Intergranular Corrosion Analysis of Metallic Materials: Testing Methods, Standards, and Evaluation

Intergranular corrosion (IGC) analysis is a critical materials testing service used to evaluate the susceptibility of stainless steels, nickel alloys, and aluminum alloys to localized attack at grain boundaries. This article provides a comprehensive technical overview of intergranular corrosion mechanisms, testing standards such as ASTM A262, ASTM G28, and ISO 3651, sample requirements, evaluation methods, and typical industrial applications. Understanding IGC testing helps manufacturers ensure material quality, qualify welding procedures, and prevent premature failure of corrosion-sensitive components.

Technical Overview

Intergranular corrosion (IGC) is a localized form of corrosive attack that proceeds preferentially along the grain boundaries of a metal or alloy, while the grains themselves remain largely unaffected. Because the grain boundaries act as the primary load-bearing connections between grains, IGC can dramatically reduce mechanical strength and ductility with minimal visible material loss, sometimes causing catastrophic failure without significant weight loss or obvious external signs of degradation.

The most common cause of intergranular corrosion in austenitic stainless steels is sensitization, a metallurgical phenomenon that occurs when the material is exposed to temperatures between approximately 425 °C and 815 °C. Within this range, chromium carbides (Cr23C6) precipitate at grain boundaries, depleting the adjacent zones of chromium to levels below the approximately 12% threshold required for passive film stability. These chromium-depleted regions become anodic relative to the grain interiors and are preferentially dissolved in corrosive media. Similar mechanisms apply to ferritic stainless steels, duplex stainless steels, nickel-based alloys (where chromium carbide, molybdenum-rich phases, or sigma phase may be involved), and 5xxx series aluminum alloys exposed to sensitizing thermal cycles.

Intergranular corrosion analysis combines accelerated laboratory corrosion testing with metallographic evaluation and microscopy techniques to determine whether a material, weldment, or heat-treated component is susceptible to IGC. Testing is essential for material qualification, incoming inspection, welding procedure qualification, failure analysis, and research into new alloy compositions. Accredited third-party laboratories perform these tests in accordance with internationally recognized standards, including ASTM A262, ASTM G28, ASTM G67, ASTM A763, ISO 3651, EN ISO 3651-2, JIS G0571-G0575, and GB/T 4334.

Testing Samples

Austenitic Stainless Steels

Austenitic stainless steels such as AISI 304, 304L, 316, 316L, 321, and 347 are the most frequently tested materials for IGC susceptibility. Sensitization may occur during welding (heat-affected zones), slow cooling from solution annealing, or service exposure to elevated temperatures. Both base metal and cross-weld specimens can be evaluated.

Ferritic and Duplex Stainless Steels

Ferritic grades (e.g., 430, 444, 446) can sensitize due to chromium carbide and nitride precipitation, and are typically tested per ASTM A763. Duplex stainless steels such as 2205 and 2507 are evaluated for grain boundary attack, sigma phase formation, and secondary austenite precipitation, which also affect corrosion resistance and toughness.

Nickel-Based Alloys

Alloys such as nickel-chromium-molybdenum grades (e.g., alloy 625, C-276-type compositions) and nickel-chromium-iron alloys are tested per ASTM G28 to detect sensitization caused by carbide precipitation, molybdenum-rich intermetallic phases, or other grain boundary precipitates introduced by improper heat treatment or welding.

Aluminum Alloys

5xxx series aluminum-magnesium alloys with magnesium content above approximately 3% can sensitize during prolonged elevated-temperature exposure, forming a continuous grain boundary beta-phase (Al3Mg2) network. These alloys are evaluated using the nitric acid mass loss test (NAMLT) described in ASTM G67.

Sample Requirements

• Specimen dimensions are typically defined by the applicable standard; common sizes include coupons of approximately 20–30 mm by 50–100 mm, or full-thickness sections of the actual product.
• Specimens must be degreased, cleaned, weighed to 1 mg precision, and identified prior to testing.
• Welded samples should include the weld metal, heat-affected zone, and adjacent base metal to capture all metallurgical conditions.
• Test surface area must be calculated accurately for corrosion rate determination.
• The ratio of solution volume to specimen surface area (typically at least 20 mL/cm²) must be maintained per standard requirements.

Testing Items

• Sensitization assessment and etch structure classification of austenitic stainless steels
• Susceptibility to intergranular attack in as-received, sensitized, and welded conditions
• Corrosion rate determination by mass loss measurement (mm/year or mils/year)
• Bend (drop-break) test evaluation after corrosive exposure to detect intergranular cracking
• Metallographic examination of grain boundary attack depth and morphology
• Chromium carbide precipitation and sigma phase evaluation
• Nitric acid mass loss testing (NAMLT) for 5xxx series aluminum alloys
• SEM (scanning electron microscopy) and EDS analysis of grain boundary precipitates and attack morphology
• Post-test verification of material acceptance criteria for specification compliance

Testing Methods

Oxalic Acid Etch Test (ASTM A262 Practice A / JIS G0571)

A rapid screening test in which polished specimens are electrolytically etched in a 10% oxalic acid solution at ambient temperature under controlled current density. The etched microstructure is examined microscopically at 250–500× and classified as step, dual, or ditch structure. Materials exhibiting a step or dual structure are considered immune or unlikely to be susceptible and may be accepted without further testing; ditch structures require confirmation by one of the quantitative acid immersion tests. This practice applies only to specific austenitic stainless steel grades listed in the standard.

Ferric Sulfate–Sulfuric Acid Test (ASTM A262 Practice B / ASTM G28 Method A)

Specimens are immersed for 120 hours in a boiling 50% sulfuric acid solution containing ferric sulfate, which maintains a high redox potential that selectively attacks chromium-depleted grain boundary regions. Results are expressed as corrosion rates calculated from mass loss. This test is applicable to austenitic stainless steels, nickel-based alloys, and other chromium-bearing alloys.

Nitric Acid Test (Huey Test, ASTM A262 Practice C)

Specimens undergo five consecutive 48-hour immersion periods in boiling 65% nitric acid, with mass loss measured after each period. The Huey test is particularly effective at detecting attack associated with sigma phase and chromium carbide precipitation, and is commonly specified for alloys intended for nitric acid service. Results are reported as average corrosion rates.

Copper–Copper Sulfate–16% Sulfuric Acid Test (Modified Strauss Test, ASTM A262 Practice E)

Specimens are embedded in copper shot or copper turnings and exposed for a minimum of 15 hours in a boiling 16% sulfuric acid solution containing dissolved copper sulfate. Metallic copper deposits on the specimen surface and establishes a low redox potential that attacks chromium-depleted zones. After exposure, specimens are bent through 180° and examined for intergranular cracking or fissuring.

Copper–Copper Sulfate–50% Sulfuric Acid Test (ASTM A262 Practice F)

Similar to Practice E but using a 50% sulfuric acid concentration with a 120-hour exposure, this test is applied to sensitization-prone molybdenum-bearing austenitic stainless steels such as 316, 316L, 317, and 317L. Evaluation is performed by bend testing after immersion.

ISO 3651-2 and GB/T 4334 Test Methods

ISO 3651-2 specifies a boiling sulfuric acid–copper sulfate test (16% H2SO4, 20-hour exposure) for austenitic stainless steels, with acceptance determined by bend testing. The Chinese standard GB/T 4334 series covers equivalent methods including the oxalic acid etch test, ferric sulfate–sulfuric acid test, 65% nitric acid test, Nital electroetching, and copper–copper sulfate–sulfuric acid tests, and is widely used for domestic qualification of stainless steel products.

Nitric Acid Mass Loss Test for Aluminum Alloys (ASTM G67, NAMLT)

Specimens of 5xxx series aluminum alloys are immersed in concentrated (70%) nitric acid at 30 °C for 24 hours following a sensitizing heat treatment where required. Mass loss per unit area (mg/cm²) quantifies the degree of grain boundary beta-phase sensitization and intergranular attack susceptibility.

Metallographic and Microanalytical Evaluation

• Optical metallography to measure the depth of intergranular penetration and classify attack morphology.
• Scanning electron microscopy (SEM) of fractured bend specimens to confirm intergranular fracture modes.
• Energy-dispersive spectroscopy (EDS) to identify chromium carbides, sigma phase, or other grain boundary precipitates.
• Electrochemical reactivation (EPR) techniques as complementary laboratory methods for quantifying the degree of sensitization.

Applications

• Petrochemical and chemical processing: qualification of piping, pressure vessels, heat exchangers, and reactors exposed to corrosive media.
• Oil and gas: evaluation of downhole tubing, subsea components, and offshore platform equipment operating in chloride-containing environments.
• Nuclear power: verification of stainless steel and nickel alloy components where sensitization affects stress corrosion cracking resistance.
• Pharmaceutical and food processing: validation of hygienic equipment fabricated from low-carbon and stabilized stainless steels.
• Power generation: assessment of boiler, superheater, and condenser tubing after fabrication welding or long-term service exposure.
• Welding procedure qualification: verification that welding heat input and post-weld heat treatment do not produce sensitized zones.
• Material certification: compliance verification of stainless steel plate, sheet, pipe, fittings, and forgings against purchase specifications.
• Failure analysis: determination of whether intergranular corrosion contributed to in-service component degradation or cracking.

FAQ

What causes intergranular corrosion in stainless steel?

The primary cause is sensitization, in which chromium carbides precipitate at grain boundaries when the steel passes through the 425–815 °C temperature range during welding, improper heat treatment, or elevated-temperature service. This depletes chromium adjacent to the boundaries, leaving those regions unable to form a protective passive film and vulnerable to selective dissolution.

How can intergranular corrosion be prevented?

Prevention strategies include using low-carbon grades (L grades with carbon ≤ 0.03%), stabilized grades containing titanium or niobium (e.g., 321 and 347), solution annealing after fabrication to redissolve carbides, minimizing heat input and time at sensitizing temperatures during welding, and applying post-weld solution annealing where feasible.

Which test method should be selected for a given material?

Method selection depends on alloy type, service environment, and governing specification. Practice A (oxalic acid etch) is a fast screening tool for austenitic grades; Practice B or Practice E is common for general qualification; Practice C (Huey) is preferred for nitric acid service and sigma-phase detection; Practice F suits molybdenum-bearing grades; ASTM G28 applies to nickel alloys; and ASTM G67 addresses 5xxx aluminum alloys.

How are test results evaluated and reported?

Results are reported as calculated corrosion rates from mass loss (for Practices B, C, and ASTM G28), pass/fail bend test outcomes (for Practices E and F), etch structure classifications (Practice A), or mass loss per unit area (ASTM G67). Acceptance criteria are defined by the applicable product specification, material standard, or customer requirement, and reports typically include specimen details, test conditions, exposure durations, and metallographic evidence.

How long does intergranular corrosion testing take?

Screening by oxalic acid etch can be completed within one day. Quantitative immersion tests range from 15 hours (Practice E) to 120 hours (Practices B and F), and the Huey test requires five consecutive 48-hour boil periods totaling approximately 240 hours of exposure. Including preparation, metallography, and reporting, most projects are completed within one to two weeks.

Can welded components be tested for IGC susceptibility?

Yes. Cross-weld specimens containing base metal, heat-affected zone, and weld metal are routinely tested. This is particularly important for qualification of welding procedures, since the heat-affected zone is the region most likely to experience sensitization during thermal cycles.