Etching In Metallography

Etchants are composed of organic or inorganic acids, alkalis, or other complex substances, in a solvent such as water, alcohol, glycerine or glycol.

Over the years, many different reagents have been developed for specific purposes and materials. Therefore they must be chosen carefully to reveal the desired structure. The specimen is either held by tongs and immersed, with the polished face down into a small petri dish partly filled with the etching reagent, or swabbed with cotton wool, which has been saturated with the etching reagent. When etching, progress should be observed and timed, depending on the materials and reagent, this could vary from a few seconds to a minute or more.

After etching, the specimen should be thoroughly washed in water or alcohol and then dried using a hot air blower. The specimen should be carefully stored and cared for in a desiccator, to prevent oxidation and scratching.

Adjusting Flowrate - Step 3

Flowrate of electrolyte should be adjusted before operation. In other words, electrolyte should be reached upper level of the mask without any turbulance. Otherwise, polishing operation cannot be done properly. To adjust flowrate, press Pump button without placing sample. If the flow rate is not enough, increase the set value until the flow is sufficient.

Preparing Specimen - Step 4

Surface of specimen should not be rough for electrolyic preparation. Specimen should be grinded with 600 grit SiC grinding paper before electrolytic preparation.

Determining Correct Voltage - Step 5

Voltage is one of the most important parameter for electrolytic polishing. ELOPREP has scanning function for easy and exact determination of the parameters. With the scanning function of ELOPREP, correct voltage of polishing can be defined automatically. If you do not have the knowledge for the polishing voltage to be set for your sample, you can use the scanning function to determine exact voltage. You do not have to make time consuming trials and error testing for the correct voltage.

• Place specimen on the mask. Be sure that the aperture is covered and closed completely.
• Place the anode arm on the top of the sample to ensure good contact.
• Enter Scan menu, set scanning voltage and flowrate parameters and press Start.
• After a while, ideal polishing voltage for your specimen will be seen on the screen.

Electrolytic Polishing & Etching - Step 6

ELOPREP has 4 different operation modes; only polishing, only etching, automatic etching after polishing and external etching.

• Place specimen on the mask. Be sure that the aperture is covered and closed completely.
• Place the anode arm on the top of the sample to ensure good contact.
• Enter Polishing&Etching menu. Set parameters: Voltage, flowrate, mask type and time.

For only polishing: Set only polishing time and polishing voltage. Do not set etching time and etching voltage, these should be zero “0”. Press start and operation will be completed automatically. The specimen must be washed after polishing operation is completed.

Polished surface and microstructure (Microstructure of Al-Cu-Steel)

For only etching: Set only polishing time and polishing voltage. Do not set etching time and etching voltage, these should be zero “0”. Press start and operation will be completed automatically. The specimen must be washed after etching operation is completed. Microstructure of etched surface can be seen as below:

Microstructure of etched surface (Microstructure of Al-Cu-Steel)

For automatic etching after polishing: Set polishing voltage, polishing time, etching voltage and etching time. Do not set etching time and etching voltage. Press start. Etching will be done automatically after polishing operation is completed. The specimen must be washed after etching operation is completed. For external etching: If polishing and etching electrolytes different than each other, in this case external etching unit can be used.

• Fill external etching unit with electrolyte.
• Connect cables of external etching unit to control unit.
• Enter External Etching menu. Set etching voltage and time.
• Hold the sample with scissors.
• Plunge the sample into the external etching unit. As soon as the sample contacts with the electrolyte the process will start automatically.
• As soon as the set etching time is reached an audible signal notifies the operator that the process is completed. Wash your specimen after etching is completed.