ASTM A262 Intergranular Corrosion Testing of Stainless Steel — Complete Technical Guide
ASTM A262 intergranular corrosion testing is the standard qualification method used across the chemical processing, power generation, petroleum, and pharmaceutical industries to confirm that austenitic stainless steels are free from sensitization — a damaging metallurgical condition that makes grain boundaries vulnerable to selective corrosive attack. The standard specifies five discrete test practices, each designed to reveal different forms of grain-boundary degradation under conditions that simulate specific service environments.
Understanding which ASTM A262 practice to apply, and why, requires a solid grasp of the underlying metallurgical phenomenon of sensitization, how carbide precipitation occurs during welding and heat treatment, and how each test practice replicates a distinct corrosive mechanism. This guide covers all five practices in depth, explains the sensitization mechanism from a metallurgical standpoint, provides guidance on test selection, and offers practical notes on interpreting test results.
Whether you are a corrosion engineer qualifying a batch of 316L for a sulphuric acid service vessel, a welding engineer validating a weld procedure on 321 stainless steel, or a quality inspector reviewing material test reports, this guide provides the technical depth you need to work confidently with ASTM A262.
What Is Sensitization and Why Does It Matter?
Austenitic stainless steels derive their corrosion resistance primarily from a passive chromium-oxide film that forms spontaneously on the metal surface. This film is stable and self-healing provided the underlying metal contains at least 10.5% chromium by weight. When the chromium content drops below this threshold — even locally — corrosion resistance is compromised.
Sensitization is the metallurgical process through which this threshold is breached at grain boundaries. When austenitic stainless steel is held in the temperature range of approximately 425 °C to 815 °C (800 °F to 1500 °F), chromium atoms in the austenite matrix diffuse toward the grain boundaries and combine with carbon to form chromium carbides, primarily Cr23C6. The carbide particles themselves are not harmful, but the depletion of chromium from the adjacent metal is. A narrow “chromium-depleted zone” forms along each boundary, and this zone — containing as little as 5–8% Cr — becomes anodic relative to the surrounding matrix, making it a preferred site for galvanic corrosion.
The Sensitization Temperature Range
The sensitization curve (or “C-curve” in time-temperature-sensitization diagrams) shows that maximum susceptibility occurs around 650–700 °C. At this temperature, carbide precipitation is fastest, and even a few minutes of exposure can produce a sensitized microstructure. Above 815 °C, carbides dissolve back into solution and sensitization is eliminated (which is the basis of solution annealing heat treatment). Below 425 °C, carbon diffusion is too slow for significant carbide formation even over long periods.
Factors Controlling Sensitization Kinetics
The speed and severity of sensitization depend on several metallurgical variables:
| Factor | Effect on Sensitization Rate | Engineering Response |
|---|---|---|
| Carbon content | Higher C accelerates carbide precipitation | Use low-carbon grades (304L, 316L) — max 0.030% C |
| Chromium content | Higher Cr provides more reservoir against depletion | Specify higher-Cr alloys for high-temperature service |
| Grain size | Coarse grains have less boundary area; depletion is faster per unit area | Control grain size through cold work and annealing |
| Stabilising elements (Nb, Ti) | Nb and Ti preferentially combine with C, preventing Cr-carbide formation | Use stabilised grades 321 (Ti) or 347 (Nb) for high-temperature service |
| Nitrogen content | N slows C diffusion and carbide formation | Specified in some high-nitrogen grades (e.g., 316N) |
| Cold work | Increases dislocation density — can accelerate or modify precipitation | Consider post-cold-work annealing for critical applications |
All Five ASTM A262 Practices — Technical Overview
ASTM A262 is not a single test — it is a collection of five distinct electrochemical and immersion test methods, designated Practice A through Practice F (Practice D was withdrawn). Each practice uses a different corrosive medium and evaluation criterion, targeting different modes of intergranular attack. The five practices are summarised below, followed by detailed coverage of Practice E.
| Practice | Common Name | Test Medium | Duration | Evaluation Method | Primary Use Case |
|---|---|---|---|---|---|
| A | Oxalic Acid Etch | 10% Oxalic Acid (electrolytic) | ~2 min | Microstructure classification | Screening only |
| B | Streicher Test | Fe&sub2;(SO&sub4;)₃ + H&sub2;SO&sub4; | 24–120 h | Mass loss rate | Quantitative |
| C | Huey Test | 65% HNO₃ | 5 × 48 h | Mass loss per interval | Sigma phase / oxidising |
| E | Strauss Test | CuSO&sub4; + 16% H&sub2;SO&sub4; + Cu | 15 h | 180° bend + crack inspection | Qualification (pass/fail) |
| F | — | CuSO&sub4; + 50% H&sub2;SO&sub4; + Cu | 120 h | 180° bend + crack inspection | Low-C grades |
ASTM A262 Practice E — The Strauss Test in Depth
Practice E, universally known as the Strauss Test, is the most widely applied of the ASTM A262 practices in industrial fabrication and material qualification. It is a two-stage test: an immersion stage followed by a mechanical deformation stage. The result is purely qualitative — pass or fail — based on the appearance of the bent specimen surface.
Test Principle and Chemistry
The copper-copper sulfate-sulfuric acid solution creates a mildly reducing environment at a specific electrochemical potential. In this environment, the bulk austenitic matrix is protected from corrosive attack, but chromium-depleted grain boundary zones — where the Cr content has fallen below the passivating threshold — are selectively dissolved. The metallic copper granules deposited in the test vessel maintain a controlled electrode potential, suppressing attack on the sound matrix while permitting dissolution of sensitised boundaries.
After immersion, the grain boundaries in a sensitised specimen will have suffered significant dissolution. When the specimen is subsequently bent to 180 degrees, the grain boundaries — now weakened by dissolution — crack, and these cracks are visible to the naked eye or under low-power magnification (10× to 20×).
Specimen Preparation for Practice E
The test specimen must be cut from the material with the final dimensions specified in ASTM A262. For sheet and plate products, specimens are typically 25 mm wide × 75 mm long, with thickness equal to the material thickness up to a specified maximum. The specimen surface must not be heavily cold worked during cutting, and the edges may need to be deburred. Prior to testing, specimens are degreased and cleaned; no aggressive pickling is performed, as this may itself alter the grain boundary condition.
Evaluating the Bend Test Result
After boiling, the specimen is rinsed and allowed to cool before bending. Bending is performed in a standard vice or press brake. The outer tensile surface of the bent specimen is examined. The evaluator must distinguish between:
- Cracks (fissures): Linear, angular discontinuities that align with grain boundaries — indicating intergranular attack. These constitute a FAIL.
- Orange peel: A rough, dimpled surface texture — acceptable. This is caused by grain rotation during bending and is not indicative of IGC.
- Minor surface roughness or mechanical marks: From cutting or handling — not to be confused with IGC cracks.
When there is uncertainty about whether a surface feature constitutes an IGC crack, the bent specimen can be cross-sectioned and metallographically examined to confirm the nature of the cracking.
ASTM A262 Practice A — Oxalic Acid Etch Structures
Practice A is the fastest and least expensive ASTM A262 test method. It is an electrolytic etch using 10% oxalic acid, performed at a current density of 1 A/cm² for 90 seconds. The etched surface is then examined under an optical microscope at a minimum of 250× magnification. The microstructure observed is classified into one of three categories:
Step Structure
All grain boundaries appear as steps, with no dissolved ditches. This indicates the absence of continuous chromium carbide networks. A step structure is acceptable — the material does not need further testing by an immersion practice.
Dual Structure
Some grain boundaries show ditching, while others show a step structure. Isolated carbide particles may be present but without continuous attack. A dual structure is a conditional result — the material should be subjected to the applicable immersion practice for a definitive determination.
Ditch Structure
Continuous networks of dissolved grain boundary material (ditches) are visible. This indicates severe sensitisation and the presence of continuous carbide precipitation. A ditch structure means the material must be tested by an immersion practice and will likely fail; however, Practice A alone cannot be the basis for rejection in most specifications.
Choosing the Correct ASTM A262 Practice
Selecting the wrong ASTM A262 practice is a common and costly error. Each practice is sensitive to specific types of grain boundary attack under specific corrosive conditions, and using a practice that does not replicate the intended service environment can produce misleading results — either false acceptance of sensitised material or unnecessary rejection of sound material.
Key Selection Criteria
The following decision points guide practice selection:
| Service / Material Condition | Recommended Practice(s) | Notes |
|---|---|---|
| Rapid batch screening, standard grades | A (then B or E if step/dual) | Practice A can accept but not reject |
| Sulfuric acid or mixed acid service | B | Mass loss quantifies degree of attack |
| Nitric acid service; sigma-phase concerns | C | Only practice that detects sigma-phase IGC |
| General qualification of standard 304 / 316 | E | Most common industrial qualification practice |
| Qualification of 304L, 316L, 321, 347 | F or E (per spec requirement) | Must sensitise 304L/316L before Practice A |
| Weld HAZ qualification (procedure approval) | E (most common) | Specimen cut from HAZ of weld test coupon |
| Nickel-iron-chromium alloys | B | Consult standard Appendix for applicable alloys |
Grade Applicability and Sensitising Requirements
ASTM A262 includes normative tables listing which stainless steel grades are applicable to each practice, and whether a sensitising heat treatment is required prior to testing. The key principles are as follows:
Standard Carbon Grades (304, 316, 317)
These grades are tested in the as-received condition — no sensitising heat treatment is applied. The material’s own carbon content is sufficient that, if sensitisation has occurred during production or heat treatment, it will be revealed by the test. If the product is being tested after welding, the specimen is cut from the weld HAZ in the as-welded condition.
Low-Carbon Grades (304L, 316L, 317L)
Low-carbon grades contain ≤0.030% C, which is insufficient to form a sensitised microstructure under most practical exposures. To reveal susceptibility under worst-case conditions, ASTM A262 requires that these grades be subjected to a sensitising heat treatment of 675 °C for 1 hour, air cool before testing by Practice A. For immersion practices (E, F), some specifications test the material both in the as-received and sensitised condition.
Stabilised Grades (321, 347)
Type 321 (titanium-stabilised) and 347 (niobium-stabilised) are designed to be resistant to sensitisation. However, they can still be susceptible to “knife-line attack” — a very narrow zone of attack immediately adjacent to the weld fusion line, where peak temperatures were high enough to dissolve stabilising carbides without re-precipitation of the protective NbC or TiC. These grades must also receive a sensitising heat treatment before Practice A, typically 675 °C for 1 hour. For Practice E, a more severe sensitising treatment may be required depending on the purchasing specification.
| Grade | UNS No. | Applicable Practices | Sensitising HT Required for Practice A? | Max C% |
|---|---|---|---|---|
| 304 | S30400 | A, B, C, E | NO (as-received) | 0.08 |
| 304L | S30403 | A, B, E, F | YES (675 °C/1h) | 0.030 |
| 316 | S31600 | A, B, C, E | NO (as-received) | 0.08 |
| 316L | S31603 | A, B, E, F | YES (675 °C/1h) | 0.030 |
| 317 | S31700 | A, B, C, E | NO (as-received) | 0.08 |
| 317L | S31703 | A, B, E, F | YES (675 °C/1h) | 0.030 |
| 321 | S32100 | A, B, C, E | YES (675 °C/1h) | 0.08 |
| 347 | S34700 | A, B, C, E | YES (675 °C/1h) | 0.08 |
Relationship of ASTM A262 to Other Corrosion Standards
ASTM A262 is not the only standard addressing intergranular and grain-boundary corrosion in stainless steels and related alloys. Understanding how it relates to companion standards helps engineers specify the correct test for the material and service combination:
| Standard | Scope | Primary Application |
|---|---|---|
| ASTM G48 | Pitting and crevice corrosion of SS and Ni alloys (FeCl₃ solution) | Duplex, super-duplex, 6Mo grades |
| ASTM A763 | IGC of ferritic stainless steels | Grades 409, 430, 446 |
| ISO 3651-1 | Nitric acid test for austenitic SS | European equivalent to Practice C |
| ISO 3651-2 | Sulphuric acid-copper sulphate test | European equivalent to Practice E |
| ASTM G28 | IGC of wrought nickel-rich alloys | Alloys 625, 825, C-276, C-22 |
Practical Engineering Notes — Avoiding Sensitisation in Service
ASTM A262 testing is the detection tool — preventing sensitisation in the first place is the engineering goal. Several material and fabrication strategies are available:
Solution annealing (heating to 1040–1120 °C followed by rapid water quench) dissolves all carbides and restores full corrosion resistance. This treatment is effective for new fabrications but is difficult to apply to large assemblies or in-situ repairs. Stabilisation annealing (at 870–900 °C) encourages preferential precipitation of NbC or TiC in stabilised grades, tying up carbon before it can combine with chromium.
For TIG (GTAW) welding of austenitic stainless steel, controlling heat input is a primary tool for minimising sensitisation of the HAZ. Low heat input reduces the time the HAZ spends in the sensitisation range, reducing carbide precipitation. The relationship between heat input, cooling rate, and sensitisation is the central reason why corrosion performance of stainless weldments is sensitive to welding procedure parameters.
ASTM A262 in the Context of Welding Qualification
ASTM A262 Practice E is a mandatory or supplementary test in many pressure vessel and piping fabrication codes when austenitic stainless steel is used in corrosive chemical service. Under ASME Section IX, corrosion testing is an additional essential variable for certain process industries, often specified in the engineering design package or client specification rather than mandated by the code itself.
When ASTM A262 is specified as part of a welding procedure qualification record (PQR), specimens are cut from the weld test coupon in the following locations:
- Base metal (BM) specimen: Cut from well away from the weld to confirm the base material is not sensitised as-received.
- HAZ specimen: Cut to include the heat-affected zone, oriented so the HAZ runs along the length of the specimen. This is the critical specimen for weld decay assessment.
- Weld metal (WM) specimen: Cut through the deposited weld metal. Weld metal IGC susceptibility depends on the filler composition and solidification mode.
A complete Practice E test on a weld procedure qualification will include specimens from all three locations. If any specimen fails the bend test (cracks observed), the weld procedure is considered unacceptable for IGC-sensitive service and must be revised — typically by reducing heat input, changing filler metal, or switching to a low-carbon or stabilised base material.
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Frequently Asked Questions
What is ASTM A262 and what does it test?
ASTM A262 is an ASTM International standard that provides five test practices (A through F) for detecting susceptibility to intergranular attack in austenitic stainless steels and certain nickel alloys. The standard is used to verify that materials have been correctly heat treated and are free from sensitisation — a condition where chromium carbides precipitate at grain boundaries, depleting the adjacent metal of chromium and making it susceptible to preferential corrosion. It is widely referenced in material purchase specifications for chemical processing, pharmaceutical, and power generation equipment.
What is sensitisation in stainless steel?
Sensitisation is a heat-induced metallurgical change that occurs when austenitic stainless steel is held in the temperature range of approximately 425 °C to 815 °C. At these temperatures, chromium combines with carbon to form chromium carbides (Cr23C6) that precipitate along grain boundaries. This removes chromium from the adjacent matrix, creating a narrow chromium-depleted zone that is highly vulnerable to selective corrosion — known as intergranular attack (IGA). Sensitisation is particularly associated with the heat-affected zone of welds, which is why ASTM A262 is routinely specified in welding procedure qualifications for corrosive service. See the WeldFabWorld article on stainless steel weld decay for a deeper explanation.
What is ASTM A262 Practice E (the Strauss Test)?
Practice E, commonly called the Strauss Test, immerses specimens in a boiling copper-copper sulfate-16% sulphuric acid solution for 15 hours, with metallic copper present in the vessel. After testing, specimens are bent 180 degrees over a mandrel of specified diameter. The bend surface is examined for cracks indicating intergranular attack. A cracked specimen is considered sensitised and fails the test. This is one of the most widely used IGC qualification tests in industrial practice because it is rapid (15 hours vs. 120 hours for Practice B), produces a clear pass/fail result, and is applicable to the most common austenitic grades.
How do I choose between ASTM A262 practices A, B, C, E, and F?
Practice A (oxalic acid etch) is used as a rapid screening tool — it can accept but not reject material. If Practice A shows a suspect or step structure, one of the immersion practices (B, C, E, or F) must follow. Practice B (Streicher) applies to a wide range of austenitic grades and measures mass loss. Practice C (Huey/nitric acid) is best for detecting sigma phase and intermetallic attack in highly oxidising environments. Practice E (Strauss) is the most widely used for qualification of 304, 316, and stabilised grades. Practice F is specific to low-carbon grades using a 50% sulphuric acid-copper sulphate solution for 120 hours. Always consult the specific material specification and purchase order — many projects mandate a specific practice regardless of grade.
What is the difference between 304 and 304L in terms of IGC susceptibility?
Type 304L has a maximum carbon content of 0.030% compared to 0.080% for standard 304. The lower carbon content in 304L dramatically reduces the availability of carbon to form chromium carbides during welding or heat treatment, making 304L inherently more resistant to sensitisation and intergranular attack. However, even 304L should be tested per ASTM A262 if used in critical chemical processing or high-temperature service, as other factors such as sigma phase or nitrogen content can still influence IGC susceptibility. ASTM A262 also requires 304L to be subjected to a sensitising heat treatment (675 °C / 1 h) before Practice A testing, to reveal any latent susceptibility under accelerated conditions.
Can ASTM A262 testing be used to detect weld decay?
Yes. Weld decay is a specific form of intergranular corrosion that occurs in the heat-affected zone (HAZ) of a weld, where the base metal has been heated into the sensitisation temperature range. ASTM A262 testing — particularly Practice E — is routinely used to evaluate weld procedure qualifications and to inspect HAZ specimens cut from welded test coupons, confirming that the material and welding procedure do not produce a sensitised microstructure. The test specimens are cut so that the HAZ runs longitudinally through the specimen, ensuring that the most susceptible region is subjected to both the corrosive immersion and the subsequent bend test.
What does a step structure, dual structure, and ditch structure mean in Practice A?
In Practice A (oxalic acid etch), the microstructure after etching is classified into three categories. A step structure, where all grain boundaries appear as sharp steps with no ditches, is acceptable — the material is considered free of sensitisation and no further testing is typically required. A dual structure, where some grain boundaries show ditching around carbide particles while others remain stepped, is conditional — the material should be tested by an immersion practice. A ditch structure, where continuous networks of dissolved ditches are visible along grain boundaries, indicates severe sensitisation. The material must be tested by an immersion practice and is likely to fail — but rejection cannot be based on Practice A alone.
Is ASTM A262 applicable to duplex stainless steel?
No. ASTM A262 is specifically written for austenitic stainless steels and is not applicable to duplex (ferritic-austenitic) grades such as 2205 or 2507. Duplex stainless steels have different corrosion failure modes — primarily pitting, crevice corrosion, and sigma-phase embrittlement — and are evaluated using different standards, most notably ASTM G48 for pitting and crevice resistance and ASTM A923 for detection of detrimental phases in duplex grades. See the WeldFabWorld guide to duplex stainless steel welding for more information on duplex-specific testing requirements.