Macro-etching WAAM steel

The following is an SOP on the macro-etching of WAA-manufactured steel samples. Macro-etching enables viewing of a material's macrostructure, including grains, weld beads, composition gradients, and other information. There are several methods of macro-etching; this page will cover the use of hydrochloric acid.

Loss Prevention

• In keeping with safety principles for all laboratory spaces, safety glasses must be worn at all times.
• Ensure that the fume hood is operating prior to the use of any solvents or acids. This is done by pressing the green "START" button at the top of the control bank at the right side of each fume hood.
• Hydrochloric Acid (HCl) is corrosive and must be used with caution. Users are required to wear lab coats (available on the wall to the left of the fume hoods in M&M U108) and nitrile gloves (found on the counter to the right of the fume hoods). Use tongs to handle samples that have been in contact with acid.
• Do not touch the face or eyes at any time during this procedure, until gloves have been removed and hands have been thoroughly washed.
• To avoid splashing of acid, pour into any required glassware in a slow and controlled manner. Clean up all spills and drops. Neutralize any excess acid with the use of an appropriate base. The resulting mixture must be washed down the drain in the sink under the fume hood. Following this, run cold water down the drain for a few minutes to dilute the waste material.
• A hot plate is required for this procedure. Ensure that the plate is cool prior to use. Check that the red light above the power control knob is off, then hold the back of your hand near the surface of the plate, at a safe distance. If the plate is hot, wait until it is fully cooled before use. Do not touch the plate during operation. Use tongs to handle any samples that have been heated. Remember to turn off the hot plate when the procedure is complete.
• Use a thermometer, located by the sink beside the auto-polishers in M&M U108, to test the temperature of any solutions used in the procedure.

Procedure

1. Polish samples down to 1200 grit. The SOP for sample polishing can be found here. Samples may be too large to mount for use on the auto-polishers - if this is the case, the hand-polishing tables offer an acceptable substitute.
   • Thoroughly clean samples with soapy water, followed by ethanol. Dry samples using the hair dryer prior to immersing in HCl.
2. Prepare a solution of 50% HCl and 50% water, using a volume sufficient for the samples' surfaces of interest to be fully immersed. This must be done under the fume hood, while it is running.
   • Pour water into the required glassware first, to avoid boiling of water when added to acid.
   • Constantly stir the mixture while adding acid, to ensure uniform dissolution and controlled mixing.
3. Place the vessel on the hot plate and turn on the control knob. Monitor the solution with a thermometer until it reaches 70 degrees Celsius. Then, turn the control knob to the "3" position; this is sufficient power to maintain the required temperature for etching.
4. Slowly immerse samples into the heated acid solution. The surfaces of the steel will effervesce throughout the procedure. Leave the samples in solution for 15 minutes, while monitoring the temperature.
5. Carefully remove the samples from the acid bath, one at a time. Each sample must be promptly and thoroughly rinsed with cold water, then cleaned with ethanol and dried. Place samples away from the fume hood.
6. Turn off the hot plate and allow the solution to reach room temperature. Remove the vessel from the hot plate.
7. SLOWLY add a base (NaOH or NaHCO₃ are acceptable) to the vessel of acid solution. Proceed until the solution stops bubbling. Stir consistently. Turn on the cold water and slowly pour the neutralized solution down the drain. Rinse the vessel thoroughly with water. Once all glassware, tongs, thermometers, etc. are cleaned, allow the water to run for several minutes to dilute the solution poured down the drain.
8. Remove nitrile gloves and dispose of in a garbage can. Place all glassware on the racks by the auto-polishers to dry.