Intergranular Corrosion Testing Methods: Standards, Practices, and Evaluation Procedures

ASTM A763 – Ferritic Stainless Steels

ASTM A763 provides Practices W, X, Y, and Z, which apply ferric sulfate-sulfuric acid, ferric sulfate-50% sulfuric acid, copper-copper sulfate-16% sulfuric acid, and nitric acid exposures respectively, adapted to the sensitization behavior of ferritic grades.

ASTM G28 – Nickel-Rich, Chromium-Bearing Alloys

ASTM G28 Method A exposes specimens to a boiling ferric sulfate-50% sulfuric acid solution for 24 or 120 hours, depending on the alloy, with acceptance based on the calculated corrosion rate. Method B uses a more aggressive boiling mixed acid (ferric chloride, nitric acid, and hydrofluoric acid) for 24 hours and is applied to alloys such as 625, C-276, and other high-performance materials.

ISO 3651-1 and ISO 3651-2

ISO 3651-1 specifies the general principles, while ISO 3651-2 defines the specific test media: a 16% sulfuric acid-copper sulfate solution with metallic copper contact, a 35% sulfuric acid solution, a 50% sulfuric acid-ferric sulfate solution, and a 50% sulfuric acid-copper sulfate solution. Evaluation combines mass loss and bend testing.

GB/T 4334 and JIS G0571–G0575

The Chinese national standard GB/T 4334 series mirrors the major international practices, offering methods in sulfuric acid-copper sulfate (with and without copper contact), ferric sulfate-sulfuric acid, nitric acid, and oxalic acid etching. The Japanese standards JIS G0571 through G0575 provide equivalent procedures for the Japanese market, ensuring global harmonization of acceptance criteria.

ASTM G108 – Electrochemical Potentiokinetic Reactivation (EPR)

EPR testing is a rapid, quantitative electrochemical method for evaluating the degree of sensitization in austenitic stainless steels such as 304 and 304L. Single-loop and double-loop variants measure the reactivation charge density or the ratio of reactivation to activation currents, providing a numerical degree of sensitization without the long boiling acid exposures required by chemical immersion tests.

Evaluation of Results

Regardless of the method, results are interpreted through one or more of the following criteria: (1) corrosion rate from mass loss, where the rate is calculated from specimen dimensions and exposure time and compared against specification limits; (2) a 180-degree bend test over a defined mandrel, where visible cracking or fissuring on the tension surface indicates intergranular attack; and (3) metallographic cross-section examination to measure attack depth, confirm grain dropping, or classify etch structures. The applicable acceptance criteria are established by the governing product specification, purchase order, or design code.

Applications

Intergranular corrosion testing is applied across a wide range of industries and scenarios:

• Chemical and petrochemical processing equipment: reactors, columns, piping, and vessels exposed to corrosive acids
• Oil and gas: refinery equipment, sour service components, and offshore platform hardware
• Power generation: nuclear plant cooling circuits, steam generators, condenser tubing, and boiler components
• Pharmaceutical and food processing: hygienic piping, sanitary fittings, and vessels requiring corrosion-resistant surfaces
• Pulp and paper: digesters and bleaching plant equipment
• Marine engineering: seawater cooling systems, fasteners, and structural components
• Aerospace: aluminum-magnesium structural alloys assessed for sensitization
• Material acceptance and incoming inspection of mill products prior to fabrication
• Welding procedure qualification and welder performance verification
• Heat treatment validation, including solution annealing and stabilization treatments
• Failure analysis and root cause investigation of service cracking incidents

FAQ

What is the difference between the Huey, Strauss, and Streicher tests?

The Huey test (ASTM A262 Practice C, boiling 65% nitric acid for 240 hours) is highly sensitive to sigma phase and is preferred for nitric acid service. The Strauss test (Practice E, boiling copper-copper sulfate-16% sulfuric acid for 24 hours with copper contact, evaluated by bend test) primarily detects chromium carbide precipitation. The Streicher test (Practice B, boiling ferric sulfate-50% sulfuric acid for 120 hours) measures chromium depletion by mass loss. The correct choice depends on the alloy, the metallurgical phenomenon of concern, and the governing specification.

When should the oxalic acid etch test be used?

The oxalic acid etch test (ASTM A262 Practice A) is a rapid screening tool that takes only minutes. It classifies the etch structure as step, dual, or ditch. Specimens showing step or dual structures may be accepted for certain grades without further testing, while ditch structures require a confirmatory hot-acid test. It is never used alone for final acceptance.

What specimen preparation is required?

Typical specimens are cut to the dimensions specified by each standard (commonly around 25–30 mm by 50–100 mm for flat products), with surfaces ground to a uniform finish, deburred, degreased, cleaned, dried, and weighed to the required precision. Welded specimens must retain the weld, heat-affected zone, and base metal so that all regions are exposed to the test solution. Dimensional measurement of surface area is required for corrosion rate calculations.

Can sensitizing heat treatment be applied before testing?

Yes. Some specifications require a laboratory sensitizing heat treatment (for example, heating within the 650–675°C range followed by controlled cooling) before exposure, in order to evaluate the material's intrinsic susceptibility under worst-case thermal conditions. Whether sensitization is applied, and its exact parameters, must be defined by the purchase specification or test request.

How long does an intergranular corrosion test take?

Duration depends on the method: the oxalic acid etch takes less than one hour, the Strauss test requires 24 hours of boiling, ASTM G28 Method A requires 24–120 hours, the Streicher test requires 120 hours, and the Huey test requires 240 hours spread across five 48-hour periods. Additional time must be allowed for specimen preparation, post-exposure bend testing, metallography, and reporting.

What results constitute a failure?

A test is generally considered failed if the calculated corrosion rate exceeds the limit in the product specification, if the post-exposure bend specimen exhibits intergranular cracks or fissures visible at a defined magnification, or if metallographic examination reveals grain boundary attack or grain dropping beyond the acceptance depth. Accredited third-party laboratories report all measured values and observations, but the final accept/reject decision is made against the criteria of the governing standard or specification.

Why is intergranular corrosion testing important after welding?

Welding creates thermal cycles that pass portions of the heat-affected zone through the sensitization temperature range, potentially forming chromium carbides at grain boundaries. Testing cross-weld specimens verifies that the base metal, filler metal, and welding procedure together provide adequate resistance to intergranular attack in service, and it is a routine requirement for welding procedure qualifications in corrosion-critical applications.