ISO 12732:2006

Overview

ISO 12732:2006 specifies the Double Loop Electrochemical Potentiokinetic Reactivation (DL‑EPR) test (based on Čihal’s method) for measuring the degree of sensitization (DOS) in stainless steels and nickel‑based alloys. The standard describes the DL‑EPR principle, apparatus and cell design, test solutions, specimen preparation, test procedure, result evaluation and reporting. Results provide an index of potential susceptibility to intergranular corrosion, pitting, and intergranular stress‑corrosion cracking, although prediction of these corrosion modes requires complementary testing.

Key Topics and Technical Requirements

• Principle: Polarize a specimen through an activation/passivation cycle then perform a cathodic sweep (double loop) to measure reactivation current/charge from depleted (sensitized) zones.
• Measured metrics: Reactivation peak current and integrated reactivation charge (Qr) compared to activation values (Ip/Ia or Qr/Qa) - normalized for grain size when relevant - to provide a DOS index.
• Apparatus/Instrumentation:
  • Scanning potentiostat with high accuracy (control to ±1 mV) and wide potential range (document guidance included).
  • Current measurement sensitivity appropriate for µA–mA current densities typical of EPR tests.
  • Cell materials inert to test solution (e.g., borosilicate glass, PTFE); Luggin capillary to minimize IR drop.
• Test solutions:
  • Typical H2SO4 + KSCN mixtures (H2SO4 ~0.5–2.5 M; KSCN ~0.001–0.05 M). KSCN is deliquescent.
  • Super‑duplex alloys: 3.0 M HCl (without KSCN) has been used effectively.
• Specimen preparation:
  • Defined surface finish (example: Ra < 0.25 µm recommended) and careful cleaning (ultrasonic wash, alcohol, acetone).
  • Sensitizing heat treatments and control specimens are described in Annex C.
• Practical cautions:
  • Heat‑affected weld zones may have non‑uniform sensitization; interpret DL‑EPR data cautiously.
  • Test cell volume and electrode mounting (flushed port/holder options in Annex A) affect results.

Applications - Who Uses ISO 12732

• Corrosion engineers, metallurgists and materials testing laboratories performing corrosion susceptibility assessments.
• Quality control and failure‑investigation teams in petrochemical, power generation, nuclear, marine, aerospace and manufacturing sectors.
• Weld inspectors and R&D groups evaluating heat treatments, welding procedures, and alloy selection for resistance to localized corrosion.

Related Standards

• ISO 8044 - Corrosion of metals and alloys: basic terms and definitions.
• ISO 643 - Steels: micrographic determination of apparent grain size.
• Annex D of ISO 12732 references correlations with ASTM G108 for sensitization metrics.

Keywords: ISO 12732, DL‑EPR, double loop EPR, degree of sensitization, stainless steel corrosion testing, nickel‑based alloys, Cihal method, intergranular corrosion, electrochemical reactivation.