Intergranular Corrosion Testing of Stainless Steel to ASTM Standards: Methods, Practices, and Accept

Intergranular corrosion (IGC) testing of stainless steel to American standards, primarily ASTM A262, is the internationally recognized method for evaluating the susceptibility of austenitic and ferritic-austenitic stainless steels to chromium carbide precipitation at grain boundaries. This article provides a comprehensive technical guide covering sample preparation, the five ASTM A262 test practices, acceptance criteria, and typical industrial applications. Understanding these test methods is essential for material qualification, weld procedure validation, and quality control in chemical, petrochemical, and nuclear industries.

Technical Overview

Intergranular corrosion is a localized form of attack that occurs along the grain boundaries of stainless steel when the material becomes sensitized. Sensitization typically happens when the steel is heated within the temperature range of approximately 425 to 815 °C (800 to 1500 °F), during which chromium carbides (Cr23C6) precipitate at grain boundaries and deplete the adjacent zones of chromium to levels below the 10.5% threshold required for passivation. The chromium-depleted regions become anodic relative to the grain interiors, and corrosive media selectively attack these boundaries, potentially causing grain dropping, loss of mechanical strength, and catastrophic intergranular cracking.

In American standard practice, the primary test specification for detecting susceptibility to intergranular attack in austenitic stainless steels is ASTM A262, titled "Standard Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels." This standard defines five distinct practices (A, B, C, E, and F), each designed to evaluate different metallurgical conditions and chromium depletion levels. Complementary American standards include ASTM G28 for detecting susceptibility to intergranular corrosion in wrought, nickel-rich, chromium-bearing alloys, and ASTM A763 for wrought ferritic stainless steels. Together, these specifications form the complete framework for intergranular corrosion qualification of corrosion-resistant alloys in the American standard system.

The selection of the appropriate practice depends on the alloy grade, the expected service environment, and the specific metallurgical concern. Some practices serve as rapid screening tools, while others provide quantitative corrosion rate measurements or pass/fail bend evaluations. The tests simulate the electrochemical conditions that drive chromium-depletion attack without requiring long-term exposure in actual process media.

Testing Samples

Plate and Sheet Specimens

Specimens cut from plate and sheet products should have a total exposed surface area of approximately 15 to 35 cm² (roughly 30 cm² is typical), depending on the specific practice. The sample thickness should generally not exceed the mill product thickness. For evaluation of rolled products, the specimen faces must represent the principal rolling surfaces. Sharp edges should be rounded by grinding to eliminate edge effects, and the surface should be finished to a uniform grit, typically 120 grit or finer, to remove scale, embedded iron, and any cold-worked layer that could affect results.

Pipe and Tube Specimens

For seamless and welded pipe and tubing, arc-shaped segments or flattened coupon sections are prepared with a comparable total surface area. When welded joints are evaluated, the specimen must include the weld, the heat-affected zone (HAZ), and adjacent base metal so that knife-line attack and HAZ sensitization can be detected. The internal surface of tubing intended for corrosive service is frequently the critical examination surface and must be prepared without contamination.

Bar, Forging, and Cast Specimens

Specimens from bars, forgings, and castings are sectioned so that the principal surfaces correspond to the surfaces exposed in service. For large sections, samples are usually taken at the mid-radius or quarter-thickness location to represent bulk material behavior. Castings may require evaluation of both as-cast and heat-treated conditions because segregation during solidification can influence intergranular attack susceptibility.

Sensitized and As-Supplied Conditions

Depending on the purchasing specification, specimens may be tested in the as-received condition (after solution annealing or stabilizing treatment) or after an additional laboratory sensitizing heat treatment. A common sensitization treatment for evaluation purposes is heating at 650 to 675 °C (1200 to 1250 °F) for 30 minutes to 2 hours, followed by cooling in air or water. Testing sensitized specimens provides a conservative assessment of the material resistance if it experiences slow cooling or thermal cycling during fabrication or service.

Testing Items

The American standard intergranular corrosion test program for stainless steel typically includes the following testing items, depending on the material grade and specification requirements:

• Oxalic acid etch screening test (ASTM A262 Practice A) for austenitic stainless steels
• Ferric sulfate–sulfuric acid immersion test (ASTM A262 Practice B) for weight-loss-based corrosion rate determination
• Nitric acid (Huey) test (ASTM A262 Practice C) for steels intended for nitric acid service
• Copper–copper sulfate–16% sulfuric acid test with bend evaluation (ASTM A262 Practice E, Strauss-type test)
• Copper–copper sulfate–50% sulfuric acid test (ASTM A262 Practice F) for molybdenum-bearing grades such as 316 and 317
• Intergranular corrosion testing of nickel-rich alloys per ASTM G28 Method A or Method B
• Sensitization heat treatment of specimens prior to testing, when specified
• Post-test bend testing, metallographic examination, and corrosion rate calculation in mdd or mils per year

Testing Methods

ASTM A262 Practice A: Oxalic Acid Etch Test

Practice A is a rapid electrolytic etching screening method performed at room temperature in a 10% oxalic acid solution. The polished specimen is etched anodically at a current density of 1 A/cm² for 1.5 minutes. The etched microstructure is then examined under a microscope at 250× to 500× and classified as a step structure, dual structure, or ditch structure. A step or dual structure indicates the material is acceptable and may be exempted from further testing, whereas a ditch structure indicates possible susceptibility and requires confirmation by one of the quantitative immersion practices (B, C, E, or F). Practice A cannot be used for final acceptance by itself and is not applicable to certain grades, such as 316 and 317, in some conditions.

ASTM A262 Practice B: Ferric Sulfate–Sulfuric Acid Test

Practice B is a 120-hour boiling immersion test in a solution of 50% sulfuric acid containing 25 g of ferric sulfate. The specimen is weighed before and after exposure, and the corrosion rate is calculated from the measured weight loss. The ferric ions provide an oxidizing potential that attacks chromium-depleted grain boundary regions. Acceptance is based on the calculated corrosion rate not exceeding the maximum value specified for the grade, commonly expressed in milligrams per square decimeter per day (mdd) or mils per year. Practice B is suitable for chromium carbide sensitization evaluation but is not designed to detect sigma-phase or chromium depletion associated with molybdenum-bearing segregation.

ASTM A262 Practice C: Nitric Acid Test (Huey Test)

Practice C, known as the Huey test, involves boiling the specimen in 65% nitric acid for five consecutive 48-hour periods, with fresh solution each period. The corrosion rate from each period is calculated and averaged, and the results are evaluated against the specified maximum rate. Because nitric acid strongly attacks sigma phase, chromium-depleted zones, and other secondary phases, Practice C is the most discriminating test and is mandatory for materials intended for nitric acid service, such as 304L and 310 stainless steels. Excessive weight loss in specific periods or intergranular voids visible on metallographic examination indicate susceptibility.

ASTM A262 Practice E: Copper–Copper Sulfate–16% Sulfuric Acid Test

Practice E, based on the Strauss test, exposes the specimen for 24 hours to a boiling solution of 16% sulfuric acid containing copper sulfate and metallic copper shot or copper pieces in contact with the specimen. After exposure, the specimen is bent 180 degrees over a mandrel with a diameter equal to the specimen thickness. The bent surface is examined at low magnification (typically 10× to 20×) for cracks or fissures caused by intergranular attack. The appearance of cracks indicates susceptibility. Practice E evaluates chromium carbide precipitation and is widely used for acceptance of low-carbon and stabilized grades such as 304L, 321, and 347.

ASTM A262 Practice F: Copper–Copper Sulfate–50% Sulfuric Acid Test

Practice F uses a more aggressive boiling solution of 50% sulfuric acid with copper sulfate and metallic copper contact for 120 hours, followed by a 180-degree bend test identical to Practice E. The higher acid concentration and longer exposure make Practice F suitable for detecting intergranular susceptibility in molybdenum-bearing austenitic stainless steels, principally Type 316 and Type 317, whose higher alloy content requires a more severe environment to reveal chromium-depletion attack.

Complementary Methods: ASTM G28 and ASTM A763

For nickel-rich chromium-bearing alloys such as Alloy 400, 600, 800, and 825, ASTM G28 Method A (ferric sulfate–sulfuric acid, 120 hours) and Method B (mixed acid with ferric sulfate, 24 hours) are applied with grade-specific acceptance criteria. For ferritic stainless steels, ASTM A763 Practices W, X, Y, and Z provide equivalent immersion and etch procedures tailored to ferritic grade compositions. Specimen preparation, solution chemistry control, solution volume per unit surface area (normally at least 20 mL/cm², or per practice requirements), and continuous boiling reflux are mandatory procedural elements for valid results in all methods.

Applications

Intergranular corrosion testing to American standards is a mandatory quality gate in numerous demanding industries:

• Chemical and petrochemical processing: qualification of reactors, heat exchangers, piping, and vessels exposed to organic acids, sulfur-bearing media, and process condensates
• Nuclear power: acceptance testing of reactor internals, primary piping, and cladding materials where grain boundary attack could compromise structural integrity
• Pharmaceutical and food processing: verification of stainless steel piping, tanks, and hygienic equipment that must retain a clean, passive surface
• Oil and gas: evaluation of offshore platform components, downhole tubulars, and refinery equipment subject to polythionic acid attack during shutdowns
• Pulp and paper: material selection and incoming inspection for digesters and bleaching plant equipment in hot acid environments
• Fabrication and welding qualification: validation of weld procedures and post-weld heat treatment to ensure the heat-affected zones do not become sensitized
• Material certification and export compliance: third-party test reports per ASTM A262 are frequently required by international buyers and engineering specifications

FAQ

What is the difference between Practices B, C, E, and F of ASTM A262?

Practice B provides a quantitative corrosion rate in ferric sulfate–sulfuric acid over 120 hours and primarily detects chromium carbide sensitization. Practice C (Huey test) is the most severe, uses boiling 65% nitric acid for five 48-hour cycles, and detects both carbide precipitation and sigma phase. Practices E and F are pass/fail copper–copper sulfate–sulfuric acid immersion tests finished with a bend examination; Practice E uses 16% sulfuric acid for standard grades, while Practice F uses 50% sulfuric acid for molybdenum-bearing grades such as 316 and 317.

Can Practice A be used for final acceptance?

No. The oxalic acid etch test in Practice A is a screening method only. Specimens showing a step or dual structure are considered acceptable for the corresponding immersion test, while a ditch structure requires the material to undergo the full quantitative immersion practice specified in the purchase order. Final acceptance always relies on Practices B, C, E, or F.

Why is a sensitization treatment sometimes performed before testing?

Low-carbon (L grades) and stabilized (321, 347) stainless steels may pass the test in the annealed condition but still contain metallurgical susceptibility if exposed to slow cooling or subsequent welding. A laboratory sensitization heat treatment at approximately 650 to 675 °C demonstrates whether the material can tolerate thermal cycles without grain boundary attack, providing a conservative margin for fabrication and service.

How are test results reported?

For weight-loss practices, results are reported as corrosion rates in mdd (mg/dm²/day) or mils per year, compared against grade-specific maximum allowable values. For bend-based practices, results are reported as pass or fail based on the absence or presence of intergranular cracks after bending. Reports should identify the standard practice used, solution composition, exposure duration, specimen heat-treatment condition, calculated rates, and metallographic or bend examination findings.

How long does a complete ASTM A262 test take?

Practice A requires only a few minutes of etching plus metallographic examination. Practice E requires 24 hours of boiling exposure plus bend evaluation. Practices B and F require 120 hours of exposure. Practice C requires approximately 240 hours (five 48-hour cycles). Including specimen machining, conditioning, weighing, and examination, a typical project cycle ranges from three working days to two weeks depending on the practice selected.