Phosphating coating is a technical means of magnet surface treatment and effectively alleviates the disadvantage of NdFeB magnets that are prone to oxidation.
The Meaning of Phosphate Coating
Phosphate coating is to put the magnet into a solution containing phosphate, the solution reacts with the surface of the magnet to form a layer of phosphating film. Phosphate film is generally gray and difficult to dissolve in water, enhancing the antioxidant properties of magnets. Phosphate coating has been used to deal with the short-term storage of NdFeB magnets.
Image Source: Mpcomagnetics & Couragemagnet
Characteristics of Phosphate Coating
The phosphating treatment process is simple, the raw materials used are mainly acid, alkali, and phosphating solution, so the cost of phosphating treatment is low, and will not significantly increase the production cost of magnet manufacturers. Moreover, phosphate-treated magnets can also be further treated with other types of coatings to obtain better oxidation resistance.
Because of the good adhesion of the phosphating solution to the porous surface of the magnet, the phosphate-treated magnet is resistant to salt spray, acid, alkali, and corrosion. However, compared with metal-coated NdFeB magnets, it is less resistant to moisture and heat.
Functions of Phosphate Coating
Short-term Anti-oxidation | NdFeB without surface treatment is easily oxidized when exposed to the air. When NdFeB magnets are produced, temporarily unsure of the subsequent surface treatment method, it is generally necessary to use the phosphating process for NdFeB magnets to carry out simple anti-oxidation treatment. It can effectively reduce the loss of magnetic properties of magnets during the storage period.
After phosphating treatment, the NdFeB magnets have uniform color and clean surface. They can be vacuum encapsulated to extend the storage time, and the storage effect is better than oil immersion and other storage methods.
Image Source: Phosphating
The phosphating film can be completely cleaned by simple pickling and will not adversely affect subsequent magnet coating treatments, such as nickel plating.
Optimizing Surface Properties of Magnets | Due to the different use environments, some NdFeB magnets need to be treated with epoxy glue and epoxy paint. Epoxy glue or epoxy paint ring belongs to organic material, which needs the good-wetting-property base surface to function well.
As the phosphating film has good wetting properties, so phosphate-treated magnets can ensure bonding magnets and epoxy glue/epoxy paint tightly.
The Process of Phosphate Coating
The process of magnet phosphating treatment is: degreasing → washing → pickling → washing → surface adjustment → phosphating treatment → washing → drying.
Degreasing and pickling have the same function. They are used to remove oil, rust, etc. on the surface of the magnet, and improve the adhesion of the phosphating film.
Surface adjustment promotes the formation of a dense phosphating film and accelerates the phosphating speed, thereby facilitating the formation of the phosphating film.
Phosphating treatment uses phosphating liquid to treat magnets. Phosphating solutions are divided into zinc-based, iron-based, manganese-based, binary, ternary, multi-component, and other varieties. Each system has its advantages and disadvantages. Magnet manufacturers will choose the appropriate type of phosphating solution depending on different customers’ requirements.
The phosphating process in terms of temperature is divided into three types: high, medium, and room temperature. The high-temperature process generally generates thick film and the room-temperature process generates thin film. Magnet manufacturers mostly use the room temperature process (15 ℃ -35 ℃).
| Operating Parameters For Some Thick Phosphating Process | ||||
| Phosphating System | Accelerator | Concentration | Treatment Time | Coating Weight |
| Zn-Ph | None | 20-50 | 30-60 (min.) | 20-45 (g/m2) |
| Zn-Ph | Nitrate | 50-80 | 5-15 (min.) | 10-35 (g/m2) |
| Mn-Ph | None | 20-50 | 30-60 (min.) | 20-45 (g/m2) |
| Mn-Ph | Nitrate | 30-60 | 5-15 (min.) | 8-30 (g/m2) |
The Effect of Phosphate Coating
Sometimes the magnet will still rust after phosphating. There are many reasons for this phenomenon.
The concentration of the phosphating agent is not enough, which causes the phosphating film to be too thin or loose and cannot be well insulated.
The water-washing effect after phosphating is not good, resulting in acidic substances remaining on the magnetic material. The acid destroys the phosphating film and causes rust.
The degreasing effect should also be checked, as residual oil will also affect the phosphating effect.
