Microscopic Examination of Intergranular Corrosion in Stainless Steel: Standards, Methods, and Evalu

Intergranular corrosion (IGC) microscopic examination is a critical metallurgical test used to detect sensitization and chromium carbide precipitation at grain boundaries in stainless steels. This article explains the testing standards, sample preparation, evaluation methods, and acceptance criteria for stainless steel intergranular corrosion inspection. It serves as a practical reference for material engineers, quality inspectors, and procurement specialists in petrochemical, nuclear, and pressure vessel industries.

Technical Overview

Intergranular corrosion is a localized form of attack that occurs preferentially along the grain boundaries of stainless steel, while the grains themselves remain largely unaffected. It is caused by sensitization, a metallurgical condition that develops when austenitic stainless steel is exposed to temperatures between approximately 450 °C and 850 °C, typically during slow cooling after welding, stress relieving, or improper heat treatment.

At these temperatures, chromium carbides (Cr23C6) precipitate at grain boundaries because carbon diffuses rapidly to the boundary regions. This precipitation consumes chromium from the adjacent matrix and creates a chromium-depleted zone. When the local chromium content falls below approximately 12%, the passive film can no longer form effectively in corrosive media, leaving the grain boundary region vulnerable to rapid dissolution. In severe cases, entire grains detach from the surface—a phenomenon known as grain dropping—which dramatically reduces load-bearing capacity and can cause catastrophic failure with little visible external warning.

Microscopic examination is the definitive method for identifying the etch structures and boundary attack associated with sensitization. It is widely used for material qualification, welding procedure validation, failure analysis, and routine quality control of stainless steel products.

Testing Samples

Austenitic Stainless Steel Samples

Common test materials include grades 304, 304L, 316, 316L, 321, and 347, supplied as plate, sheet, pipe, bar, forgings, or finished components. Samples are typically taken from the most representative or most severely thermally affected location, such as the heat-affected zone of a weld or the mid-thickness of a hot-rolled section.

Ferritic and Duplex Stainless Steel Samples

Ferritic grades (e.g., 430, 444) and duplex grades (e.g., 2205, 2507) can also undergo sensitization through different precipitation mechanisms, including chromium nitrides and sigma phase. Sample selection for these materials must account for their specific heat treatment histories and the higher annealing temperatures used in processing.

Welded and Heat-Treated Samples

For welded assemblies, specimens should include the weld metal, the heat-affected zone, and the base metal in a single cross section so that all regions can be evaluated together. Samples that have undergone sensitizing heat treatment—such as 675 °C for one hour, as specified in some acceptance tests—may also be prepared to assess the material's inherent resistance to carbide precipitation.

• Typical specimen size: 10 mm × 10 mm to 30 mm × 30 mm, or full cross sections of tube and bar products
• Surface condition: ground and polished for electrolytic etching, or finished to a specified roughness for immersion testing
• Volume-to-surface-area ratio must comply with standard requirements for boiling acid immersion tests
• Samples must be degreased and cleaned before testing to avoid contamination of the test solution

Testing Items

The microscopic examination evaluates several characteristic features that indicate the degree of sensitization and susceptibility to intergranular attack:

• Step structure: uniform grain boundary steps without ditches, indicating a solution-annealed, non-sensitized condition
• Dual structure: intermittent ditches along grain boundaries with no grain completely enclosed, indicating partial sensitization
• Ditch structure: one or more grains completely surrounded by ditches, indicating significant chromium carbide precipitation and susceptibility to intergranular attack
• End-grain pits: isolated pits associated with inclusions in transverse sections, particularly relevant to nitric acid service evaluation
• Islands: separated patches of ditch structure within an otherwise step-structure matrix
• Grain dropping evidence: detached or loosened grains observed after immersion testing, confirming severe intergranular attack
• Depth of intergranular penetration measured from the exposed surface after corrosion testing

Testing Methods

Oxalic Acid Etch Test (ASTM A262 Practice A / GB/T 4334.1)

This rapid screening method uses electrolytic etching in a 10% oxalic acid solution at a current density of 1 A/cm² for 90 seconds at room temperature. The etched surface is then examined under an optical microscope at 250× to 500× magnification. Step structures and qualifying dual structures allow the material to be accepted without further testing, while ditch structures require supplementary corrosion testing by one of the quantitative methods below.

Ferric Sulfate–Sulfuric Acid Test (ASTM A262 Practice B / ISO 3651-2 Method B)

The specimen is immersed in a boiling 50% sulfuric acid solution containing ferric sulfate for 120 hours. The test measures the average corrosion rate from weight loss. In strongly oxidizing environments this solution selectively attacks chromium-depleted grain boundaries, making the method suitable for evaluating extra-low-carbon and stabilized grades.

Nitric Acid Huey Test (ASTM A262 Practice C / GB/T 4334.3)

The specimen is subjected to five consecutive 48-hour boiling periods in 65% nitric acid. Weight loss is measured after each period to calculate the maximum and average corrosion rates. Following the test, the specimen is examined microscopically for intergranular attack and grain dropping. This test is essential for material intended for nitric acid service, as it also detects attack from sigma phase and other nitric-acid-sensitive constituents.

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

The specimen is exposed for 24 hours to a boiling 16% sulfuric acid solution saturated with copper sulfate, with copper chips in contact with the sample to accelerate the galvanic reaction. After exposure, the specimen is bent 180° over a specified mandrel and inspected for cracks, fissures, or exfoliation. Microscopic cross-sectional examination is performed when the bend results are ambiguous or when the material lacks sufficient ductility for bending.

Microscopic Evaluation Procedure

After etching or immersion testing, the specimen is sectioned, mounted, ground, and polished using standard metallographic techniques that avoid heating or mechanical deformation of the grain boundary regions. Examination is performed with a calibrated metallurgical microscope in both bright-field and, where necessary, differential interference contrast modes. The observed etch structure is classified according to the reference photomicrographs and acceptance criteria of the governing standard, and a documented report includes the structure classification, magnification, etching conditions, and pass or fail judgment.

Applications

Intergranular corrosion microscopic examination is applied across industries where stainless steel components are exposed to corrosive media at elevated temperatures:

• Petrochemical and chemical processing: reactors, heat exchangers, piping, and column internals exposed to acids and chlorides
• Nuclear power: primary circuit piping and components where IGC resistance is a licensing requirement
• Pressure vessels and boilers: qualification of plate, forgings, and welded joints to ASME and EN specifications
• Food, beverage, and pharmaceutical industries: sanitary tubing and equipment requiring long-term corrosion integrity
• Failure analysis: identifying sensitization as the root cause of cracking or leakage in service components
• Welding procedure qualification: verifying that heat input and post-weld treatment do not sensitize the heat-affected zone
• Incoming material inspection: confirming that supplied stainless steel meets the specified sensitization resistance before fabrication

FAQ

What causes intergranular corrosion in stainless steel?

It is caused by sensitization—chromium carbide precipitation at grain boundaries when the steel passes through the 450–850 °C range—which creates chromium-depleted zones that cannot maintain a passive film in corrosive media.

How can intergranular corrosion be prevented?

Effective measures include using extra-low-carbon grades such as 304L and 316L (carbon ≤ 0.03%), using stabilized grades such as 321 (titanium-stabilized) or 347 (niobium-stabilized), performing solution annealing at 1050–1150 °C followed by rapid water quenching, and controlling welding heat input and cooling rates.

What magnification is used for the oxalic acid etch examination?

The etched surface is examined at a minimum magnification of 250×, and 500× is commonly used to confirm the presence of ditches and to classify dual or ditch structures accurately.

What is the difference between the screening test and the immersion tests?

The oxalic acid etch test is a rapid, low-cost screening method that classifies the microstructure visually, while the boiling acid immersion tests (Practices B, C, and E) are quantitative acceptance tests that measure weight-loss corrosion rates or bend-test integrity over extended exposure periods.

Does a ditch structure always mean the material is rejected?

No. A ditch structure in the screening test indicates susceptibility and requires confirmation by the appropriate immersion test. Final acceptance is based on corrosion rate limits, bend test results, and the acceptance criteria specified in the material purchase specification.