ASTM A262 Intergranular Corrosion Testing: Practices, Procedures, and Applications

ASTM A262 intergranular corrosion testing is the industry-standard method for evaluating the susceptibility of austenitic stainless steels to grain-boundary attack caused by sensitization. This technical guide explains the five test practices (A, B, C, E, and F), specimen preparation, evaluation criteria, and typical industrial applications. Whether for material qualification, weld procedure validation, or failure analysis, understanding ASTM A262 helps ensure the reliable corrosion performance of stainless steel components.

Technical Overview

ASTM A262 is the internationally recognized standard practice for detecting the susceptibility of austenitic stainless steels to intergranular attack (IGA). Intergranular corrosion is a localized form of degradation that occurs along grain boundaries when the protective chromium-rich passive film is compromised in these regions, while the bulk of the grain remains unaffected. In severe cases, entire grains can drop out of the metal surface, leading to rapid loss of mechanical strength and premature component failure with little visible warning.

The dominant mechanism behind intergranular corrosion in austenitic stainless steels is sensitization. When a steel containing more than approximately 0.02–0.03% carbon is exposed to the sensitization temperature range of roughly 425 °C to 815 °C (800 °C to 1500 °F), chromium carbides (Cr23C6) precipitate preferentially at the grain boundaries. This precipitation depletes the zones adjacent to the boundaries of chromium below the level of approximately 12% required to maintain passivity, leaving those zones vulnerable to corrosive attack in oxidizing and mildly reducing environments.

Common causes of sensitization include slow cooling from solution annealing, improper heat treatment, stress-relieving operations, and the thermal cycles experienced by the heat-affected zone (HAZ) during welding. ASTM A262 provides five standardized evaluation practices — Practices A, B, C, E, and F — that use different chemical environments and evaluation techniques to reveal susceptibility to intergranular attack associated with chromium carbide precipitation, sigma phase, or other grain-boundary phenomena. Related standards include ASTM G28 for nickel-rich alloys, ASTM A923 for duplex stainless steels, and ASTM A763 for ferritic stainless steels.

Testing Samples

Applicable Product Forms

Specimens may be prepared from virtually any mill product or fabricated component made of austenitic stainless steel, including:

• Plate, sheet, and strip
• Seamless and welded pipe and tubing
• Bars, rods, and wire
• Forgings and castings
• Fittings, flanges, and fasteners
• Welded joints and weld overlay claddings, including cross-sections containing base metal, heat-affected zone, and weld metal

Typical grades tested include 304, 304L, 316, 316L, 317, 317L, 321, 347, and the corresponding cast grades such as CF-8 and CF-3M. Extra-low-carbon (L) grades and stabilized grades (321, 347) are frequently tested to verify their resistance to sensitization.

Specimen Preparation

Proper specimen preparation is critical for reproducible results. Standard laboratory practice includes:

• Cutting specimens to a size that provides a measurable surface area, typically 5 to 20 cm², with all surfaces accessible to the test solution
• Removing scale and decarburized layer by machining or grinding, followed by finishing to a uniform surface finish (typically 120 grit or finer) without overheating the specimen
• Measuring dimensions accurately to calculate the total exposed surface area
• Degreasing, cleaning, rinsing, and drying the specimens, then weighing them to the nearest 0.001 g on an analytical balance
• Ensuring an adequate solution volume-to-surface-area ratio in the test flask so that corrosion products do not saturate the environment

Test Conditions and Sensitization Treatments

Specimens are normally tested in the as-received condition (for example, in the solution-annealed or as-welded state). However, many material specifications require a laboratory sensitization heat treatment prior to testing — commonly 675 °C (1250 °F) for one hour followed by air cooling — to demonstrate that the material will resist carbide precipitation during future fabrication or service. The applicable heat treatment condition must be clearly defined by the purchaser or the governing specification.

Testing Items

An ASTM A262 test program typically evaluates and reports the following items, depending on the practice selected:

• Susceptibility to intergranular attack in the specified environment
• Etch structure classification: step, dual, or ditch structure (Practice A)
• Mass loss of each specimen after exposure
• Corrosion rate for each test period and the average rate over all periods, expressed in mm/month, mils/month, or equivalent units (Practices B, C, and F)
• Maximum corrosion rate observed in any single period, with a comparison of the first versus the fifth period to reveal accelerating attack (Practice C)
• Bend test results after exposure, including the presence or absence of fissures and cracks (Practice E)
• Metallographic evidence of grain-boundary ditching or grain dropping, where required
• Full documentation of material grade, heat number, heat treatment condition, specimen dimensions, test duration, and solution composition

Testing Methods

Practice A – Oxalic Acid Etch Test (Rapid Screening)

Practice A is a rapid, low-cost screening test based on electrolytic etching. The polished specimen face is etched anodically in a 10% oxalic acid solution at room temperature at a current density of approximately 1 A/cm² for 1.5 minutes. The etched surface is then examined under a microscope at 250× to 500× magnification and classified as one of three structures:

• Step structure: steps between grain boundaries with no ditches; indicates the material is immune to intergranular attack and is acceptable for the acceptance test in question
• Dual structure: some ditches present, but no grain is completely surrounded; acceptable for certain grades and practices as defined in the standard
• Ditch structure: one or more grains completely surrounded by ditches; the specimen is presumed susceptible and must be verified by a full acceptance test (Practice B, C, E, or F)

Practice A is a screening test only; it cannot be used as an acceptance test by itself, except where explicitly permitted by the material specification.

Practice B – Ferric Sulfate–Sulfuric Acid Test

Practice B exposes weighed specimens to a boiling solution of 50% sulfuric acid containing ferric sulfate (approximately 25 g per 600 mL of solution) for a continuous 120-hour period in a glass reflux flask. The ferric ions act as an oxidizing agent that drives the attack of chromium-depleted grain boundaries. After exposure, the specimens are cleaned, reweighed, and the corrosion rate is calculated. Practice B is effective for detecting susceptibility associated with chromium carbide precipitation in most austenitic grades, including the L and stabilized grades, and is widely specified for chemical plant materials.

Practice C – Nitric Acid Test (Huey Test)

Practice C immerses specimens in boiling 65% nitric acid for five consecutive 48-hour periods, with fresh solution prepared for each period. Specimens are weighed after each period, and both the average and maximum corrosion rates are reported. Because nitric acid also attacks sigma phase and other secondary phases, Practice C is uniquely capable of detecting intergranular and grain-boundary attack beyond simple carbide sensitization, and it is the standard test for material intended for nitric acid service. A corrosion rate that increases from period to period is a strong indication of progressive grain-boundary attack.

Practice E – Copper–Copper Sulfate–16% Sulfuric Acid Test (Strauss Test)

Practice E, often called the Strauss or modified Strauss test, places specimens in direct metallic contact with metallic copper in a boiling solution of 16% sulfuric acid containing copper sulfate, typically for a 24-hour exposure. The copper-copper ion couple maintains the electrochemical potential in the range where chromium-depleted boundaries are attacked. After exposure, each specimen is bent 180° around a mandrel with a diameter equal to the specimen thickness and examined at low magnification (approximately 10× to 20×). The appearance of cracks or fissures on the tension surface indicates intergranular attack and constitutes failure. Practice E is the most widely used acceptance test for qualification of austenitic stainless steels and their weldments.

Practice F – Copper–Copper Sulfate–50% Sulfuric Acid Test

Practice F uses a significantly more aggressive environment: a boiling 50% sulfuric acid solution with copper sulfate and metallic copper contact for 120 hours. Evaluation is by mass loss and calculated corrosion rate, similar to Practice B. Because molybdenum-bearing grades such as 316, 316L, 317, and 317L can pass Practice E even when sensitized, Practice F provides a more sensitive weight-loss-based evaluation for these alloys.

Corrosion Rate Calculation

For weight-loss-based practices, the corrosion rate is calculated using the general expression: Corrosion Rate = (K × W) / (A × T × D), where W is the mass loss, A is the exposed surface area, T is the exposure time, D is the material density, and K is a unit-conversion constant. Acceptance limits — for example, maximum allowable average corrosion rate or no cracking after bend — are defined by the governing material specification, the purchaser, or the applicable construction code rather than by ASTM A262 itself.

Applications

ASTM A262 testing is specified worldwide wherever austenitic stainless steel components are exposed to corrosive media and where intergranular corrosion could compromise safety, reliability, or product purity. Typical applications include:

• Chemical and petrochemical processing: qualification of reactors, heat exchangers, columns, piping, and vessel internals
• Oil and gas: offshore and refinery equipment, flowlines, and claddings exposed to sour and acidic environments
• Nuclear power: qualification of primary circuit piping and reactor internals where grain-boundary attack is a licensing concern
• Pharmaceutical and bioprocessing: verification of material condition for product-contact surfaces requiring corrosion-free service
• Food and beverage, pulp and paper, and general chemical plant construction
• Welding procedure qualification and welder performance qualification, particularly for the heat-affected zone
• Verification of solution-annealing effectiveness and detection of improper heat treatment at mill or fabrication stages
• Failure analysis and root-cause investigation of service corrosion failures
• Research and development of new stainless steel compositions and processing routes

FAQ

Which ASTM A262 practice should I choose?

The choice depends on the alloy and its intended service. Practice A is ideal for rapid, low-cost screening of large lots. Practice E is the most common acceptance test for general qualification of standard austenitic grades and weldments. Practice B is preferred when a quantitative weight-loss criterion is required. Practice F is recommended for molybdenum-bearing grades such as 316L and 317L. Practice C should be used when sigma-phase susceptibility is a concern or when the material will serve in nitric acid. When in doubt, follow the practice mandated by the material specification or the end-user.

What do step, dual, and ditch structures mean?

These terms describe the appearance of grain boundaries after oxalic acid etching in Practice A. A step structure shows clean boundaries with no attack, indicating a fully solution-annealed, non-sensitized material. A ditch structure shows one or more grains completely encircled by deep grooves, indicating chromium carbide precipitation and probable susceptibility. A dual structure is intermediate and may or may not be acceptable depending on the grade and the acceptance test involved.

Is ASTM A262 applicable to duplex or ferritic stainless steels?

No. ASTM A262 is written specifically for austenitic stainless steels. For duplex stainless steels, ASTM A923 is the appropriate standard; for ferritic stainless steels, ASTM A763 applies; and for nickel-based alloys, ASTM G28 is used. Testing a non-austenitic alloy to ASTM A262 can produce misleading results.

What causes sensitization and how can it be prevented?

Sensitization is caused by chromium carbide precipitation at grain boundaries during exposure to 425–815 °C. It can be prevented by using extra-low-carbon (L) grades, using stabilized grades containing titanium or niobium (321 or 347), applying a proper solution anneal followed by rapid cooling, minimizing weld heat input, and avoiding stress-relief heat treatments within the sensitization range.

Can welded components be tested?

Yes. Cross-section specimens containing base metal, heat-affected zone, and weld metal are routinely tested because the HAZ is the region most likely to become sensitized during welding. Practice E followed by a bend test is the most common approach for welded qualification coupons.

What are the typical acceptance criteria?

ASTM A262 itself does not fix acceptance limits; they are set by the purchaser or the material specification. Typical criteria include a step or otherwise acceptable etch structure for Practice A, a maximum allowable corrosion rate for Practices B, C, and F (with limits on the maximum single-period rate in Practice C), and the absence of cracking after a 180° bend for Practice E.

How long does testing take?

Practice A requires only about 30 minutes including etching and microscopy. Practice E requires approximately 24 hours of boiling exposure plus specimen preparation and bend testing. Practices B and F each require 120 hours of boiling exposure, and Practice C requires approximately 240 hours spread over five 48-hour periods. Reporting times also include specimen machining, conditioning, and documentation.

What happened to Practice D?

Earlier editions of ASTM A262 included a Practice D (nitric–hydrofluoric acid test) intended for molybdenum-bearing steels. It has been discontinued in current editions, and users are directed to Practice F or ASTM G28 for such alloys. Reports referencing Practice D should be reviewed against the edition of the standard in force at the time of testing.