In many applications, NdFeB magnets must be protected by surface treatment in order to improve the corrosion resistance. There are several types of surface treatments with different character and function for option.
|Coating material||Suggested environment temperature||Coating thickness||Coating colour||Features|
|Phosphated||/||< 1 µm||/||Temporary storage and short term transportation protection|
|Ni||<200℃||5 – 20 µm||Light yellow||Good resistance to corrosion, suitable for dew atmosphere and long time using|
|Zn||<150℃||5 – 8 µm||Silver white||General resistance to corrosion and salt spray|
|Sn||<150℃||5 – 10 µm||Silver||Excellent corrosion resistance, ductility and weldability|
|Ag||<200℃||5 – 10 µm||Silver||Enhanced corrosion resistance, increasing the conductivity|
|Au||<200℃||5 – 10 µm||Golden||Suitable for decorating application|
|Cu||<200℃||5 – 10 µm||Yellow||Good electrical conductivity, used for local anti-carburizing|
|Ni-Cu-Ni||<200℃||10 – 30 µm||Silver||Excellent resistance to corrosion|
|Epoxy||<130℃||10 – 25 µm||Black/grey||Good resistance to corrosion, high temperature, high humidity, especially salt spray|
[NdFeB magnet electroplating process]
1. Process flow (suitable for automatic production line): sealing Bai hole → solidification → vibrating light → feeding → ultrasonic oil removal → water washing → water washing → pickling → water washing → water washing → ash removal → water washing → water washing → activation → water washing → water washing → pre nickel plating → bright nickel → recycling → water washing → water washing → water washing → sealing → water washing → hot drying → blanking → drying
2. Nd-Fe-B magnet electroplating process:
(1) Sealing hole: powder metallurgy Nd-Fe-B permanent magnetic material is the same as other powder materials, with many pores. In the process of electroplating, the solution infiltrates into the pores and is not easy to clean, which causes transverse corrosion of Nd-Fe-B and its coating in the process of use and storage, reduces the adhesion of the coating, blisters the coating, peeling, and pulverization of alloy materials.
The experimental results show that: if the sealing treatment is not carried out, there will be acid substances remaining in the NdFeB micropores during the pretreatment process, and after half a year of transverse corrosion, there will be pulverization and peeling phenomenon. Therefore, in order to obtain a good coating, sealing treatment must be carried out before plating. Different from the common powder metallurgy ferroalloy, the content of Neodymium in NdFeB alloy is as high as 30% (mass fraction). Neodymium is a highly active metal with a standard equilibrium potential of -2.431v, which can react with water, oxygen, etc. If the usual zinc stearate is used as sealing agent and working at about 180 ℃, the NdFeB permanent magnet will be seriously corroded, the size of the workpiece will be affected, and even the workpiece will be scrapped. Therefore, the hole sealing should be carried out under the condition of less than 60 ℃ and the presence of corrosion inhibitor.
(2) Curing: in the oven, at about 60 ℃, curing for 30-40 minutes, or room temperature for more than 2 hours.
(3) At room temperature until the presence of light and vibration inhibitor on the metal surface is satisfactory.
(4) In order to ensure the coating quality, it is necessary to strengthen the oil removal measures, and it is not suitable for electrolytic oil removal. Of course, according to the specific situation, this process can also be omitted.
(5) Pickling: pickling is a key step to affect the quality of coating. Improper treatment is easy to cause corrosion and affect the quality of coating
The corrosion inhibitor must be added.
(6) Ash removal: in addition to NdFeB, NdFeB alloy also contains a small amount of metal and nonmetal components insoluble in acid. Therefore, ash hanging is often produced after pickling. This kind of gray black material is insoluble in common inorganic acids (such as HF, HCl, H2SO4, H3PO4, etc.).
(7) Activation process;
(8) Pre nickel plating process;
(9) Bright nickel process.
Post time: Jan-04-2021