Intergranular Corrosion Bend Test: Principles, Standards, and Evaluation Methods

The intergranular corrosion bend test is a standardized qualification method used to evaluate the susceptibility of stainless steels, duplex steels, and nickel alloys to grain-boundary attack caused by sensitization. By exposing specimens to a boiling corrosive medium and subsequently bending them through 180 degrees, the test reveals intergranular fissures that indicate compromised structural integrity. This article explains the underlying metallurgical principles, applicable standards such as ASTM A262, GB/T 4334, and ISO 3651-2, sample requirements, test procedures, acceptance criteria, and typical industry applications.

Technical Overview

Intergranular corrosion (IGC) is a localized form of attack that preferentially occurs along grain boundaries, where thermal exposure during welding, stress relief, or improper heat treatment can cause chromium carbide precipitation. This phenomenon, known as sensitization, depletes the grain-boundary regions of chromium and leaves them vulnerable to dissolution in corrosive media. The affected material may retain an apparently intact surface while losing nearly all of its ductility and cohesion along the grain boundaries.

The intergranular corrosion bend test addresses this risk through a two-stage evaluation. First, a specimen is immersed in a boiling acidic solution that selectively attacks chromium-depleted zones. The specimen is then subjected to a controlled bend, typically through 180 degrees, which mechanically opens any weakened grain boundaries into visible fissures or cracks. The presence of intergranular cracking after bending constitutes evidence of sensitization and is grounds for rejection under most purchase specifications.

The bend test is incorporated directly into several internationally recognized standards, which define the corrosive medium, exposure time, bend geometry, and acceptance criteria:

• ASTM A262, Practices E and F — copper–copper sulfate–sulfuric acid tests for austenitic and molybdenum-bearing stainless steels, followed by a bend test
• ISO 3651-2 — determination of resistance to intergranular corrosion of austenitic, ferritic, and duplex stainless steels, including bend evaluation
• GB/T 4334, Method E — the copper–copper sulfate–16% sulfuric acid test with post-exposure bending for stainless steels
• JIS G 0575 — the copper sulfate–sulfuric acid test for stainless steels with bend acceptance
• ASTM G28 — corrosion testing of nickel-based alloys, where bend evaluation may supplement mass-loss measurement

It should be noted that not all intergranular corrosion practices rely on bending. The Huey test (ASTM A262 Practice C) and the Streicher test (ASTM A262 Practice B) quantify susceptibility through corrosion-rate measurement, while the oxalic acid etch test (ASTM A262 Practice A) serves as a rapid screening method based on etch-structure classification. The bend test remains the definitive acceptance criterion where direct evidence of grain-boundary embrittlement is required.

Testing Samples

Austenitic Stainless Steels

Grades such as 304, 304L, 316, 316L, 321, and 347 are the most frequently tested materials. Low-carbon and stabilized grades are evaluated to confirm that solution annealing or stabilization with titanium or niobium has effectively prevented sensitization, particularly after fabrication welding.

Ferritic and Duplex Stainless Steels

Ferritic grades and duplex grades such as 2205 and 2507 are covered under ISO 3651-2. Duplex materials require verification that the ferrite–austenite phase balance has been maintained and that heat treatment has not promoted intermetallic precipitation at grain boundaries.

Nickel-Based Alloys

Corrosion-resistant alloys such as Alloy 600, 625, 800, 825, and C-276 are tested per ASTM G28. Although mass loss is the primary metric for these alloys, bend testing of exposed specimens provides a supplementary check for grain-boundary embrittlement in critical service applications.

Welded Joints and Heat-Affected Zones

Weldments represent the highest-risk locations for sensitization because the heat-affected zone passes through the sensitizing temperature range. Test coupons containing the weld, heat-affected zone, and base metal are prepared so that the bend axis intersects the weld, ensuring that all three regions are simultaneously evaluated.

Product Forms and Specimen Geometry

Acceptable sample forms include plate, sheet, strip, tube, pipe, bar, wire, and forgings. Typical bend specimens measure approximately 10 mm in width and 75 to 100 mm in length, with a thickness of 5 mm or less. Thicker sections are machined on one or both surfaces, and specimen orientation relative to the rolling or welding direction is recorded as required by the governing standard.

Testing Items

A complete intergranular corrosion bend test program typically encompasses the following evaluation items:

• Bend ductility assessment after exposure to the boiling corrosive medium, normally through a 180-degree bend
• Visual and low-magnification inspection of the bent outer surface at 10x to 20x magnification for intergranular fissures or cracks
• Metallographic cross-sectional examination of any cracks to confirm an intergranular propagation path and exclude mechanical tearing or slag inclusions
• Optional determination of corrosion rate by mass loss when companion practices such as ASTM A262 Practices B and C are performed
• Optional sensitizing heat treatment, commonly 650 degrees Celsius for 2 hours followed by air cooling, when specified by the product standard or purchaser
• Issuance of a pass or fail verdict against the acceptance criteria of the applicable specification

Testing Methods

Specimen Preparation

Specimens are cut to the specified dimensions with edges rounded or smoothed by grinding to eliminate stress concentrators that could initiate mechanical cracks unrelated to corrosion. Surfaces are descaled, ground, and degreased to remove oxide, oil, and contaminants, and specimens are weighed when mass-loss measurement is required.

Optional Sensitization Treatment

When the specification calls for a sensitized condition, specimens are held at approximately 650 degrees Celsius for a defined period, typically 1 to 2 hours, and air cooled. This treatment deliberately precipitates chromium carbides in susceptible material and is used to evaluate the inherent sensitization resistance of the grade or the effectiveness of stabilization.

Corrosion Exposure

The classic Strauss-type exposure specified in ASTM A262 Practice E, GB/T 4334 Method E, and JIS G 0575 uses a boiling solution of 16% sulfuric acid saturated with copper sulfate, with copper chips placed in contact with the specimens to establish the copper–copper ion redox potential. Exposure is maintained at reflux for 24 hours in ASTM and GB/T procedures and for 20 hours under ISO 3651-2. For molybdenum-bearing grades, ASTM A262 Practice F specifies a 50% sulfuric acid–copper sulfate solution with a 120-hour boiling exposure. Nickel alloys are exposed per ASTM G28 in ferric sulfate–sulfuric acid solution for the prescribed duration.

Bend Test Procedure

After exposure, rinsing, and drying, each specimen is bent through 180 degrees in a press brake, vise, or dedicated bend fixture around a mandrel whose diameter is defined by the governing standard, typically equal to one or two times the specimen thickness. Welded specimens are oriented so that the root or face of the weld, as specified, is placed in tension on the outer bend radius. The bend is performed in a continuous motion without shock loading.

Examination and Acceptance

The tension surface of the bent specimen is examined visually and at 10x to 20x magnification. The absence of cracks or fissures indicates satisfactory resistance to intergranular corrosion. If cracking is observed, a metallographic cross-section is prepared perpendicular to the crack to verify intergranular morphology; cracks arising from machining marks, inclusions, or bending artifacts are adjudicated separately. Retest provisions, usually involving duplicate specimens, apply when the first result is borderline or disputed. All work is performed by accredited third-party laboratories or qualified in-house facilities under controlled documentation.

Applications

The intergranular corrosion bend test is specified across industries where grain-boundary degradation could compromise pressure integrity or product purity:

• Chemical and petrochemical processing — reactors, columns, heat exchangers, and piping exposed to organic acids, sulfur-bearing media, and chloride-limited acidic streams
• Nuclear power — primary circuit piping and internals where sensitization contributes to intergranular stress corrosion cracking susceptibility
• Oil and gas, including LNG — qualification of duplex and austenitic components, clad plates, and subsea hardware
• Pharmaceutical and food processing — verification of hygienic tubing and vessels where intergranular attack would create contamination sites
• Welding procedure qualification — confirmation that heat input and postweld treatment have not sensitized the heat-affected zone
• Heat treatment validation — verification of solution annealing and stabilization practices during mill production and component fabrication
• Incoming material inspection and supplier certification for pressure equipment built under ASME, EN, and other construction codes

FAQ

What is the purpose of the bend step in intergranular corrosion testing?

The bend step mechanically loads the grain boundaries after the corrosive exposure has selectively attacked any chromium-depleted zones. Even when the corrosion damage is invisible to the naked eye, bending opens the weakened boundaries into detectable fissures, providing a direct and conservative measure of sensitization.

Which standards require a bend test as the acceptance criterion?

ASTM A262 Practices E and F, GB/T 4334 Method E, ISO 3651-2, and JIS G 0575 all incorporate a post-exposure bend test. In contrast, ASTM A262 Practices B and C and ASTM G28 rely primarily on corrosion rate calculated from mass loss.

What does cracking after bending indicate?

Cracking or fissuring on the outer bend surface indicates that the material has been sensitized and that grain boundaries were attacked during the corrosion exposure. A confirmed intergranular crack path in metallographic cross-section is normally a cause for rejection.

What specimen thickness can be tested?

Bend specimens are generally limited to a thickness of about 5 mm. Thicker products are machined on one or both surfaces to the testable thickness, and both the as-received and machined conditions are documented in the test report.

Can welded samples be evaluated?

Yes. Welded coupons are prepared so that the weld metal, heat-affected zone, and base metal all lie within the bend region. This configuration is widely used for welding procedure qualification and welder performance qualification.

How is the final test verdict determined?

The verdict is pass or fail: a specimen passes when no intergranular fissures are found after the 180-degree bend and inspection at the mandated magnification. Results, specimen history, sensitization condition, solution composition, exposure duration, and bend geometry are all recorded to ensure traceability and reproducibility.