I have prepared an etching solution of 60% HCl and 40% HNO3 according to ASM Handbook of metallography. after only 5 seconds of etching, heavy pitting occurs on the samples. the concentrations of HCl and HNO3 are 37% and 65% respectively. what combination of these acids would be proper?
what other solution (containing HF, NaOH, etc.) would be useful?
The reagent you are using is very strong, so the material is stinging. Use the reagent from "Keller's (ASTM E407-07)". It is a mixture of HF, HCl, HNO3 and water. Attach the newly polished specimen, for at least 5 minutes.
1. Use Keller's reagent (2-3% nitric, 1-2% hydrochloric and 1% fluoric acid) by immersion only to form a surface film for 10 to 20 seconds. Wash in a stream of warm water afterwards.
Keller's is a general purpose micro etchant, show microstructure and grains. Contains hydrofluoric acid.
This etchant is listed in ASTM Standard E 407 as formula number 3 Keller’s
or
2. Use Caustic Etching (NaOH as you mentioned, with a concentration of around 10-15%) Solution by immersing the sample in this etchant for 5 to 15 minutes and rinse.
Caustic Etching solution is a good general purpose macro etchant to etch aluminum alloys for visual inspection. It is ideal for aluminum welds.
The formula of this product is listed in ASTM Standard E 340 table 1.
For microetching of Al alloys Keller's reagent (2-3% nitric, 1-2% hydrochloric and 1% fluoric acid) is generally used by immersion only to form a surface film for 10 to 20 seconds. Subsequently wash in a stream of warm water afterwards.
If it contains that much silicon maybe it is better to use an alkaline etchant like NaOH, KOH or TMAH, possibly at elevated temperatures. Silicon is generally quite stable in acidic environment.
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