Corrosion protection and
testing of sintered Nd-Fe-B
magnets
Elisa Isotahdon
Department of Materials Science
Tampere University of Technology (TUT)
Magnetic Materials in Electrical Machine Applications 2012
Pori, Finland, 13.-15.6.2012
Contents
• Background
• Corrosion mechanism
• Protection with coatings
• Corrosion testing methods
• Ongoing study in TUT
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Background
• Well-known weakness of sintered Nd-Fe-B
permanent magnets is their low corrosion
resistance
• Corrosion behaviour different from traditional
engineering materials (e.g. steels)
– Several phases in microstructure
– Rare-earth component
– Functionality (not structural materials)
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Corrosion mechanism
• Dominant corrosion mechanism is the selective
corrosion of Nd-rich grain boundary phase,
which leads to detachment of Nd2Fe14B grains
• Pulverization
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Corrosion protection
• Corrosion cell
– Potential difference between the Nd-rich phase
(anode) and the matrix phase (cathode)
– Electrical connection between phases (humidity)
• Decreasing the potential difference
– cobalt alloying
• Preventing the connection between phases by
adding barrier for humidity
– Coatings!
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Different coating types
• Metallic: Ni, Sn, Zn, Al, multilayers
– If the coating is destroyed locally, there is a possibility
of forming a new corrosion cell where the coating is
the cathode
• accelerates the corrosion of the magnet!
• Organic
– Mostly epoxies
– Diffusion
– Lower resistance against high temperatures
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Adhesion
• It is difficult to achieve good adhesion to sintered
magnet
• Hydrogen embrittlement in electroplating
procedures
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Traditional
accelerated corrosion
tests
Salt spray tests
Electrochemical
measurements
•Marine environment
•Coatings
•Weight loss is measured
•Good for uncoated magnets
Other heat-humidity
tests
HAST
BCT (ASTM)
Corrosion testing
After 96 hours of salt spray
Uncoated Nd-Fe-B Nickel Tin
rent resin
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Black epoxy Zinc flake coating (organic binder) Transparent resin
What is worth measuring?
• No quantitative parameter for coating failures
• Measurement of total losses of magnetization
• After 500 hours in 85 °C/85 RH-%:
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Uncoated Transparent resin Tin coating
Ongoing study in TUT
• Two type of Nd-Fe-B magnets as substrates
• Corrosion behaviour of nickel, tin and six
different organic coatings is studied
• Material characterization of coated samples
before and after corrosion tests
– SEM studies
• Adhesion and its effect on corrosion
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Ongoing study in TUT
• Electrochemical corrosion measurements
– Polarization
– Impedance spectroscopy
• Quantitative information
• Salt spray tests, 85/85 tests
• Temperature cycling
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