Intergranular corrosion (IGC) testing is a critical quality control and materials qualification process for evaluating the susceptibility of advanced structural materials—such as stainless steels, nickel-based alloys, and aluminum alloys—to localized attack along grain boundaries. This article provides a comprehensive technical overview of intergranular corrosion mechanisms, international testing standards including ASTM A262, ASTM G28, ISO 3651, and GB/T 4334, specimen preparation requirements, evaluation criteria, and practical applications across petrochemical, nuclear, aerospace, and marine industries.
Intergranular corrosion (IGC) testing is a critical quality control and materials qualification process for evaluating the susceptibility of advanced structural materials—such as stainless steels, nickel-based alloys, and aluminum alloys—to localized attack along grain boundaries. This article provides a comprehensive technical overview of intergranular corrosion mechanisms, international testing standards including ASTM A262, ASTM G28, ISO 3651, and GB/T 4334, specimen preparation requirements, evaluation criteria, and practical applications across petrochemical, nuclear, aerospace, and marine industries.
Technical Overview
Intergranular corrosion (IGC) is a localized form of corrosion that preferentially attacks the grain boundaries of a metal or alloy, while the grain interiors remain largely unaffected. In advanced structural materials, this phenomenon is most commonly associated with sensitization—a metallurgical condition in which thermal exposure, typically in the temperature range of 425 °C to 815 °C, causes chromium carbides (Cr23C6) to precipitate at grain boundaries. This precipitation depletes the adjacent zones of chromium to levels below the ~12% threshold required for passive film stability, leaving the grain boundaries vulnerable to aggressive corrosion in service environments.
IGC is particularly dangerous because it can proceed with little or no visible surface evidence while severely degrading mechanical properties. Grain boundary attack ultimately leads to "grain dropping," intergranular cracking, embrittlement, and catastrophic component failure. For this reason, intergranular corrosion testing is a mandatory qualification step for structural materials used in pressure vessels, chemical reactors, nuclear components, and other safety-critical applications. Accredited third-party laboratories routinely perform these tests to verify that materials have been correctly solution annealed, stabilized, or otherwise processed to resist sensitization.
Testing Samples
Austenitic Stainless Steels
Austenitic grades such as 304, 304L, 316, 316L, 321, and 347 are the most frequently tested materials. Unstabilized grades are especially prone to sensitization during welding or slow cooling through the critical temperature range. Test specimens are typically prepared from plates, sheets, pipes, tubes, bars, forgings, and welded joints.
Duplex Stainless Steels
Duplex alloys such as 2205 and 2507 combine austenitic and ferritic microstructures. Testing focuses on detecting intermetallic phases and grain boundary precipitation that may form during improper heat treatment or fabrication.
Nickel-Based Alloys
Alloys such as Alloy 600, 625, 800, 825, and C-276 are tested for grain boundary attack and selective precipitation of carbides or intermetallic phases, which can compromise corrosion resistance in severe chemical environments.
Ferritic and Martensitic Stainless Steels
High-chromium ferritic grades are evaluated using dedicated test practices designed to reveal sensitization caused by grain boundary carbide and nitride precipitation.
Aluminum Alloys
5xxx and 2xxx series aluminum alloys are tested for exfoliation and intergranular susceptibility related to grain boundary precipitate networks and sensitization during low-temperature service exposure.
Testing Items
Intergranular corrosion test programs typically include the following evaluation items:
- Determination of susceptibility to intergranular attack after accelerated exposure in standardized corrosive media
- Measurement of corrosion rate based on specimen mass loss over the exposure period
- Post-exposure bend testing (180° bend) to reveal intergranular cracking and loss of ductility
- Metallographic examination of etch structures and grain boundary attack depth
- Degree of sensitization (DOS) assessment for qualifying heat treatment and welding procedures
- Evaluation of heat-affected zones (HAZ) in welded specimens and weldment cross-sections
- Verification of stabilization effectiveness in titanium- or niobium-stabilized alloys
Testing Methods
Intergranular corrosion testing is governed by well-established international standards. The appropriate method is selected based on alloy type, expected service environment, and specification requirements.
ASTM A262 – Austenitic Stainless Steels
This standard defines the most widely used practices for detecting susceptibility to intergranular attack in austenitic stainless steels:
- Practice A – Oxalic acid etch test: a rapid electrochemical screening method that classifies etch structures as acceptable or suspect
- Practice B – Ferric sulfate–50% sulfuric acid test: measures corrosion rate by mass loss over a 120-hour exposure
- Practice C – Nitric acid (Huey) test: five 48-hour boiling exposures in 65% nitric acid, used to detect chromium carbide precipitation and sigma phase
- Practice E – Copper–copper sulfate–16% sulfuric acid (Strauss) test: boiling exposure followed by a bend test to reveal intergranular cracking
- Practice F – Copper–copper sulfate–50% sulfuric acid test: a more aggressive variant for specific grades and applications
ASTM G28 – Nickel-Rich Alloys and Duplex Steels
Method A employs a boiling ferric sulfate–50% sulfuric acid solution for 120 hours, while Method B uses a mixed acid (ferric chloride–nitric–hydrofluoric) environment suited to more corrosion-resistant alloys. Results are expressed as corrosion rates calculated from mass loss.
ISO 3651-1 and ISO 3651-2
The ISO 3651 series specifies the determination of resistance to intergranular corrosion of stainless steels. Part 1 addresses austenitic and ferritic-austenitic (duplex) steels using sulfuric acid–copper sulfate and nitric acid media, while Part 2 covers ferritic, austenitic, and ferritic-austenitic steels in the as-delivered condition.
GB/T 4334 – Chinese National Standard
GB/T 4334 defines multiple test methods for stainless steel intergranular corrosion susceptibility, including the 10% oxalic acid etch test, the 50% sulfuric acid–ferric sulfate test, the 65% nitric acid test, and the copper–copper sulfate–sulfuric acid test with bend evaluation. It is widely specified for projects in the Chinese market and is broadly harmonized with ASTM and ISO approaches.
ASTM A763 and ASTM G67 – Specialized Practices
ASTM A763 covers practices for detecting sensitization in ferritic stainless steels, while ASTM G67 (the NAMLT test) measures the mass loss of 5xxx series aluminum alloys in concentrated nitric acid to quantify susceptibility to intergranular and exfoliation attack.
Applications
Intergranular corrosion testing supports material qualification and quality assurance across a broad range of demanding industries:
- Petrochemical and chemical processing: qualification of reactor vessels, heat exchangers, piping, and column internals exposed to acidic chlorides and oxidizing media
- Nuclear power: evaluation of primary circuit piping, steam generator tubing, and reactor internals for stress corrosion and sensitization resistance
- Oil and gas: verification of corrosion-resistant alloys (CRA) used in downhole tubing, subsea components, and sour service environments
- Aerospace: assessment of high-strength aluminum alloys and superalloys in airframe and engine structural applications
- Pharmaceutical and food processing: certification of sanitary stainless steel surfaces requiring long-term corrosion integrity
- Marine and offshore engineering: qualification of duplex and super duplex steels for seawater handling systems and structural components
- Pressure vessel and boiler manufacturing: compliance with material specifications and construction codes requiring IGC testing of plate, pipe, and welded joints
FAQ
What causes sensitization in stainless steels?
Sensitization occurs when chromium carbides precipitate at grain boundaries during exposure to temperatures between approximately 425 °C and 815 °C, depleting adjacent regions of chromium and creating zones that cannot maintain a passive film. Welding, slow cooling after heat treatment, and prolonged service at elevated temperatures are common causes.
How is a sensitized material corrected?
Sensitization can be eliminated by solution annealing, which dissolves chromium carbides and restores a homogeneous chromium distribution. Alternatively, stabilized grades (containing titanium or niobium) or low-carbon grades (e.g., 304L, 316L) can be specified to suppress carbide precipitation.
Which ASTM A262 practice should be selected?
Selection depends on the alloy and intended service. Practice A is a low-cost screening test, Practice C (Huey) is used where nitric acid service or sigma phase concern exists, and Practices B, E, and F are specified according to the applicable product specification and expected corrosion environment.
What are the typical specimen requirements?
Specimen dimensions and surface preparation requirements are defined by each standard. Full cross-section specimens are generally required for wrought products, and as-welded specimens should include the weld metal, heat-affected zone, and base metal. Surface finishing, degreasing, and accurate weighing are essential before exposure.
How are test results evaluated?
Evaluation combines quantitative and qualitative criteria: corrosion rate calculated from mass loss, presence or absence of cracking after bend testing, and metallographic evidence of grain boundary attack. Acceptance limits are defined by the governing material specification or end-user requirement.
How long does intergranular corrosion testing take?
Test durations vary by method: the oxalic acid etch screen takes under an hour of electrolytic etching, the Strauss test involves a 24-hour boiling exposure, while the Huey test requires five consecutive 48-hour boiling periods. Total turnaround, including specimen preparation and evaluation, typically ranges from several days to two weeks.