Intergranular corrosion (IGC) quality inspection is a critical materials testing service used to verify the resistance of stainless steels to grain boundary attack caused by sensitization during welding, heat treatment, or service exposure. This article provides a comprehensive technical overview of intergranular corrosion testing, covering sample requirements, test media, international standards such as ASTM A262 and ISO 3651, evaluation criteria, and practical applications across the chemical, petrochemical, nuclear, and pharmaceutical industries.
Technical Overview
Intergranular corrosion is a localized form of corrosive attack that proceeds along the grain boundaries of a metal while the bulk grains remain largely unaffected. In austenitic, ferritic, and duplex stainless steels, this phenomenon is most commonly triggered by a metallurgical condition known as sensitization. When stainless steel is heated within the critical temperature range of approximately 425 °C to 815 °C (800 °F to 1500 °F)—for example, during slow cooling from solution annealing, stress relief heat treatment, multi-pass welding, or prolonged high-temperature service—chromium carbides (Cr23C6) precipitate preferentially at the grain boundaries.
The precipitation of chromium carbides consumes chromium from a narrow zone adjacent to the grain boundary, depleting the local chromium content below the approximately 12% threshold required to maintain a stable passive oxide film. These chromium-depleted zones become electrochemically active relative to the grain interiors and are preferentially dissolved in corrosive media. The result is a progressive loss of cohesion between grains, a severe reduction in mechanical strength, and, in advanced stages, intergranular cracking or disintegration of the material with little visible evidence on the surface.
Intergranular corrosion quality inspection is therefore an essential element of material qualification, incoming material control, weld procedure qualification, and failure analysis. Laboratory testing accelerates the sensitization mechanism under controlled aggressive conditions and evaluates the material response through weight-loss measurement, bend ductility testing, or metallographic examination. Testing is typically performed in accordance with internationally recognized standards, including ASTM A262 for austenitic stainless steels, ASTM A763 for ferritic stainless steels, ASTM G28 for nickel-rich chromium-bearing alloys, ISO 3651-1 and ISO 3651-2, and the Chinese national standard series GB/T 4334.
- Sensitization mechanism driven by chromium carbide precipitation at grain boundaries
- Chromium-depleted zones with local chromium content below the passivation threshold
- Critical sensitization temperature range of approximately 425 °C to 815 °C
- Accelerated laboratory testing combined with quantitative and metallographic evaluation
- Conformance assessment against ASTM, ISO, EN, and GB/T standard requirements
Testing Samples
The validity of an intergranular corrosion test depends strongly on correct sampling, specimen orientation, and surface preparation. Samples must be representative of the delivered metallurgical condition of the product, and the test report must clearly state whether specimens were tested in the as-received condition or after a laboratory sensitizing heat treatment. Typical product forms accepted for testing are described below.
Plate, Sheet, and Strip
Flat-rolled products are sampled with the test surface parallel to the rolling plane. Specimens are cut to the dimensions specified by the applicable test practice, with all edges ground smooth to eliminate shear-affected zones that could distort the corrosion response. Full-thickness specimens are preferred so that sensitization gradients through the thickness are captured.
Bars, Rods, and Sections
For bars and structural sections, specimens are taken in both longitudinal and transverse orientations whenever product dimensions permit, because grain structure and sensitization behavior can be direction-dependent. Round bars are commonly tested using a full cross-sectional disc or a milled segment depending on bar diameter.
Pipes and Tubes
Seamless and welded pipes and tubes are sampled as full ring sections, longitudinal strips, or flattened panels. Welded tubular products receive particular attention because the heat-affected zone (HAZ) on either side of the weld is the region most susceptible to sensitization during fabrication welding. The specimen must retain the weld, HAZ, and base metal so that all zones are evaluated simultaneously.
Forgings and Castings
Forged components are sampled from locations representative of heavy sections where cooling rates are slow, while castings are sampled with consideration of segregation and dendritic structures. Large grain size and chemical segregation in castings can amplify sensitization, so test locations are agreed between the purchaser and the testing laboratory in advance.
Welded Joints and Weldments
Weld procedure qualification coupons, production weld test plates, and repaired components are tested to verify that the welding thermal cycle has not sensitized the heat-affected zone. Cross-section specimens containing base metal, HAZ, and weld metal are exposed to the test medium and subsequently examined for intergranular attack, knife-line attack, or cracking at the fusion boundary.
Specimen Preparation Requirements
- Surface grinding to a uniform finish, typically 120 grit or finer, with removal of scale, oxide, and decarburized layers
- Dimensional measurement and weighing to 1 mg precision or better before exposure for weight-loss testing
- Optional sensitizing heat treatment, for example 650 °C for 30 minutes followed by water quenching, when specified by the purchaser or product specification
- Degreasing and drying immediately before immersion in the test solution
- Identification of specimen orientation, product form, heat number, and metallurgical condition in the test report
Testing Items
Intergranular corrosion quality inspection encompasses several measurable and observational outputs. Depending on the selected test practice and the governing specification, one or more of the following items are evaluated and reported.
- Susceptibility classification to intergranular attack based on etch structure or post-exposure examination
- Average corrosion rate calculated from mass loss, specimen dimensions, exposure time, and material density, expressed in mm/year or mils per year (mpy)
- Bend test performance, in which the exposed specimen is bent 180 degrees over a specified mandrel and examined for fissures or cracking indicative of intergranular attack
- Metallographic classification of oxalic acid etch structures as step, dual, or ditch type
- Depth and morphology of intergranular attack measured on polished cross-sections under optical microscopy
- Presence or absence of knife-line attack adjacent to welds in stabilized grades
- Comparative evaluation of base metal, heat-affected zone, and weld metal after exposure
- Conformance statement against the acceptance criteria of the specified standard or purchase order
Testing Methods
Several standardized test methods exist, each designed around specific alloy systems and forms of intergranular attack. The most widely applied practices derive from ASTM A262 for austenitic stainless steels, supplemented by ASTM A763 for ferritic grades, ASTM G28 for nickel alloys, and ISO 3651 and GB/T 4334 as parallel international and Chinese frameworks. Method selection depends on the alloy composition, the expected corrosion mechanism, and the acceptance criteria imposed by the end user.
Oxalic Acid Etch Test (ASTM A262 Practice A)
The oxalic acid etch test is a rapid screening method in which polished specimens are anodically etched at a current density of approximately 1 A/cm2 in a 10% oxalic acid solution at room temperature. The etched microstructure is examined at 250× to 500× magnification and classified as step, dual, or ditch structure. A step or dual structure indicates the specimen is acceptable and does not require further testing; a ditch structure indicates susceptibility and mandates confirmation by a quantitative immersion test such as Practice B, C, or E. The method is fast and economical but is applicable only to specific alloy grades and does not itself constitute an acceptance test.
Ferric Sulfate–Sulfuric Acid Test (ASTM A262 Practice B)
In this weight-loss based immersion test, specimens are exposed for 120 hours in a boiling solution of 50% sulfuric acid containing 25 g of ferric sulfate. The oxidizing ferric ions maintain a high corrosion potential that selectively attacks chromium-depleted grain boundaries. The test evaluates the average corrosion rate, and acceptance is based on comparison with the maximum allowable rate specified for the alloy. Practice B is widely used for austenitic grades including Types 304, 304L, 316, and 316L, and is suitable for detecting chromium carbide precipitation.
Nitric Acid Test (ASTM A262 Practice C, Huey Test)
The Huey test consists of five consecutive 48-hour boiling periods in 65% nitric acid, with the solution renewed and the specimen reweighed after each period. Because nitric acid is strongly oxidizing, it detects not only chromium carbide precipitation but also sigma phase, molybdenum-rich phases, and end-grain effects that other practices may not reveal. The result is reported as the average corrosion rate over the five periods, and the maximum individual period rate is also reviewed. The Huey test is commonly specified for alloys intended for nitric acid service.
Copper–Copper Sulfate–Sulfuric Acid Test (ASTM A262 Practice E, Strauss Test)
The Strauss test exposes specimens for a minimum of 15 hours to a boiling 16% sulfuric acid solution containing copper sulfate, with metallic copper turnings in contact with the specimen to establish the redox couple. After exposure, the specimen is bent 180 degrees over a mandrel with a diameter equal to twice the specimen thickness and inspected for intergranular fissures or cracking. This practice is particularly effective for detecting sensitization caused by chromium carbide precipitation and is frequently applied to welded components and stabilized grades.
Copper–Copper Sulfate–50% Sulfuric Acid Test (ASTM A262 Practice F)
Practice F employs a more aggressive boiling solution of 50% sulfuric acid with copper sulfate and metallic copper contact, typically for 120 hours. It was developed for heavily sensitized conditions and for alloys such as extra-low-carbon molybdenum-bearing grades where Practice E may not provide sufficient discrimination. Evaluation combines weight-loss measurement with bend testing.
Nitric–Hydrofluoric Acid Test (ASTM A262 Practice G and ISO 3651-2 Method B)
Practice G uses a boiling 10% nitric acid plus 3% hydrofluoric acid solution with two 2-hour exposure periods. It is intended for molybdenum-bearing austenitic stainless steels such as Types 316 and 317, for which the low-potential ferric sulfate test can be overly severe. Acceptance is based on the ratio of the corrosion rate of the test specimen to that of a solution-annealed reference specimen of the same heat. Hydrofluoric acid handling requires specialized apparatus and rigorous safety controls.
Methods for Ferritic and Duplex Stainless Steels
ASTM A763 provides practices W, X, Y, and Z covering ferritic stainless steels, using ferric sulfate–sulfuric acid, oxalic acid etch, nitric acid, and hydrochloric acid–ferric chloride solutions respectively. Duplex stainless steels are assessed through modified versions of austenitic practices, with attention to preferential attack on ferrite or austenite phases and to sigma and chi phase precipitation, which can dominate sensitization behavior in duplex grades.
Relevant Standards Framework
- ASTM A262 – Detecting susceptibility to intergranular attack in austenitic stainless steels (Practices A, B, C, E, F, G)
- ASTM A763 – Detecting susceptibility to intergranular attack in ferritic stainless steels (Practices W, X, Y, Z)
- ASTM G28 – Detecting susceptibility to intergranular corrosion in wrought nickel-rich, chromium-bearing alloys
- ISO 3651-1 and ISO 3651-2 – Determination of resistance to intergranular corrosion of stainless steels
- GB/T 4334 series – Chinese national standards for intergranular corrosion testing of stainless steels, aligned in principle with the ASTM and ISO practices
Applications
Intergranular corrosion quality inspection supports material integrity across a broad range of demanding industries where localized grain boundary attack could compromise pressure containment, product purity, or structural reliability.
- Chemical and petrochemical plants – qualification of reactors, heat exchangers, columns, piping, and gasketed flange materials exposed to organic and inorganic acids
- Nuclear power generation – certification of primary circuit piping, core internals, and cladding materials where sensitization and irradiation-assisted grain boundary attack are concerns
- Oil and gas processing – evaluation of refinery equipment, sour service components, and offshore platform piping after welding or thermal exposure
- Pharmaceutical and food processing – verification of sanitary-grade tubing, vessels, and fittings where product contact surfaces must remain passive and contamination-free
- Pulp and paper industry – assessment of digesters, bleach plant washers, and liquor handling systems in chloride and oxidizing environments
- Aerospace and transportation – qualification of high-alloy fasteners, fittings, and welded structural elements
- Welding procedure qualification – mandatory IGC testing of procedure qualification records (PQR) for stainless steel welding in code-governed construction
- Failure analysis and litigation support – determination of whether in-service cracking is attributable to sensitization-induced intergranular corrosion
FAQ
What causes intergranular corrosion in stainless steel?
Intergranular corrosion is primarily caused by sensitization, in which chromium carbides precipitate at grain boundaries when the steel passes through the 425 °C to 815 °C temperature range. This precipitation depletes chromium in the adjacent zones below the level needed for passivation, leaving the grain boundaries vulnerable to selective dissolution. Sigma phase precipitation and segregation of impurity elements such as sulfur and phosphorus can also contribute to grain boundary attack in certain alloys.
Which intergranular corrosion test method should be selected?
Method selection depends on the alloy system and the failure mechanism of concern. For a fast, low-cost screening of austenitic grades, Practice A (oxalic acid etch) is appropriate. For quantitative acceptance of Types 304 and 316 family materials, Practice B (ferric sulfate–sulfuric acid) is common. Practice C (Huey) is preferred for nitric acid service and for detecting sigma phase, while Practice E (Strauss) with the bend test is favored for welded fabrications and stabilized grades. Molybdenum-bearing alloys are best evaluated with Practice G or ISO 3651-2 Method B. The purchase specification or end user normally dictates the governing practice and acceptance criteria.
What is a sensitizing heat treatment and when is it required?
A sensitizing heat treatment is an artificial laboratory exposure, commonly 650 °C for 30 minutes followed by water quenching, applied to specimens before testing to simulate the most adverse thermal condition the material could experience during fabrication. It is specified when the material will be welded or thermally processed in service, or when the specification requires worst-case evaluation. Extra-low-carbon and stabilized grades are frequently tested both with and without sensitization to demonstrate their resistance.
How are test results evaluated and reported?
Results are reported as either a pass or fail classification based on bend test integrity and visual examination for cracking, or quantitatively as a corrosion rate in mm/year or mpy calculated from mass loss. Oxalic acid etch screening results are classified as step, dual, or ditch structure. Metallographic cross-sections may be included to document the depth and morphology of attack. A complete test report identifies the material grade and heat number, product form, specimen orientation, test practice and solution, exposure duration, sensitization condition, calculated corrosion rates, and the conformance conclusion against the specified acceptance limit.
How can intergranular corrosion be prevented?
Prevention is achieved through metallurgical and fabrication controls: selecting extra-low-carbon grades such as 304L and 316L with carbon content typically below 0.03%; choosing stabilized grades such as Type 321 (titanium-stabilized) or Type 347 (niobium-stabilized), whose stabilizing elements form carbides preferentially to chromium; applying post-weld solution annealing to dissolve precipitated carbides; using low heat input welding with fast cooling through the sensitization range; and, where applicable, adopting modern dual-stabilized ferritic or duplex grades engineered for weldability.
Can intergranular corrosion testing be performed on welded components?
Yes. Welded test coupons and production samples are routinely tested with the specimen oriented so that base metal, heat-affected zone, and weld metal are all exposed simultaneously. After the immersion period, the specimen is bent and examined for intergranular fissures, with particular attention to the heat-affected zone and to knife-line attack immediately adjacent to the fusion line in stabilized alloys. Testing of welded coupons is a standard requirement of many welding procedure qualifications and pressure equipment codes.