Qualitative intergranular corrosion (IGC) testing is a critical quality-control procedure used to verify the metallurgical integrity of stainless steels, nickel alloys, and aluminum alloys. By combining accelerated immersion tests with bend tests and metallographic examination in accordance with ASTM A262, ASTM G28, ISO 3651, and related standards, accredited third-party laboratories can deliver reliable pass/fail judgments on a material's susceptibility to grain boundary attack. This article explains the corrosion mechanism, sample requirements, test methods, evaluation criteria, and typical industrial applications of qualitative IGC detection.
Technical Overview
Intergranular corrosion (IGC) is a localized degradation mechanism in which corrosive attack is concentrated at the grain boundaries of a metal while the grain interiors remain largely unaffected. Because grain boundaries possess higher energy and frequently differ in chemical composition from the grain matrix, they can become preferential anodic sites when the metal is exposed to specific aggressive media. The result is a progressive loss of cohesion between grains, which can cause severe embrittlement, grain dropping, and catastrophic failure with little or no visible warning on the component surface.
The most common cause of IGC in austenitic stainless steel is sensitization. When the steel is heated within the range of approximately 425 °C to 815 °C — for example during welding, slow cooling from solution annealing, or prolonged high-temperature service — chromium carbides (Cr23C6) precipitate at the grain boundaries. This precipitation depletes the adjacent zones of chromium to below the level of roughly 12% required to maintain a stable passive film, leaving those chromium-depleted regions vulnerable to rapid dissolution. Analogous mechanisms exist in other alloy families, including sigma phase precipitation in high-chromium grades, grain boundary segregation in nickel alloys, and anodic precipitate networks in 2xxx, 5xxx, and 7xxx series aluminum alloys.
- Common causes of intergranular susceptibility include sensitization during welding, slow cooling through the critical temperature range, improper or omitted solution annealing, sigma phase or other deleterious phase precipitation, and unfavorable grain boundary segregation.
- Qualitative detection is inherently a pass/fail evaluation: specimens are exposed to an accelerated corrosive medium and then examined by bending and/or microscopy to determine whether intergranular fissuring, grain dropping, or cracking is present.
- Governing standards include ASTM A262 (stainless steels), ASTM G28 (nickel-rich chromium-bearing alloys), ASTM G67 and ASTM G110 (aluminum alloys), ISO 3651-1 and ISO 3651-2, JIS G 0571 through G 0575, and the GB/T 4334 series.
Testing Samples
Austenitic Stainless Steels
Austenitic grades are the most frequently tested materials because they are the most prone to sensitization. Typical candidates include Types 304, 304L, 316, 316L, 321, 347, 310, and super austenitic grades such as 904L. Samples may be taken from plate, sheet, strip, pipe and tube, bar, forgings, castings, or fabricated components, including cross-sections containing weld metal, the heat-affected zone (HAZ), and base metal so that the full welded joint can be evaluated.
Ferritic, Duplex, and Martensitic Stainless Steels
Ferritic and duplex stainless steels can also suffer intergranular attack after high-temperature exposure. Test programs for these grades typically select media and evaluation criteria suited to their composition and service environment, following the guidance of the applicable ASTM, ISO, EN, or GB standards.
Nickel-Chromium and Nickel-Based Alloys
Solid-solution strengthened nickel alloys such as Alloy 600, 690, 800, 825, and C-276 type materials are tested under ASTM G28 to assess grain boundary attack related to carbide precipitation or segregation. Sample orientation, product form, and any required sensitizing heat treatment are defined by the governing material specification.
Aluminum Alloys
Heat-treatable 2xxx and 7xxx series alloys and strain-hardened Al-Mg alloys of the 5xxx series, such as 5083, are evaluated for intergranular and exfoliation-type attack using the procedures of ASTM G67 and ASTM G110, typically followed by cross-sectional metallographic examination.
Sample Preparation Requirements
- Specimen dimensions, orientation, and surface finish are prescribed by the specific test standard; surfaces are usually ground to a uniform finish and thoroughly degreased and dried before weighing and exposure.
- Where the specification requires it, laboratory sensitizing heat treatments (for example, holding at approximately 650 °C followed by cooling) are applied to low-carbon or stabilized grades to evaluate worst-case susceptibility.
- Welded specimens must retain the complete weld cross-section so that weld metal, fusion line, HAZ, and base metal are all exposed to the test medium.
Testing Items
- Assessment of intergranular corrosion susceptibility in the as-received material condition
- Sensitization evaluation after a specified laboratory sensitizing heat treatment
- Susceptibility testing of welded joints, including weld metal and heat-affected zone
- Etch-structure classification (step, dual, or ditch structure) by the oxalic acid screening method
- Bend test evaluation for intergranular fissures and cracks after acid exposure
- Metallographic examination for grain boundary attack, attack depth, and grain dropping
- Verification of the effectiveness of stabilization (titanium or niobium additions) or low-carbon composition
- Condition assessment of components after extended high-temperature service or suspected sensitization
Testing Methods
The general workflow for qualitative IGC testing consists of specimen preparation, optional sensitizing heat treatment, exposure to an accelerated corrosive medium under strictly controlled boiling conditions, and post-exposure evaluation by bending and/or optical microscopy. The choice of method depends on the alloy system, the expected precipitate type, the product form, and the acceptance criteria of the governing specification.
Oxalic Acid Etch Screening Test
Described in ASTM A262 Practice A and equivalent procedures in JIS G 0571 and the GB/T 4334 series, this rapid electrolytic etch uses a 10% oxalic acid solution at a current density of 1 A/cm² for 90 seconds. The etched surface is examined microscopically and classified as a step structure, dual structure, or ditch structure. Step and dual structures are generally acceptable, while a ditch structure indicates the need for confirmatory immersion testing. The method is a screening tool, principally for 18Cr-8Ni type austenitic grades, and is not by itself an acceptance test.
Strauss Test — Copper Sulfate–Sulfuric Acid with Copper Contact
Covered by ASTM A262 Practice E, ISO 3651-2, JIS G 0575, and corresponding GB/T 4334 procedures, the Strauss test immerses specimens in a boiling solution of 16% sulfuric acid and copper sulfate in contact with metallic copper turnings for 15 to 24 hours. After exposure, each specimen is bent 90° to 180° around a mandrel, and the tension surface is inspected. The appearance of cracks or fissures is qualitative evidence of intergranular susceptibility, making this the classic qualitative acceptance test for sensitization.
Streicher Test — Ferric Sulfate–50% Sulfuric Acid
Specified in ASTM A262 Practice B, ASTM G28 Method A, JIS G 0572, and GB/T 4334 counterparts, this test involves boiling exposure for 120 hours in a 50% sulfuric acid solution with ferric sulfate. Although primarily evaluated quantitatively by weight loss and corrosion rate, the method is frequently combined with microscopic examination of the specimen surface and cross-section to provide a qualitative verdict on grain boundary attack.
Huey Test — Boiling 65% Nitric Acid
Defined in ASTM A262 Practice C, JIS G 0573, and the GB/T 4334 series, the Huey test subjects specimens to five consecutive 48-hour boiling cycles in 65% nitric acid. It is particularly sensitive to sigma phase and other chromium-rich precipitates in addition to chromium carbide sensitization. Beyond corrosion-rate measurement, qualitative evidence such as grain dropping, intergranular fissuring, and visible grain fall-out on the specimen surface is recorded.
Tests for Molybdenum-Bearing Grades
For molybdenum-containing austenitic stainless steels such as Types 316 and 317, ASTM A262 Practice F (copper sulfate in 50% sulfuric acid with copper contact) and nitric–hydrofluoric acid procedures referenced in the GB/T 4334 series are applied, since the standard oxalic acid screening classification is not directly applicable to these compositions.
Nickel Alloy Testing
ASTM G28 Method A employs the ferric sulfate–sulfuric acid medium for 24 to 120 hours, while Method B uses a boiling mixed acid solution containing sulfuric acid and ferric sulfate with copper contact for highly alloyed materials. Evaluation combines weight-loss measurement with metallographic examination of grain boundary attack and bend testing where required by the specification.
Aluminum Alloy Testing
ASTM G67, the nitric acid mass loss test, exposes 5xxx series aluminum alloy specimens to concentrated nitric acid for 24 hours to reveal susceptibility to intergranular attack and exfoliation. ASTM G110 immerses aluminum alloys in a sodium chloride–hydrogen peroxide solution followed by metallographic sectioning to quantify and qualify the depth of grain boundary penetration.
Evaluation and Acceptance Criteria
- Bend test acceptance: the specimen is bent around a mandrel of specified radius, and any cracks or fissures on the outer tension surface that are confirmed metallographically as intergranular constitute failure.
- Metallographic acceptance: cross-sections are examined at defined magnifications, and the presence of continuous grain boundary attack, grain dropping, or attack exceeding the depth limit set by the specification constitutes failure.
- Etch-structure classification: ditch structures in the oxalic acid screen trigger mandatory confirmatory immersion testing under the governing acceptance method.
Applications
- Chemical and petrochemical processing: qualification of reactors, heat exchanger tubing, columns, and piping exposed to hot acids and chlorides.
- Nuclear power: acceptance testing of steam generator tubing, reactor internals, and other corrosion-critical components where sensitization is unacceptable.
- Oil and gas: evaluation of stainless and nickel alloy piping, cladding, and downhole hardware in sour and acidic service.
- Food, beverage, and pharmaceutical industries: verification of hygienic tubing and equipment fabricated by welding, where sensitized HAZs can compromise both corrosion resistance and cleanliness.
- Marine and shipbuilding: assessment of Al-Mg alloys such as 5083 plate in structural applications where intergranular and exfoliation corrosion can degrade load-bearing capacity.
- Aerospace: incoming material inspection and heat-treatment verification for 2xxx and 7xxx series aluminum structures.
- Fabrication quality assurance: weld procedure qualification, heat-treatment cycle validation, material certificate verification, and root-cause investigation in failure analysis, often performed by accredited third-party laboratories.
FAQ
What is the difference between qualitative and quantitative intergranular corrosion testing?
Qualitative testing renders a pass/fail judgment based on the presence or absence of intergranular cracks after a bend test or on microscopic evidence of grain boundary attack and grain dropping. Quantitative testing expresses the result numerically as corrosion rate in millimeters per year or mils per year, calculated from mass loss over the exposure period. Many test programs combine both approaches.
Why does welding often trigger intergranular corrosion?
During welding, portions of the heat-affected zone pass through the sensitization temperature range of roughly 425 °C to 815 °C, allowing chromium carbides to precipitate at grain boundaries and creating chromium-depleted zones. This is why welded joints, including the HAZ, are routinely included in IGC test specimens.
How is a material judged to be susceptible?
A material is considered susceptible when, after exposure to the specified medium, it exhibits cracks or fissures on the tension surface of a bent specimen that metallographic examination confirms as intergranular, or when cross-sectional microscopy reveals continuous grain boundary attack or grain dropping beyond the acceptance limit of the governing standard.
How do I select the right test method?
Method selection depends on the alloy composition, the type of precipitate expected, the product form, and the acceptance requirements of the purchase specification. The oxalic acid etch provides fast screening for 18Cr-8Ni grades, the Strauss test is the standard qualitative acceptance method for sensitization, the Huey test is preferred where sigma phase or nitric acid service is a concern, ASTM G28 applies to nickel alloys, and ASTM G67 or G110 applies to aluminum alloys. Accredited third-party laboratories can advise on the most appropriate route.
Are sensitizing heat treatments always performed before testing?
No. Sensitizing treatments are applied only when the material specification or client requirement calls for a worst-case evaluation, most commonly for low-carbon and stabilized grades that would otherwise resist sensitization under standard mill conditions.
How long does qualitative IGC testing take?
The oxalic acid screening etch can be completed within a few hours, the Strauss test typically requires one to two days of boiling exposure plus bend evaluation, while the Huey test with five 48-hour cycles can take ten to fifteen days of exposure. Metallographic preparation and examination, along with any required sensitizing treatment, add to the overall project timeline.