Phosphating – Process, Advantages and Disadvantages
Phosphating is a surface technology process that modifies the surface layer of a metallic surface with an aqueous phosphate solution. Despite its high environmental incompatibility, the treatment is still frequently used to improve corrosion properties, electrical insulation and wear resistance.
During phosphating, the workpieces are immersed in an aqueous electrolyte solution containing free orthophosphoric acid, phosphates, zinc, calcium, sodium or manganese and accelerators. The treatment is used for many metallic materials, such as unalloyed steels, zinc, magnesium and aluminum. The process is only suitable for non-precious materials, as the material must be dissolved.
The process
Before phosphating, workpieces must be pickled in order to obtain a clean surface. Following the pickling process, the components are wetted with the phosphate solution by dipping, brushing or spraying. The phosphate layer is firmly anchored to the base metal through a chemical reaction. This layer contains many cavities and capillaries, which can optimally absorb oils and lubricants and thus ensure a very good lubricating effect. The absorption capacity also gives the layer the ability to provide corrosion protection and serve as an adhesive base for paints and varnishes.
A distinction is made between non-layer-forming and layer-forming phosphating.
- Layer-forming phosphating: cations originate from the phosphate solution
- Non-layer-forming phosphating: cations originate from the base material
Common types of phosphating are:
- Thin film
- Iron phosphate (pickling of aluminum and steel)
- Zinc phosphate (pre-treatment of subsequent coating)
- Thick film (Ca-phosphate)
Compared to iron phosphate, zinc phosphate offers greater corrosion protection.
Applications
Phosphate conversion coatings have a wide range of applications across various industries. Here are some of the key applications:
- Automotive Industry: Phosphate coatings are extensively used in the automotive industry. They are applied to car bodies and parts to improve paint adhesion and prevent corrosion. Components like gears, bearings, and fasteners often receive treatments to reduce wear and tear.
- Aerospace Industry: In the aerospace industry, phosphate coatings are used to protect aircraft parts from corrosion and wear. They also improve the adhesion of paint and other coatings, which is critical in this industry.
- Appliance Manufacturing: In the manufacture of appliances, phosphate coatings are used to enhance the adhesion of paint, thereby improving the appearance and lifespan of the product.
- Military and Defense: Military equipment and vehicles often receive phosphate coatings for enhanced corrosion resistance and paint adhesion. This helps to extend the service life of the equipment.
- Metalworking and Machining: In metalworking and machining, the process is used to reduce friction and wear on tools and equipment. Thereby, it can improve the efficiency of the machining process and extend the life of the tools.
- Biomedical Applications: Calcium phosphate coatings are used in the biomedical field, particularly in the manufacture of orthopedic implants like hip and knee replacements. The coating promotes bone growth and adhesion, improving the success rate of these procedures.
- Electronics Industry: Aluminum phosphate coatings are used in the electronics industry to provide electrical insulation and high-temperature resistance.
- Oil and Gas Industry: Nickel phosphate coatings are used in the oil and gas industry for their corrosion resistance.
Advantages
Phosphating is a widely used surface treatment due to its good adhesion to metallic surfaces and the properties of the conversion layer. The process is relatively simple and inexpensive, and the surface layer has good material properties:
Optimum adhesion base
Phosphating creates a very good adhesive base for further surface treatments. It is therefore a pre-treatment for further corrosion protection. It ensures a microporous/microcapillary layer structure, which provides good conditions for anchoring further treatments.
Temporary corrosion protection
Metal surfaces treated by phosphating can have better corrosion properties than the base material if the latter is susceptible to corrosion. However, the conversion layer is only temporary. Zinc phosphatings exhibit better corrosion protection than iron phosphatings. In addition, damaged layers are hardly penetrated by corrosion.
Electrical resistance
Phosphates have a high electrical resistance. For this reason, phosphated metals are particularly suitable for insulation.
Disadvantages of the process
Phosphates are considered to be particularly environmentally unfriendly. For this reason, waste from the process must be disposed of correctly as hazardous waste. However, due to their price and the simplicity of the process, they are still frequently used, for example for automotive paintwork or screws.
No further heat treatments should be carried out after phosphating, as the coating favors the formation of delta ferrite at elevated temperatures. Phosphating is not suitable as protection against wear. Tested processes such as BORINOX® or BoroCoat® are better suited as treatments to demonstrably improve wear protection.
