Intergranular Corrosion Test Result Analysis for Stainless Steel: Methods, Criteria, and Interpretat

Intergranular corrosion (IGC) testing is a critical quality control step for evaluating the sensitization resistance of stainless steels used in corrosive environments. This article provides a professional analysis of intergranular corrosion test results for stainless steel, covering sensitization mechanisms, standard test methods such as ASTM A262 and GB/T 4334, acceptance criteria, and practical guidance on interpreting corrosion rates, bend test outcomes, and metallographic findings.

Technical Overview

Intergranular corrosion is a localized form of attack that proceeds along the grain boundaries of a metal while the grains themselves remain largely unaffected. In stainless steels, the dominant mechanism is sensitization: when the material is exposed to temperatures between approximately 450 °C and 850 °C during welding, slow cooling, or improper heat treatment, chromium carbides (typically Cr23C6) precipitate at the grain boundaries. This precipitation depletes the chromium content in the adjacent zones to below the ~12% threshold required for passivation, leaving these narrow regions vulnerable to preferential dissolution in corrosive media.

The consequences of intergranular corrosion are severe: grain boundaries lose cohesion, individual grains may drop out entirely (a phenomenon known as "grain falling" or grain dropping), and the material suffers a drastic reduction in mechanical strength and toughness even though total weight loss appears minimal. Because the degradation is hidden beneath a seemingly intact surface, dedicated IGC testing and rigorous result analysis are essential before stainless steel components enter service in chemical, petrochemical, nuclear, and food-processing applications.

Result analysis for intergranular corrosion testing involves three complementary dimensions:

• Quantitative evaluation through corrosion rate calculation based on mass loss measurements.
• Qualitative evaluation through bend testing to detect intergranular cracking.
• Metallographic evaluation of the etched microstructure to classify the mode and depth of grain boundary attack.

A sound conclusion requires integrating all three dimensions, because each standard test method emphasizes a different aspect of sensitization.

Testing Samples

The validity of any intergranular corrosion result depends first on correct sampling and specimen preparation. Samples must represent the actual metallurgical condition of the product, including any sensitizing thermal cycles experienced during welding or heat treatment.

Austenitic Stainless Steels

Austenitic grades such as 304, 304L, 316, 316L, 321, and 347 are the most frequently tested materials. Their relatively high carbon content and widespread use in welded constructions make them the primary candidates for sensitization. Both base metal and welded joints (including the heat-affected zone) should be sampled when evaluating welded fabrications.

Ferritic and Duplex Stainless Steels

Ferritic grades and duplex grades such as 2205 and 2507 can also undergo grain boundary attack, although their sensitization mechanisms differ from austenitic steels. Duplex steels may additionally form harmful sigma phase and chromium nitrides at ferrite–austenite boundaries, which requires careful interpretation during metallographic examination.

Product Forms and Specimen Preparation

Acceptable sample forms include plates, sheets, strips, tubes, pipes, bars, forgings, and castings. Standard specimen dimensions are typically defined by the applicable test method, with total surface area recorded precisely because the corrosion rate calculation is normalized to exposed area. Preparation involves:

• Cutting specimens to the required dimensions while avoiding overheating or deformation that could alter the sensitized condition.
• Surface grinding or polishing to a uniform finish and removal of scale, oxide, and embedded contaminants.
• Degreasing, rinsing, drying, and accurate weighing on an analytical balance to the nearest 0.1 mg or better.
• Marking specimens with a method that does not compromise the test surface.

Testing Items

A complete intergranular corrosion test and result analysis program typically covers the following items:

• Corrosion rate determination: calculated from specimen mass loss over a defined exposure period in the boiling test solution.
• Bend test evaluation: a 180-degree bend applied to the exposed specimen, followed by examination for intergranular fissures and cracks.
• Macroscopic and microscopic examination: visual inspection for cracking, exfoliation, or grain dropping, followed by optical microscopy of cross sections.
• Microstructure classification: categorization of oxalic acid etch structures as step, dual, or ditch structures.
• Attack depth measurement: quantification of the maximum intergranular penetration depth from the exposed surface.
• Weld zone evaluation: separate assessment of weld metal, fusion line, and heat-affected zone in welded specimens.

Testing Methods

Method selection depends on the steel grade, intended service environment, and applicable product specification. The most widely used standards are ASTM A262, GB/T 4334, ISO 3651-2, JIS G 0571–G 0575, and ASTM G28 (for nickel-rich alloys). Test solutions are maintained at boiling temperature under reflux condensers to keep concentration constant throughout the exposure.

Oxalic Acid Etch Test (ASTM A262 Practice A / GB/T 4334 Method A)

This rapid electrolytic screening test etches a polished surface in 10% oxalic acid at a current density of 1 A/cm² for approximately 90 seconds. The etched microstructure is examined at 250× to 500× magnification and classified as follows:

• Step structure: discrete steps between grains with no grain boundary ditching; indicates the absence of sensitization and qualifies the material as acceptable without further testing.
• Dual structure: isolated ditched boundaries surrounding individual grains; typically acceptable for certain grades, but the relevant practice must be consulted.
• Ditch structure: one or more continuous grain boundary ditches surrounding grains; indicates probable sensitization, requiring confirmation by a quantitative weight-loss or bend test method.

Practice A is a screening test only; it cannot be used to reject material directly but serves to exempt clearly non-sensitized samples from longer boiling tests.

Ferric Sulfate–Sulfuric Acid Test (ASTM A262 Practice B / GB/T 4334 Method B)

The specimen is exposed for 120 hours in a boiling solution of 50% sulfuric acid containing ferric sulfate. The oxidizing ferric ions accelerate attack on chromium-depleted boundaries. Results are expressed as corrosion rate calculated from mass loss. This method is suitable for most austenitic and duplex stainless steels and is frequently specified for material acceptance.

Nitric Acid Test / Huey Test (ASTM A262 Practice C / GB/T 4334 Method C)

The specimen is exposed to boiling 65% nitric acid for five consecutive 48-hour periods, with fresh solution each period and weighing after each period. This test is particularly severe and is specified for material intended for nitric acid service. It detects not only chromium carbide sensitization but also attack associated with sigma phase and, in some cases, molybdenum-containing constituents, so a failed Huey result requires metallographic confirmation of the actual attack mode.

Copper–Copper Sulfate–16% Sulfuric Acid Test (ASTM A262 Practice E / GB/T 4334 Method E)

Known as the Strauss test, this method immerses the specimen for a minimum of 24 hours in boiling 16% sulfuric acid with copper turnings or copper shot, with the specimen in metallic contact with the copper. The low-potential environment selectively attacks sensitized boundaries. Evaluation is performed by bending the specimen 180 degrees over a mandrel of a specified radius and examining the tension surface at low magnification for cracks or fissures.

Nitric–Hydrofluoric Acid Test (ASTM A262 Practice D / GB/T 4334 Method D)

This two-period exposure in 70 °C, 10% nitric–3% hydrofluoric acid is designed for molybdenum-bearing austenitic steels such as 316 and 317, for which the ferric sulfate test may be unnecessarily severe. Results are evaluated by comparing the corrosion behavior of the as-received specimen against a laboratory-sensitized reference specimen.

Result Analysis and Acceptance Criteria

The core of intergranular corrosion result analysis is converting raw test data into a pass or fail judgment that is both technically defensible and traceable to the governing specification.

Corrosion Rate Calculation

For weight-loss methods, the corrosion rate is calculated as: CR = (K × W) / (A × T × D), where W is the mass loss in grams, A is the exposed surface area in square centimeters, T is the exposure time in hours, D is the material density in g/cm³, and K = 8.76 × 10⁴ yields the rate in millimeters per year. The density values differ by grade (approximately 7.9 g/cm³ for austenitic grades and 7.7–7.8 g/cm³ for duplex grades), so using the correct value is essential. Acceptance limits are defined by the product specification or contractual agreement; typical acceptance thresholds for well-annealed material fall in the range of a few tenths of a millimeter per year, while specific values vary by grade and test method.

Bend Test Interpretation

After exposure in the Strauss-type test, the specimen is bent 180 degrees and inspected. The presence of cracks or fissures on the tension surface indicates intergranular attack. Analysts must distinguish genuine intergranular fissures from mechanical bending cracks or surface defects by examining the fracture edges under magnification: intergranular cracks follow grain boundaries and often show evidence of individual grains having been pulled out, whereas mechanical cracks are transgranular and associated with pre-existing scratches or inclusions. A crack-free bend with no evidence of grain dropping is the standard pass condition.

Common Result Patterns and Their Meaning

• Low corrosion rate with step structure and crack-free bend: material is in the solution-annealed or stabilized condition and fully resistant to intergranular corrosion.
• High corrosion rate combined with ditch structure: classic carbide sensitization, usually caused by slow cooling through the critical temperature range or inadequate solution annealing.
• High nitric acid corrosion rate without carbide precipitation: suspect sigma phase or other secondary phases; supplementary metallography and hardness testing are required.
• Attack concentrated only in the heat-affected zone of welded specimens: welding-procedure-related sensitization, pointing to corrective actions such as low heat input, faster cooling, or post-weld solution annealing.
• Localized edge attack or abnormal attack on sheared edges: often a specimen-preparation artifact rather than true sensitization; edge effects should be documented and, where permitted, edges may be masked or ground before testing.

Retesting and Dispute Resolution

Most specifications permit retesting with double the original number of specimens when results are marginal or anomalous. All retest specimens must meet the acceptance criterion for the material to pass. When disputes arise, metallographic evidence of attack depth and mode, combined with hardness testing and possibly electron microscopy of the grain boundaries, provides the technical basis for the final determination.

Applications

Intergranular corrosion testing and its result analysis are applied across many industries where stainless steel components face corrosive media at elevated temperatures:

• Chemical and petrochemical plants: reactors, columns, piping, and heat exchangers handling acids, chlorides, and sulfide-bearing streams.
• Pressure vessels and boilers: material qualification under ASME and PED frameworks, including verification of post-forming and post-weld heat treatments.
• Nuclear power: tubing and internals where Huey test results and etch structure ratings form part of procurement specifications.
• Food, beverage, and pharmaceutical manufacturing: sanitary piping and vessels requiring weld-zone corrosion resistance and cleanability.
• Welding procedure qualification: confirming that welding parameters avoid sensitization of the heat-affected zone.
• Failure analysis: determining whether service failures originated from sensitization-related intergranular attack or other corrosion mechanisms.
• Incoming material inspection: distinguishing correctly annealed or stabilized material from non-conforming stock before fabrication.

FAQ

What causes intergranular corrosion in stainless steel? Intergranular corrosion is caused by sensitization, the precipitation of chromium carbides at grain boundaries when stainless steel passes through the 450–850 °C range. This creates chromium-depleted zones that corrode preferentially because they cannot maintain a passive film.

Which test method should be selected? Oxalic acid etching (Practice A) is used only for rapid screening. Practice E (Strauss) with a bend test is the most common acceptance test for general service. Practice B (ferric sulfate–sulfuric acid) is preferred for quantitative evaluation of most austenitic grades, Practice D suits molybdenum-bearing grades, and Practice C (Huey) is specified for nitric acid service. The governing product specification normally dictates the choice.

How is a failed result interpreted? A failed result means the specimen exhibited sensitization under the specific test conditions. The analyst must confirm the attack mode metallographically, determine whether the attack is limited to the heat-affected zone, and identify the responsible metallurgical cause such as carbide precipitation or sigma phase before recommending corrective measures.

How can intergranular corrosion be prevented? Effective measures include using low-carbon L-grade steels, using titanium- or niobium-stabilized grades, applying proper solution annealing followed by rapid cooling, controlling welding heat input, and using suitable filler metals and post-weld heat treatments.

Does a high corrosion rate always mean rejection? Not automatically. Acceptance depends on the limit stated in the applicable specification. In addition, abnormally high rates caused by edge artifacts, wrong density values, or test solution errors must be ruled out through review of test records and metallographic verification before a final judgment is issued.