and the ferrous ions dissolved in the solution, maintaining a stable Fe²⁺ concentration.
• In solutions containing accelerators, the zinc content, although initially reduced, remains stable throughout the process under a specific acid ratio. This ensures consistent phosphating film composition, regardless of the treated area.
• Additionally, sedimentation in the solution is considerably lower than in solutions without accelerators, without compromising the corrosion resistance of the phosphating film. The bath solution’s lifespan is also significantly extended.

Regular testing and replenishment of the solution are necessary during production to ensure consistent film quality.

• Workpiece Handling: Large components are typically handled with specialized jigs or baskets, while smaller batch parts are treated using tumblers.
• Energy Efficiency: Recent advancements have reduced the required phosphating temperature from 92–98°C to 65–85°C, enabling sufficient film thickness without increasing energy consumption.
• Phosphating films can be tested with standard copper sulfate solutions, achieving 20–30 minutes without rusting.

Color Variations:

• High-carbon steel and high-carbon low-alloy steel produce black or deep gray-black films.
• Medium- and low-carbon steels result in silver-gray to gray films.

For black phosphating films, three methods are available:

1. Pre-dyeing before phosphating.
2. Black phosphating by adding dyeing agents to the phosphating solution.
3. Post-phosphating black dyeing.

Functions of Phosphating Treatment

1. Enhanced Corrosion Resistance

• Although thin, the phosphating film acts as a non-metallic, non-conductive isolation layer, transforming the surface from a good conductor to a poor conductor.
• This inhibits the formation of micro-galvanic cells on the metal surface, effectively preventing film corrosion.

2. Improved Adhesion

• The phosphating film forms a tightly bonded structure with the metal substrate.
• Its porous nature allows coatings to penetrate and bond more effectively, enhancing the overall adhesion between coatings and the phosphating film.

3. Clean Surface Preparation

• Phosphating films can only form on oil-free and rust-free surfaces, ensuring the treated metal workpieces have a clean, uniform, and oil- and rust-free surface.

4. Enhanced Cold Working Performance

• Phosphating improves material performance during cold processing operations.

5. Reduced Friction and Improved Sliding Properties

• Phosphating films enhance surface friction characteristics, promoting smoother sliding and improved wear resistance.