NdFeB surface treatment: phosphating
Everyone knows that sintered NdFeB is easy to be oxidized and corroded, which will cause the attenuation or even loss of magnetic properties over time, so strict anti-corrosion treatment must be carried out before use. In the previous article, yueci has introduced the knowledge related to the surface treatment of sintered NdFeB and the electroplating process . In addition to electroplating, NdFeB surface treatment can also use electroless plating, electrophoretic coating, phosphating treatment and other methods. Today, yueci will introduce you to the relevant knowledge of phosphating.
What is phosphating?
Phosphating is a process of forming a phosphate chemical conversion film by chemical and electrochemical reactions. The phosphate conversion film formed is called a phosphate film.
The purpose of phosphating treatment:
The phosphating surface treatment of sintered NdFeB permanent magnet has two purposes. The first is to prevent corrosion in the process, and the second is to improve the surface epoxy wettability.
Due to the low density and porous characteristics of the sintered NdFeB surface, the exposed magnet will be oxidized in the air, which is also called corroded. Therefore, when the NdFeB magnets are too long in turnover and storage, and the subsequent surface treatment methods are not clear, it is necessary and effective to use the phosphating process for simple anti-corrosion treatment.
The phosphating process is simple and does not require equipment investment. The phosphating production process consumes less, mainly acid, alkali and phosphating solution. The production cost is low. As a process anti-corrosion, it will not significantly increase the production cost of the magnet, and it can avoid the storage period. Loss of magnet, high rate of return. After phosphating, the product has a uniform color and a clean surface. It can be vacuum packaged to greatly extend the storage time. The storage method is better than the previous storage methods of oil immersion and oil coating. The complete phosphating film can resist oxidation and corrosion in normal atmospheric environment. The subsequent surface treatment of phosphating magnet products is very easy. The phosphating film can be completely removed by simple pickling. It will not have adverse effects on subsequent surface treatments (such as zinc plating, nickel plating, etc.), and the subsequent surface can be completed at any time Treatment process.
Some neodymium iron boron magnets need epoxy adhesive bonding, painting, etc. The bonding force of epoxy organic compounds such as glue and paint requires good wettability of the matrix. Phosphating film and epoxy organic matter have good wettability, so phosphating process needs to be used to improve the wettability of NdFeB magnet surface. Good wettability can ensure the bonding force of epoxy glue, epoxy paint and magnet.
The principle and process of phosphating
The process of phosphating treatment of the magnet surface is: degreasing → washing → pickling → washing → surface conditioning → phosphating treatment → sealing and drying.
Degreasing and pickling are the same as the pretreatment for electroplating. The surface adjustment is a special surface treatment on the surface of the NdFeB magnet to adapt to the formation of the phosphating film. The surface adjustment of neodymium iron boron magnets generally adopts weak acid soaking to improve the alloy composition of the magnet surface to achieve the purpose of facilitating the formation of phosphating film.
The phosphating process currently mainly uses commercial phosphating solution for production. Commercial phosphating solution is divided into zinc series, iron series, manganese series, binary, ternary, multi-element and other varieties. Each system has its own advantages and disadvantages. Therefore, in the production and application of NdFeB phosphating, each magnet manufacturer Depending on the purchase price and the effect of use, we will use the products we think are suitable. Some NdFeB products require phosphating after galvanizing, and phosphating after galvanizing should use zinc-based phosphating solution or multi-component phosphating solution containing zinc.
The phosphating process is divided into three types in terms of temperature: high temperature, medium temperature and normal temperature. The high temperature process generally has a thick film layer, and the phosphating film has a high density and a large thickness; the normal temperature process generally has a thin film layer; the medium temperature process is somewhere in between. Magnetic material manufacturers mostly use normal temperature processes, but normal temperature processes also require temperature control to ensure temperature stability (15°C-35°C).
Rust problem after phosphating
Some readers say that the magnetic material will still rust after phosphating. There are many reasons for this phenomenon. For example, the concentration of phosphating agent in the phosphating bath is not enough, which leads to the phosphating film being too thin or loose. , Can not isolate the air well; after phosphating, the washing is not clean, or the acidity of the washing pool is too high, causing acidic substances to remain on the magnetic material, and the acid will destroy the phosphate film and cause rust; in addition, check whether the degreasing is complete , Degreasing impure will also affect the phosphating effect.
In order to ensure the phosphating effect, special attention should be paid to the distribution ratio of the phosphating solution during the phosphating process to ensure an appropriate temperature; the phosphating time is controlled at 10-15 minutes; the hanging or reducing loading is used to ensure that there is a certain amount of magnetic material. The gap can be fully phosphated.
Source: China Permanent Magnet Manufacturer – www.rizinia.com