Intergranular Corrosion Testing of Stainless Steel: Standards, Methods, and Procedures

Intergranular corrosion (IGC) testing evaluates the resistance of stainless steels to localized attack along grain boundaries caused by sensitization during welding, heat treatment, or service exposure. This article provides a comprehensive technical guide to intergranular corrosion testing of stainless steel, covering major standards such as ASTM A262, ASTM G28, ASTM A763, ISO 3651-2, and GB/T 4334, including sample preparation, test procedures, acceptance criteria, and typical industrial applications.

Technical Overview

Intergranular corrosion (IGC) is a localized form of corrosion in which attack proceeds along the grain boundaries of a metal while the grains themselves remain largely unaffected. In stainless steels, this phenomenon is almost always associated with sensitization: when austenitic stainless steel is exposed to a temperature range of approximately 425 to 815 degrees Celsius, chromium combines with carbon to precipitate chromium-rich carbides (typically M23C6) at the grain boundaries. This leaves a thin chromium-depleted zone adjacent to the boundary, where the chromium content falls below the approximate 10.5 to 12 percent threshold required to maintain a stable passive film.

Sensitization commonly occurs during welding, in the heat-affected zone (HAZ), during slow cooling from hot working or annealing, or during prolonged service at elevated temperatures. The resulting attack can cause grains to detach from the surface, a mechanism known as grain dropping, leading to rapid loss of mechanical strength and, in severe cases, catastrophic failure with little external visual warning. For this reason, intergranular corrosion testing is a critical quality control and qualification requirement wherever stainless steel components are welded, hot formed, or exposed to corrosive media at elevated temperatures.

Intergranular corrosion susceptibility is evaluated using internationally recognized test methods, and the governing standards most frequently applied include:

• ASTM A262: Standard Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels (Practices A, B, C, E, and F)
• ASTM A763: Standard Practices for Detecting Susceptibility to Intergranular Attack in Ferritic Stainless Steels (Practices W, X, Y, and Z)
• ASTM G28: Detecting Susceptibility to Intergranular Corrosion in Wrought, Nickel-Rich, Chromium-Bearing Alloys (Methods A and B)
• ISO 3651-2: Determination of Resistance to Intergranular Corrosion of Stainless Steels, Corrosion Test in Media Containing Sulfuric Acid (Methods A, B, and C)
• GB/T 4334: Chinese national standard for testing intergranular corrosion susceptibility of stainless steels, broadly harmonized with ASTM A262
• JIS G 0571 to G 0575: Japanese industrial standards for oxalic acid, sulfuric acid-ferric sulfate, nitric acid, and copper-copper sulfate testing
• ASTM G108: Electrochemical Potentiokinetic Reactivation (EPR) method for quantifying sensitization in Type 304 and 304L stainless steels

Testing Samples

Intergranular corrosion tests can be performed on virtually all product forms of stainless steel. Specimen geometry, orientation, and surface condition significantly influence test validity, so sampling must follow the requirements of the applicable standard and product specification.

Plates, Sheets, and Strips

Flat-rolled products are the most commonly tested form. Coupons are typically cut to approximately 30 mm by 20 mm, retaining the full product thickness where possible so that the mid-thickness microstructure, where sensitization may be most severe, is evaluated. Test surfaces are ground to a uniform finish, typically 120 grit or finer.

Pipes and Tubes

Specimens are taken from the pipe or tube wall, often as longitudinal or circumferential segments or flattened sections depending on diameter and wall thickness. For welded pipe, specimens are oriented so that the weld metal, heat-affected zone, and base metal are all exposed to the test solution.

Bars, Rods, Wires, and Forgings

Solid products are sectioned so that both longitudinal and transverse surfaces are examined where required. For large forgings, specimens should be taken from locations representative of the heaviest section, since cooling rates there promote sensitization.

Welded Joints and Heat-Affected Zones

Weldment coupons are prepared to include the full weld cross section, including weld metal, fusion line, and heat-affected zone. This is essential for weld procedure qualification and for evaluating knife-line attack in stabilized grades such as 321 and 347.

Castings

Investment and sand castings are tested using coupons machined from the casting itself or from separately cast test blocks heat treated together with the production parts, in accordance with standards such as ASTM A351 supplementary requirements.

Specimen Preparation Requirements

All specimens are degreased, cleaned, dried, and weighed to an accuracy of 0.001 g or better before testing. Where required by the purchase specification, an artificial sensitizing heat treatment (for example, 650 degrees Celsius for a specified holding time) may be applied before testing to evaluate worst-case susceptibility. Dimensional measurements are recorded so that exposed surface area can be calculated for corrosion rate determination.

Testing Items

Depending on the selected method and the client specification, an intergranular corrosion test program may include the following evaluation items:

• Qualitative screening of microstructure for sensitized grain boundaries (etch structure classification)
• Quantitative corrosion rate determination from mass loss after boiling acid exposure
• Bend (ductility) testing of exposed specimens to reveal intergranular fissures and cracks
• Metallographic examination and measurement of intergranular attack depth after exposure
• Evaluation of weld metal, fusion line, and heat-affected zone susceptibility in welded samples
• Electrochemical determination of the degree of sensitization (DOS) using reactivation current ratios
• Comparison of corrosion rates against acceptance limits specified in the governing material specification
• Verification of the effectiveness of stabilizing elements (titanium, niobium) and low-carbon grades

Testing Methods

The most widely applied methods are those of ASTM A262 and its international equivalents. Each practice targets different metallurgical conditions and corrosive environments, and the correct choice depends on the steel grade, product form, and intended service.

Practice A: Oxalic Acid Etch Test (ASTM A262 Practice A)

The oxalic acid etch test is a rapid screening method in which the polished specimen surface is anodically etched at a current density of 1 A per square centimeter for 1.5 minutes in a 10 percent oxalic acid solution at room temperature. The etched microstructure is then examined under an optical microscope at 250 to 500 magnification and classified as a step structure, a dual structure, or a ditch structure. Step and dual structures indicate acceptable resistance for most grades, while a ditch structure disqualifies the material from acceptance by screening and requires confirmation using a full immersion practice such as Practice B or E. Practice A is a screening test only and is never used alone for final acceptance.

Practice B: Ferric Sulfate-Sulfuric Acid Test (ASTM A262 Practice B, ASTM G28 Method A)

The specimen is boiled for 120 hours (24 hours for nickel alloys under ASTM G28 Method A) in a solution of 50 percent sulfuric acid containing ferric sulfate. The oxidizing ferric ions maintain a strongly corrosive potential that attacks chromium-depleted zones. The corrosion rate is calculated from mass loss using the formula: Corrosion Rate = (K x W) / (A x T x D), where W is mass loss, A is exposed area, T is exposure time, D is density, and K is a unit-conversion constant. Acceptance is based on the calculated rate not exceeding the maximum limit stated in the product specification. Practice B is highly sensitive to chromium carbide precipitation and also detects sigma phase in certain molybdenum-bearing grades.

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

The Huey test exposes the specimen to boiling 65 percent nitric acid for five consecutive 48-hour periods, with fresh solution used in each period. Because nitric acid strongly attacks sigma phase as well as chromium carbides, Practice C is the preferred method for material intended for nitric acid service and for grades where intermetallic phases are a concern. The average and maximum corrosion rates over the five periods are reported and compared against specification limits, and a rising rate across successive periods is itself an indicator of progressive intergranular or interphase attack.

Practice E: Copper-Copper Sulfate-16 Percent Sulfuric Acid Test (Strauss Test, ASTM A262 Practice E, ISO 3651-2 Method A)

In the Strauss test, the specimen is placed in contact with metallic copper and boiled for 24 hours in a solution of copper sulfate in 16 percent sulfuric acid. The presence of metallic copper shifts the corrosion potential into a range that selectively attacks sensitized grain boundaries. After exposure, the specimen is subjected to a bend test, typically bent 180 degrees around a mandrel with a diameter related to the specimen thickness, and the tension surface is inspected for cracks and fissures. The appearance of intergranular fissures constitutes failure. This method closely matches many plant service environments and is widely used in Europe under ISO 3651-2.

Practice F: Copper-Copper Sulfate-50 Percent Sulfuric Acid Test (ASTM A262 Practice F)

Practice F extends the Strauss concept by using a 50 percent sulfuric acid solution with copper sulfate and metallic copper contact, boiled for 24 hours. The more aggressive medium was developed for molybdenum-bearing austenitic grades such as 316, 316L, 317, and 317L, whose higher alloy content makes the 16 percent acid test insufficiently discriminating.

Electrochemical Potentiokinetic Reactivation (EPR and DL-EPR, ASTM G108)

Electrochemical reactivation methods polarize the specimen anodically into the passive region and then reverse the scan. In sensitized material, the chromium-depleted boundaries dissolve during reactivation, producing a measurable reactivation current. In the double-loop variant (DL-EPR), the ratio of the reactivation peak current to the activation peak current quantifies the degree of sensitization. These methods are fast, largely non-destructive, and particularly valuable for research, for assessing duplex stainless steels, and for components from which destructive coupons cannot easily be removed.

Methods for Ferritic and Duplex Grades

Ferritic stainless steels are evaluated under ASTM A763, while duplex stainless steels are commonly assessed using ISO 3651-2 methods or ASTM A923, which detects detrimental intermetallic phases such as sigma that promote intergranular and phase-boundary attack in grades like 2205.

Acceptance Criteria and Evaluation

Acceptance is defined by the governing product specification or purchase order rather than by the test standard itself. Typical criteria include a maximum allowable corrosion rate in millimeters per year or mils per year for Practices B, C, and F, absence of intergranular cracks after the bend test in Practices E and F, and acceptable etch structures or attack depths in metallographic evaluation. A complete test report should identify the steel grade and heat number, heat treatment condition, test practice and solution, exposure time, mass loss and calculated corrosion rate, bend test results, and microstructural observations.

Applications

Intergranular corrosion testing is specified across a broad range of industries where welded or heat-treated stainless steel operates in corrosive environments:

• Chemical and petrochemical processing: reactors, heat exchangers, columns, and piping carrying acids, chlorides, and process chemicals
• Oil and gas: offshore platforms, subsea components, flowlines, and refinery equipment exposed to sour and acidic media
• Nuclear power: primary circuit piping, reactor internals, and steam generator components where sensitization can compound stress corrosion risks
• Food, beverage, and pharmaceutical: sanitary welded piping, tanks, and vessels requiring welded joints free of sensitized zones
• Pulp and paper: bleach plant equipment exposed to hot acidic chloride solutions
• Power generation and water treatment: boiler components, condensers, flue gas desulfurization systems, and RO plant piping
• Material qualification and quality control: incoming material verification, heat release certification, and supplier audits
• Welding procedure qualification: verifying that welding procedures do not sensitize the heat-affected zone of the base metal
• Failure analysis and research: diagnosing in-service failures and developing new low-carbon, stabilized, or high-alloy grades

FAQ

What causes intergranular corrosion in stainless steel?

Intergranular corrosion is caused by sensitization, the precipitation of chromium carbides at grain boundaries when the steel is heated between roughly 425 and 815 degrees Celsius. This depletes the boundary-adjacent zones of chromium below the level needed for passivation, leaving them vulnerable to selective attack in corrosive media. Welding, slow cooling from hot working, and elevated-temperature service are the most common causes.

Which ASTM A262 practice should be selected for my material?

Practice A is a fast screening etch for austenitic grades such as 304 and 316. Practices B and C provide quantitative corrosion rates, with Practice C preferred for nitric acid service and for detecting sigma phase. Practices E and F use a bend test for acceptance, with Practice F recommended for molybdenum-bearing grades. The final selection should follow the governing product specification and intended service environment.

If a sample shows a ditch structure in the oxalic acid test, does it automatically fail?

No. A ditch structure only means the material cannot be accepted through screening and must be subjected to the full immersion test, such as Practice B or Practice E. Many materials with ditch structures still pass the quantitative or bend test acceptance criteria.

Can sensitization be removed from stainless steel?

Yes. A solution annealing treatment, typically heating to 1050 to 1150 degrees Celsius followed by rapid water quenching, redissolves the chromium carbides and restores uniform corrosion resistance. Prevention alternatives include using low-carbon grades such as 304L and 316L, or stabilized grades containing titanium (321) or niobium (347).

How long does intergranular corrosion testing take?

The oxalic acid etch test takes only minutes after surface preparation. The Strauss test requires a 24-hour boil plus bend testing. Practice B requires 120 hours of boiling, and the Huey test requires five consecutive 48-hour periods, totaling 240 hours of exposure. Including machining, weighing, metallography, and reporting, complete programs typically take from several days to about two weeks.

Is intergranular corrosion testing required for duplex stainless steel?

Duplex grades are less prone to classical carbide sensitization but can suffer intergranular and phase-boundary attack from intermetallic precipitates such as sigma and chromium nitrides. They are commonly evaluated under ISO 3651-2, ASTM A923, or DL-EPR electrochemical methods rather than the standard austenitic practices alone.