Intergranular corrosion (IGC) sensitivity testing evaluates the susceptibility of metals—particularly stainless steels and nickel-based alloys—to localized grain boundary attack caused by sensitization during welding, heat treatment, or service exposure. This technical guide explains the sensitization mechanism, standard test methods such as ASTM A262, GB/T 4334, and ISO 3651, sample requirements, evaluation criteria, and typical industrial applications, providing engineers and quality professionals with a complete reference for IGC qualification testing.
Technical Overview
Intergranular corrosion (IGC) is a localized form of corrosion that preferentially attacks the grain boundaries of a metal while leaving the grain interiors largely unaffected. In austenitic stainless steels, the most common cause is sensitization: when the material is heated within the critical temperature range of approximately 450 °C to 850 °C (840 °F to 1560 °F)—typically during welding, slow cooling after annealing, or extended service at elevated temperatures—chromium carbides (Cr23C6) precipitate along the grain boundaries. This precipitation depletes the adjacent zones of chromium to below the ~12% threshold required for passivation, leaving these depleted corridors vulnerable to preferential corrosive attack.
IGC is particularly dangerous because the metal may show little or no visible deterioration on the surface while losing a substantial portion of its load-bearing cross-section along grain boundaries. In severe cases, sensitized material can disintegrate into powder-like grains under slight mechanical stress, making intergranular corrosion a critical failure mechanism in pressure vessels, piping systems, and chemical processing equipment.
Intergranular corrosion sensitivity testing, therefore, serves as an essential quality control and material qualification tool. Accredited third-party laboratories perform these tests to verify that delivered materials have received correct solution annealing, that welded fabrications are free of harmful sensitization, and that low-carbon or stabilized grades meet the corrosion resistance specified in purchase orders and design codes. The principal international and national standards governing this testing include ASTM A262 and ASTM A763 (USA), ISO 3651-1 and ISO 3651-2 (international), GB/T 4334 (China), and JIS G 0571 through G 0575 (Japan).
Testing Samples
Austenitic Stainless Steels
The most frequently tested material group includes grades such as 304, 304L, 316, 316L, 316Ti, 321, 347, 310S, and 904L. Both solution-annealed base metal and sensitized or as-welded conditions are tested, since sensitization in the heat-affected zone of weldments is the classic failure scenario for these alloys.
Ferritic and Duplex Stainless Steels
Ferritic grades such as 409, 430, 439, and 444 can suffer grain boundary attack due to chromium carbide or nitride precipitation and are tested in accordance with ASTM A763 or ISO 3651-2. Duplex stainless steels such as 2205 and 2507 are evaluated for intermetallic phase precipitation (sigma, chi, and chromium nitrides) using methods adapted from ASTM A262 Practice A, B, or F and related procedures.
Nickel-Based and Chromium-Bearing Alloys
Solid-solution strengthened nickel-chromium and nickel-chromium-molybdenum alloys, commonly used in aggressive chemical environments, are tested under ASTM G28 (Method A using ferric sulfate–sulfuric acid, and Method B using a mixed acid solution) to detect grain boundary precipitation of carbides and intermetallic phases.
Aluminum Alloys
5xxx series aluminum-magnesium alloys in sensitized tempers can undergo grain boundary attack from beta-phase (Al3Mg2) precipitation. ASTM G67 (NAMLT test) and ASTM G66 (ASSET test) are applied to quantify exfoliation and intergranular susceptibility.
Product Forms and Fabricated Components
Test samples are routinely prepared from plates, sheets, strips, bars, forgings, castings, pipes, tubes, fittings, flanges, fasteners, and welded joints. When testing welded components, specimens are oriented to include weld metal, heat-affected zone, and base metal so that the entire metallurgical cross-section of the joint is examined.
Testing Items
Typical test items offered in an intergranular corrosion sensitivity testing program include:
- Oxalic acid etch test (rapid screening test for austenitic stainless steels)
- Ferric sulfate–50% sulfuric acid test (weight-loss immersion test)
- 65% nitric acid test, also known as the Huey test (five 48-hour boiling cycles)
- Nitric–hydrofluoric acid test for molybdenum-bearing stainless steels
- Copper–copper sulfate–sulfuric acid test with copper chip contact (Strauss test) followed by bend evaluation
- 16% sulfuric acid–copper sulfate test and 50% sulfuric acid–copper sulfate variants
- Post-exposure bend testing, metallographic examination, and corrosion rate calculation
- Weldment and heat-affected zone susceptibility assessment
- Heat treatment verification and sensitization curve studies
Testing Methods
ASTM A262 Practice A: Oxalic Acid Etch Test
A rapid screening method in which polished specimens are electrolytically etched in 10% oxalic acid at a current density of approximately 1 A/cm² for 90 seconds. The etched microstructure is examined at 250× to 500× magnification and classified as step, dual, or ditch structure. Materials showing a step or dual structure are accepted as immune to intergranular attack; only ditch structures require further confirmatory testing under Practice B, C, E, or F. This method is fast (results within minutes) but applies only to specific alloy composition limits and cannot be used as a final acceptance test by itself.
ASTM A262 Practice B and GB/T 4334 Method B: Ferric Sulfate–Sulfuric Acid Test
Specimens with measured surface area are immersed in a boiling solution of 50% sulfuric acid containing 25 g of ferric sulfate for 120 hours. The oxidizing ferric ions accelerate grain boundary attack in sensitized material. Evaluation is by weight loss, and the corrosion rate is calculated from the formula CR = (K × W) / (A × T × D), where W is the mass loss, A is the exposed area, T is the exposure time, D is the density, and K is a unit constant. Acceptance limits are commonly defined by product specifications, for example 1.14 μm/h (approximately 0.045 mil/h) or lower for many austenitic grades.
ASTM A262 Practice C and GB/T 4334 Method C: Nitric Acid (Huey) Test
The most severe of the common ASTM A262 practices, the Huey test immerses specimens in boiling 65% nitric acid for five consecutive 48-hour periods, with weight loss measured after each period. It is particularly sensitive to chromium carbide precipitation and sigma phase, and it is widely specified for applications involving nitric acid service. An increasing corrosion rate from period to period often indicates progressive grain boundary attack.
ASTM A262 Practice E and GB/T 4334 Method E: Strauss Test
Specimens are boiled for 24 hours in a 16% sulfuric acid solution saturated with copper sulfate, with copper chips placed in contact with the samples to establish a low corrosion potential that selectively attacks chromium-depleted zones. After exposure, the specimen is bent 180° over a mandrel with a diameter equal to the specimen thickness. The presence of cracks or fissures on the tension surface, confirmed metallographically to be intergranular, indicates sensitization. Practice F applies a more aggressive 50% sulfuric acid–copper sulfate solution for 120 hours, typically used for molybdenum-bearing grades such as 316 and 317.
Nitric–Hydrofluoric Acid Test
Specified for molybdenum-containing austenitic stainless steels such as 316 and 316L, this test immerses samples in a 10% nitric–3% hydrofluoric acid solution at 70 °C for two hours. Because the etching is uniform, evaluation is performed by comparing the corrosion rate of the test specimen against that of an unsensitized reference sample; a ratio above 1.5 to 2.0 typically indicates susceptibility.
ASTM G28 for Nickel-Based Alloys
Method A uses the same ferric sulfate–50% sulfuric acid boiling solution as ASTM A262 Practice B with a 120-hour exposure, while Method B uses a boiling mixed acid of 23% sulfuric acid, 1.2% hydrochloric acid, 1% ferric chloride, and 1% cupric chloride for 24 hours. Both are weight-loss methods used to detect grain boundary precipitates in high-performance alloys.
Evaluation and Reporting
Regardless of the specific method, final evaluation combines corrosion rate calculation, bend ductility assessment, and metallographic cross-section examination to confirm whether attack is intergranular, the depth of intergranular penetration, and the presence or absence of grain dropping. A comprehensive test report includes material designation and heat number, heat treatment condition, specimen dimensions and surface area, test standard and practice used, solution concentration and boiling temperature, exposure duration, weight loss and calculated corrosion rate, bend test results, and metallographic findings with representative photomicrographs.
Applications
Intergranular corrosion sensitivity testing is a mandatory or highly recommended qualification step across many industries:
- Petrochemical and chemical processing: reactors, heat exchangers, distillation columns, and piping handling nitric, sulfuric, and organic acids
- Oil and gas: offshore platform components, downhole tubulars, and refinery equipment exposed to polythionic acid and wet CO2 environments
- Nuclear power: primary circuit piping, steam generator tubing, and reactor internals where IGC interacts with irradiation-assisted phenomena
- Power generation: boiler and superheater tubing, condensers, and flue gas desulfurization systems
- Pharmaceutical and food processing: sanitary piping, storage tanks, and vessels requiring corrosion-free product contact surfaces
- Pulp and paper: digesters, bleaching washers, and chlorine dioxide handling equipment
- Material certification and trade: mill test verification, incoming inspection, and third-party certification of plates, pipes, forgings, and fittings per ASTM, EN, GB, and JIS specifications
- Failure analysis and litigation support: confirming sensitization as the root cause of in-service cracking
FAQ
What causes intergranular corrosion in stainless steel?
The dominant mechanism is chromium carbide precipitation at grain boundaries when austenitic stainless steel passes slowly through the 450–850 °C range. The carbides remove chromium from adjacent zones, creating chromium-depleted regions that cannot form a passive film and dissolve preferentially in corrosive media. Welding, stress-relief heat treatment, and slow cooling after hot working are the most common triggers.
How can sensitization be prevented?
Effective countermeasures include using low-carbon grades (≤0.03% C, such as 304L and 316L), using titanium- or niobium-stabilized grades (321, 347) whose stabilizers form carbides preferentially, applying a post-weld solution anneal and rapid quench to redissolve carbides, controlling welding heat input and pass speed, and verifying the result through intergranular corrosion sensitivity testing.
Which test method should be selected?
Selection depends on the alloy system and the intended service environment. Oxalic acid etching (Practice A / Method A) is a fast screen; the Strauss test is appropriate for detecting carbide sensitization; the Huey nitric acid test is stricter and also flags sigma phase; ferric sulfate–sulfuric acid and nitric–hydrofluoric tests suit molybdenum-bearing grades; and ASTM G28 applies to nickel-based alloys. Purchase specifications, design codes, and end-user requirements usually dictate the exact practice and acceptance limit.
How long does a typical IGC test take?
The oxalic acid etch screen requires only hours including metallographic preparation. Strauss tests run 24 hours of boiling plus bend and metallographic evaluation. Ferric sulfate–sulfuric acid tests run 120 hours, and Huey tests require about 240 hours across five 48-hour boiling cycles. With specimen preparation and reporting, standard turnaround in accredited laboratories typically ranges from 3 to 10 working days depending on the method.
Is a bend test alone sufficient to accept the material?
No. Bend cracking indicates sensitization but is qualitative and can be masked by surface condition or thickness effects. Best practice combines weight-loss corrosion rate, bend testing, and metallographic confirmation of the crack path so that transgranular tearing, mechanical scratches, or edge effects are not misreported as intergranular attack.
Do duplex stainless steels need IGC testing?
Yes. Although duplex grades are less prone to classical chromium carbide sensitization, they can precipitate sigma phase, chi phase, and chromium nitrides at grain boundaries when improperly heat treated or welded with excessive heat input. Screening etch tests and ferric sulfate or modified acid boiling tests are commonly applied during welding procedure qualification and material acceptance.
