Protective Coatings  Surface Finish for Neodymium Magnets

Protective Coatings Surface Finish for Neodymium Magnets

A Neodymium magnet does not corrode and will retain its performance theoretically forever in dry environment (if not subjected to excessive heat, radiation or strong external magnetic fields). However, if it is placed in a wet environment, the result may be the opposite. The within-structure Iron can ‘rust’ and causes a permanent structural change in NdFeB, which will result in a permanent weakening of the magnetic performance; the worst case scenario is a total loss of magnetism.

Therefore, it is recommended to apply a coating on the magnet surface to protect the magnet from moisture (e.g. encasing, modified coatings such as zinc plus rubber, etc). The coating/surface finish should be for best corrosion protection; scratched or damaged surfaced may render the affected region more prone to corrosion. Marine environments (salt sprays, sea water) are particularly corrosive and far from ideal for NdFeB. It is the application and the overall design that determines how well the magnet will perform in damp environments.

Main Coating Types

COATING APPLIED

NICKEL

EPOXY RESIN

Ni + EPOXY

Electroless

Powder Spray
Coating

E-Coating

Nickel plating
+ Epoxy E-Coating

Coating Thickness

Range (microns)

12 to 25

25 to 40

20 to 40

15 to 25

25 to 40

Homogeneity

Excellent

Good

Poor

Excellent

Good

Effectiveness Versus Magnet Size

Small (<20 grams)

Excellent

Good

Fair

Good

Good

Large (>20 grams)

Fair to Good

Good

Fair

Good

Good

Hours before coating is likely to fail

Temp. & Humidity
(60°C, 95%RH)

>2500

>500

>1500

>2500

Temp. & Humidity
(85°C, 85%RH)

>500

>100

>300

>500

Salt Spray
(35°C, 5% NaCl)

>48

<24

>100

>200

Coating Color

Silver

Silver

Black

Black

Black

Heat Cycle

Fair

Fair

Fair

Fair

Fair

Heat Resistance

Poor

Poor

Poor

Poor

Poor

Collision Test

Fair

Fair

Fair

Fair

Fair

Film to Material Adhesion test

Fair

Fair

Fair

Fair

Fair

Glue Adhesion Test

Fair

Fair

Fair

Fair

Fair

Tolerance Accuracy

Excellent

Excellent

Fair

Fair

Fair to Poor

Additional Remarks

15-30 microns Ni-Cu-Ni Standard coating

Epoxy resins are not hermetic

Thickness buildup can be a problem