ASTM No. 97

Electrolytic

Composition

187.5g KOH, 245ml Hâ‚‚O

43%

Method

Electrolytic

Time

5–15 s at 1–10 V DC

Alternative Names

Reveals

Sigma phase (darkened preferentially); ferrite vs austenite contrast in duplex SS; chi phase; carbides

Typical Results

ASTM E407 #97 is an electrolytic KOH for sigma-phase identification in austenitic and duplex stainless steels. Sigma darkens preferentially; ferrite and austenite separate by voltage tuning. The standard etchant for sigma quantification per ASTM A923.

Application Method

Method:Electrolytic

Typical Time:5–15 s at 1–10 V DC

Preparation Notes

CAUTION: KOH is caustic. Dissolve 187.5g potassium hydroxide (KOH) in 245ml distilled water. Use in fume hood with proper PPE.

Application Notes

Sample is the anode; stainless cathode. Apply 1–10 V DC for 5–15 s. Lower voltages darken sigma selectively; higher voltages add ferrite/austenite contrast.

Troubleshooting

Strong CAUSTIC; full PPE required. Tune voltage for selectivity: sigma-only at 1–3 V; ferrite + sigma at 6–10 V.

Storage Notes

PE bottle at room temperature. Stable many months.

Alternative Etchants

10% Oxalic Acid (Electrolytic)
Murakami's Reagent (boiling)

Similar Etchants

10% Oxalic Acid (Electrolytic)