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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Summary

• Protective coatings

– Metallic and organic coatings behave very differently

– No coating at all is better than a bad coating!

• Testing methods

– Uncoated and coated magnets require different test

methods

– Loss of magnetization is important, not weight loss

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