ISO 3651-3:2017

Overview - ISO 3651-3:2017 (intergranular corrosion test for low‑Cr ferritic stainless steels)

ISO 3651-3:2017 specifies a laboratory corrosion test to assess the susceptibility of low‑Cr ferritic stainless steels to intergranular corrosion using a 0.5 % sulfuric acid / copper sulfate (CuSO4) boiling test. It applies to steels with composition limits (less than 16 % Cr; < 0.3 % Ni, Ti, Nb; and < 0.3 % (Ti + atomic‑weight‑ratio Nb)) supplied as rolled sheets or welded tubes intended for use in mildly oxidizing acidic media. The standard emphasises that results are valid only for the specific test medium and provide a basis for estimating intergranular corrosion resistance - they are not an absolute quality criterion nor are they predictive of other corrosion forms (pitting, stress corrosion, general corrosion).

Key topics and technical requirements

• Material scope: low‑Cr ferritic stainless steels with specified alloy limits.
• Specimen preparation:
  • Test piece area recommended: 5–20 cm², including weld zone, heat‑affected zone and base metal.
  • Mechanical descaling with iron‑free abrasives (≥ grade 600).
  • Degreasing (e.g., acetone or ethanol) and drying before testing.
• Sensitization heat treatment (to check intrinsic resistance):
  • Step 1: 1200 °C ± 10 °C for 30 min, water‑cool.
  • Step 2: 600 °C ± 10 °C for 30 min, water‑cool.
• Test solution & setup:
  • Prepare solution: 0.5 % H2SO4 + 24 % CuSO4 (CuSO4·5H2O); dissolve by boiling in a 1 L flask with condenser.
  • Copper filings/shavings covering specimens; ≥ 50 g Cu per litre to ensure galvanic coupling.
  • Immersion volume: at least 8 mL per cm² of specimen surface.
  • Boil for 15 h ± 3 h; use solution once.
• Evaluation:
  • Metallographic inspection (approx. ×500 magnification).
  • Cross‑section measurement: material may be reported resistant if maximum intergranular corrosion depth ≤ 5 µm (subject to agreement).

Practical applications and users

Who uses ISO 3651-3:

• Materials and corrosion engineers, quality control and metallurgical labs, manufacturers and fabricators who produce or weld low‑Cr ferritic stainless steels.
• Procurement and certification teams that need objective test data for specification compliance. Practical uses:
• Verifying sensitization susceptibility after welding or thermal processing.
• Comparing batches or welding procedures to reduce intergranular attack risk in mildly oxidizing acidic service.
• Supporting material selection and supplier acceptance testing (note: adapt the test when steels will be used in different service environments).

Related standards

• ISO 3651‑1 / 3651‑2 (other parts of the series for different stainless steels and acid test media)
• ASTM A763 (practices for detecting susceptibility in ferritic stainless steels)

Keywords: ISO 3651-3:2017, intergranular corrosion, low‑Cr ferritic stainless steels, sulfuric acid copper sulfate test, sensitization heat treatment, metallographic examination, corrosion susceptibility.