Electrolytic preparation is a fast and efficient method for metallographic preparation, providing a deformation-free surface, whereas mechanical grinding and polishing tends to leave a thin deformation layer at the surface.
Using the electrolytic polishing equipment, the specimen is configured as the anode in a suitable electrolyte, and material is removed by controlled dissolution of the surface of the specimen. Electrolytic polishing can be followed by an electrolytic etching process to bring forth contrasts in the microstructure of the specimen.
To do electrolytic preparation, the right method needs to be selected. Struers’ equipment contains ten standard methods for different materials. If these standard methods do not fit the material, a correct electrolyte can be selected from the Selection of Electrolyte list. Having selected the correct electrolyte for your purpose, it is necessary to determine the polishing voltage. This is done using a scan function that is built into the electrolytic preparation equipment.
Correct application settings
After placing a specimen on the polishing table, a pre-defined voltage range is scanned to determine the current density curve. This curve is used to define the correct polishing voltage for the polishing or thinning process. The other parameters to be determined are polishing time and the flow rate. Once the correct application settings have been determined, they can be stored in the database and recalled for subsequent polishing tasks.
Safety is important.
Safety is a key word when dealing with electrolytes, and all users must be familiar with the relevant safety procedures.
How to Do Electrolytic Preparation
Selection of Electrolyte
Several materials can be polished/etched with more than one of the electrolytes.
The right choice depends on the composition of the material and what you want to see.
Troubleshooting
Problem
Surface not polished or only partly polished
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Cause:
Current density insufficient Solution:
Adjust the voltage.
Cause:
The electrolyte is too old. Solution:
Renew the electrolyte.
Cause:
Insufficient quantity of electrolyte. Solution:
Add the electrolyte.
Spots that have not been polished
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Cause:
Gas bubbles Solution:
- Adjust the flow rate.
- Check the electrolyte temperature.
- Decrease the voltage.
Etching of the polished surface
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Cause:
Chemical attack of grain boundaries after switching of f the current. Solution:
- Remove the specimen immediately when the curren t is switched off.
- Choose a less corrosive electrolyte.
Phases in relief
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Cause:
Polishing film is inadequate. Solution:
- Increase the voltage.
- Improve the mechanical preparation of the specimen.
Cause:
The polishing time is too long. Solution:
Reduce the polishing time.
Pitting
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Cause:
The polishing time is too long. Solution:
- Improve the mechanical preparation.
- Decrease the time.
Cause:
The voltage is too high. Solution:
Lower the voltage.
Cause:
Insufficient anodic layer. Solution:
- Reduce the flow rate.
- Use a different electrolyte.
Attack at the edge of the specimen.
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Cause:
The film is too viscous or too thin. Solution:
- Decrease the voltage.
- Increase the flow rate.
The center of the specime n is deeply attacked or not polished at all.
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Cause:
Polishing the film did not for m in the center of the specimen. Solution:
- Increase the voltage.
- Reduce the flow rate.
- Add more electrolyte.
Deposits on the surface
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Cause:
Insoluble reaction products. Solution:
- Renew the electrolyte or try a different one.
- Increase the voltage.
Wavy surface or residual deformation
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Cause:
The polishing time is too short. Solution:
Increase the time.
Cause:
The flow rate is too high or too low. Solution:
Change the flow rate.
Cause:
Rough surface after grinding. Solution:
Improve the preparation prior to electrolytic polishing.
Cause:
The electrolyte is too old. Solution:
Renew the electrolyte.
Cause:
Selective polishing because of potential differences in surface (heterogeneous material). Solution:
Use another electrolyte with other polishing data or use a mechanical preparation instead.
