Overview
ISO 4212:2023 specifies an oxalic acid etching test for detecting intergranular corrosion (IGC) characteristics in austenitic stainless steels. The standard defines an electrolytic etching procedure and microscopic evaluation to determine whether a stainless steel specimen is non‑sensitized or suspected of sensitization. This method is a rapid, simple screening technique that complements ISO IGC tests (ISO 3651-1, ISO 3651-2, ISO 12732) and established practices such as ASTM A262-15 and JIS G0571.
Key Topics and Requirements
- Purpose: Screen for non‑sensitization or suspicion of sensitization by observing etched surface structure (does not by itself quantify sensitization).
- Test solution: Dissolve 100 g oxalic acid crystals in 900 ml deionized/distilled water (ISO 3696 Grade 3). For some Mo‑containing steels, ammonium peroxodisulfate (100 g/900 ml) may be used.
- Apparatus: DC power supply (example: 15 V, 20 A), variable resistor, ammeter, cathode (stainless beaker or plate), inert test vessel, holder and thermometer (±1 °C).
- Specimen preparation: Fine polishing (buffing with diamond suspension), isolation of non‑tested areas with insulating material; welded sections should include base metal, HAZ and weld metal where applicable.
- Etching conditions: Oxalic acid solution at 20–50 °C; etch as an anode for 90 s at 1 A/cm². For ammonium peroxodisulfate: 5–10 min at 1 A/cm².
- Microscopy: Examine entire etched surface; typical magnification 200×–500× for rolled, forged, cast products or weld zones.
- Etch classification:
- A (Step structure): orientation‑dependent step features.
- B (Dual structure): grooves at grain boundaries but no fully surrounded grains.
- C (Groove structure): one or more grains completely surrounded by grooves.
- Evaluation: A or B indicate no further ferric sulfate/65% HNO3/CuSO4/EPR testing required; C (grooved) indicates suspicion of sensitization and requires follow‑up testing (ISO 3651-1, ISO 3651-2, ISO 12732).
- Test report: Must reference ISO 4212:2023 and record steel grade, specimen details, sensitization heat treatment (if applied), solution and times, etching structure, solution temperature, deviations and anomalies, and test date.
Applications
- Rapid screening in corrosion laboratories and metal production QA/QC for austenitic stainless steel components.
- Weld and fabrication inspection to identify areas that may require further IGC testing.
- Material acceptance testing for manufacturers, fabricators, shipbuilding, chemical and power industry components where IGC risk is a concern.
- Metallurgists and corrosion engineers use it as a cost‑effective triage method before undertaking detailed IGC or electrochemical reactivation (EPR) tests.
Related Standards
Keywords: ISO 4212:2023, oxalic acid etching test, intergranular corrosion, austenitic stainless steel, sensitization, electrolytic etching, IGC screening, metallurgical inspection.