Purpose of phosphating treatment:
There are two purposes for phosphating surface treatment of sintered NdFeB permanent magnets. The first is to prevent corrosion during the process, and the second is to improve the epoxy wettability of the surface.
1. Process corrosion protection Due to the sintered NdFeB surface's low density and porous characteristics, the exposed magnet will oxidize in the air, which we also call corrosion. Therefore, when the NdFeB magnet has a long turnover and storage time, and the subsequent surface treatment method is unclear, it is essential and effective to use the phosphating process for simple corrosion protection. The phosphating process is simple and does not require equipment investment. The production process of phosphating consumes less, mainly acid, alkali, and phosphating liquid, and the production cost is low. As a process corrosion protection, it will not significantly increase the production cost of the magnet, it can avoid the loss of magnets during the storage period, and the return rate is very high. After phosphating, the product has a uniform color and clean surface. It can be vacuum packaged to greatly extend the storage time. The storage method is better than the previous oil immersion and oil coating storage methods. The complete phosphating film can resist oxidation corrosion in the normal atmospheric environment. The subsequent surface treatment of phosphate magnet products is very easy. The phosphating film can be completely removed by simple pickling, which will not have adverse effects on subsequent surface treatment (such as galvanizing, nickel plating, etc.), and the subsequent surface treatment process can be completed at any time.
2. Improve the wetting performance Some NdFeB magnets need epoxy glue bonding, painting, etc. The bonding strength of epoxy organics such as glue and paint requires the substrate to have good wetting performance. The phosphating film has good wetting performance with epoxy organics, so the phosphating process is needed to improve the wetting performance of the surface of NdFeB magnets. Good wetting performance can ensure the bonding strength of epoxy glue, epoxy paint, and magnets.
Principle and process flow of phosphating The process of phosphating treatment of magnet surface is:
Degreasing → Water washing → Pickling → Water washing → Surface conditioning → Phosphating treatment → Sealing and drying.
Degreasing and pickling are the same as pre-treatment before electroplating. Surface conditioning is a special surface treatment for the surface of NdFeB magnets to adapt to the formation of phosphating film.
The surface conditioning of NdFeB magnets generally adopts weak acid immersion to improve the alloy composition of the magnet surface, to achieve the purpose of phosphating film formation.
The phosphating process is currently mainly produced by commercial phosphating liquid.
Commercial phosphating liquid is divided into zinc series, iron series, manganese series, binary, ternary, and multi-element varieties. Each system has its advantages and disadvantages. Therefore, in the production and application of NdFeB phosphating, each magnet manufacturer will use the supply variety they think is suitable due to different purchase prices and use effects.
Some NdFeB products require phosphating after galvanizing. After galvanizing, phosphating should use zinc series phosphating liquid or zinc-containing multi-element phosphating liquid. The phosphating process is divided into three types in terms of temperature: high temperature, medium temperature, and room temperature. The high-temperature process generally has a thick film layer, high density, and large thickness of the phosphating film; the room-temperature process generally has a thin film layer; the medium-temperature process is between the two. Magnetic material manufacturers mostly use room temperature processes, but room temperature process also needs temperature control to ensure temperature stability (15℃-35℃).