ISO 3651-2:1998

Overview

ISO 3651-2:1998 specifies standardized laboratory methods to determine resistance to intergranular corrosion of stainless steels - specifically ferritic, austenitic and ferritic‑austenitic (duplex) grades - using corrosion tests in media containing sulfuric acid. The standard defines test purposes, specimen preparation, sensitization treatments (heat or welding), three corrosive test methods (A, B and C), and post‑test evaluation (bend or flattening tests). It applies to cast, rolled, forged products and tubes intended for mildly oxidizing acid environments (for example, sulfuric or phosphoric acid).

Key Topics and Requirements

• Purpose of the test
  • Verification of intrinsic resistance for low‑carbon (C < 0.03 %) or stabilized steels after a sensitization cycle.
  • Inspection of solution‑treatment effectiveness for thin products in the as‑delivered condition.
• Sensitization
  • Heat treatments defined for austenitic and duplex steels: T1 = 700 °C ±10 °C for 30 min (water quench) or T2 = 650 °C ±10 °C for 10 min (water quench).
  • Sensitization may alternatively be produced by welding; as‑welded specimens are tested without further heat treatment.
• Test methods (choose by agreement)
  • Method A (16% H2SO4 + CuSO4) - Monypenny/Strauss test. Solution recipe and copper filings used; specimen immersed cold, then boiled for 20 h (±5 h).
  • Method B (35% H2SO4 + CuSO4) - higher acid concentration; similar procedure, solution may be reused under conditions.
  • Method C (40% H2SO4 + Fe2(SO4)3) - ferric sulfate based; solution single‑use.
• Specimen preparation
  • Mechanical descaling (iron‑free abrasives) or controlled chemical descaling; degreasing before test.
  • Test piece surface area requirement (15–35 cm²); thickness limits (max 6 mm) and dimensional guidance for bending apparatus.
• Evaluation
  • After boiling exposure, specimens undergo a bend test (≥90°) or flattening test for small tubes; inspect for intergranular cracking under low magnification. Metallography or comparative unexposed bends can confirm ambiguous results.

Applications and Users

• Intended for corrosion testing laboratories, materials engineers, quality assurance teams, welding inspectors, and producers/end‑users of stainless steel components (tubes, forgings, plates) who need to:
  • Verify alloy susceptibility to intergranular corrosion after heat exposure or welding.
  • Validate solution annealing and fabrication practices.
  • Support product standards, procurement specifications and failure investigations for components used in acidic or mildly oxidizing environments.

Related Standards

• ISO 3651-1 - Huey test: nitric acid mass‑loss method for austenitic and duplex steels.
• Annexes A and B of ISO 3651-2 provide application examples and comparisons with national standards (informative).

Keywords: intergranular corrosion, stainless steels, sulfuric acid test, Strauss test, Monypenny Strauss, ferritic, austenitic, duplex stainless steel, sensitization, bend test, flattening test, copper sulfate, ferric sulfate.