Intergranular Corrosion Metallographic Examination: Standards, Methods, and Evaluation Criteria

Intergranular corrosion (IGC) metallographic examination is a specialized laboratory technique used to evaluate the susceptibility of stainless steels, nickel alloys, and aluminum alloys to preferential grain boundary attack caused by sensitization. Performed in accordance with internationally recognized standards such as ASTM A262, ISO 3651, GB/T 4334, and JIS G 0571, the examination combines accelerated corrosion testing with high-magnification metallographic analysis to detect, measure, and classify intergranular attack. This article explains the technical principles, sample requirements, testing procedures, evaluation criteria, and typical applications of intergranular corrosion metallographic examination.

Technical Overview

Intergranular corrosion metallographic examination is a controlled laboratory evaluation of a metallic material's resistance to localized attack along its grain boundaries. Unlike general corrosion, intergranular corrosion proceeds invisibly along the internal grain boundary network, causing a dramatic loss of cohesion between grains, grain dropping, and premature cracking, often with little or no visible deterioration on the component surface. Metallographic examination is the definitive method for confirming, quantifying, and classifying this form of attack.

The metallurgical root cause in chromium-bearing alloys is sensitization. When austenitic stainless steel is held in the temperature range of approximately 425 °C to 815 °C, for example during welding, slow cooling, or stress-relieving heat treatment, chromium-rich carbides (typically M23C6) precipitate at grain boundaries. This precipitation depletes the adjacent zones of chromium below the level required for passive-film stability, leaving those zones anodic and highly vulnerable to attack by corrosive media. Similar grain boundary phenomena occur in nickel alloys (carbide or sigma phase precipitation) and in heat-treatable aluminum alloys (grain boundary precipitates and precipitate-free zones).

The examination integrates two stages: an accelerated corrosion test that selectively attacks sensitized grain boundaries (electrolytic etching or boiling acid immersion), followed by microscopic examination of etched surfaces and polished cross sections to classify the microstructure and measure the depth of intergranular penetration. The principal governing standards include ASTM A262 for austenitic stainless steels, ASTM A763 for ferritic stainless steels, ASTM G28 for wrought nickel-rich chromium-bearing alloys, ASTM G110 for aluminum alloys, the ISO 3651 series, the GB/T 4334 series, and the JIS G 0571 to G 0575 series.

Testing Samples

Austenitic Stainless Steels

The most frequently tested materials include grades such as 304, 304L, 316, 316L, 321, 347, 310S, 904L, and super-austenitic alloys. Both solution-annealed material (to verify the delivered condition) and sensitized or as-welded material (to evaluate service susceptibility) are examined.

Ferritic and Duplex Stainless Steels

Ferritic grades such as 430, 446, and 444 are tested in accordance with ASTM A763 (Practices W, X, Y, and Z), while duplex grades such as 2205 and 2507 are evaluated primarily for sigma phase and other intermetallic precipitation that degrades boundary corrosion resistance.

Nickel-Based Alloys

Alloys such as Alloy 600, 625, 800, and 825 are examined under ASTM G28, which provides a ferric sulfate–sulfuric acid method (Method A) and a more aggressive mixed-acid method (Method B) for highly alloyed materials.

Aluminum Alloys

Heat-treatable 2XXX and 7XXX series aluminum alloys are evaluated for intergranular and exfoliation-type attack using the sodium chloride–hydrogen peroxide immersion environment described in ASTM G110.

Specimen Forms and Requirements

• Flat coupons, typically in the range of 20 mm × 30 mm with a defined thickness, prepared to the surface finish required by the applicable standard.
• Full cross sections of welded joints, including weld metal, heat-affected zone (HAZ), and base metal on a single mount.
• Tube ring sections, bar sections, fastener blanks, forgings, and castings, with orientation (longitudinal or transverse) documented.
• Specimens requiring a sensitization heat treatment are treated prior to testing when specified by the material or product specification.
• Samples for weight-loss-based corrosion rate measurement are degreased and weighed to the precision specified by the standard before immersion.

Testing Items

Depending on the specified standard and product specification, the examination may include the following determination items:

• Microstructure classification after electrolytic etching: step structure, dual structure, or ditch structure, plus end-grain pitting classification where applicable.
• Depth of intergranular penetration on polished cross sections, measured in micrometers using calibrated microscopy and image analysis.
• Detection and extent of grain dropping or grain fallout after immersion testing.
• Extent of the sensitized zone, for example the width of the attack-affected region in a weld HAZ.
• Corrosion rate calculation from mass loss for boiling-acid practices, expressed in millimeters per month or equivalent units.
• Confirmation of grain boundary precipitates, such as chromium carbides or sigma phase, with supporting micrographs.
• Conformity assessment against the acceptance criteria defined by the applicable standard or purchase specification.

Testing Methods

Specimen Preparation

Specimens are sectioned with minimal heat input, mounted, ground through successive abrasive grits, and polished in accordance with ASTM E3 and the etching guidance of ASTM E407. Careful preparation is essential because mechanical deformation or overheating artifacts can be mistaken for genuine intergranular attack. Cross sections are prepared both before and after the corrosion exposure so that baseline microstructure and post-test attack can be compared directly.

Oxalic Acid Etch Test (Screening Method)

Based on ASTM A262 Practice A, GB/T 4334.1, and JIS G 0571, the sample is electrolytically etched in a 10% oxalic acid solution at a controlled current density for a short period at ambient temperature. The etched surface is examined optically, typically at 250× to 500×, and classified as follows:

• Step structure: stepped relief between grains caused by preferential dissolution of orientation-dependent boundaries, with no ditches; indicates the absence of a continuous chromium carbide network.
• Dual structure: steps with isolated ditches, but no grain completely surrounded by ditches.
• Ditch structure: one or more grains completely surrounded by continuous ditches, indicating an interconnected sensitized boundary network.

A step or acceptable dual structure normally clears the material for the screening acceptance, whereas a ditch structure indicates potential susceptibility and requires a confirmatory boiling-acid test.

Boiling Immersion Corrosion Tests

Confirmatory quantitative testing is performed in glassware fitted with reflux condensers under rigorously controlled boiling conditions in a fume-hood environment by accredited third-party laboratories. The commonly specified practices include:

• Strauss test: copper–copper sulfate–16% sulfuric acid with metallic copper contact (ASTM A262 Practice E, ISO 3651-2 Method A, GB/T 4334.5, JIS G 0575).
• Ferric sulfate–sulfuric acid test: boiling 50% sulfuric acid with ferric sulfate addition (ASTM A262 Practice B, ISO 3651-2 Method C variant, GB/T 4334.2).
• Huey test: boiling 65% nitric acid in five consecutive 48-hour periods (ASTM A262 Practice C, GB/T 4334.3, JIS G 0573), assessed by corrosion rate.
• Nitric–hydrofluoric acid test for molybdenum-bearing grades (ASTM A262 Practice D, GB/T 4334.4, JIS G 0574).
• Copper–copper sulfate–50% sulfuric acid test for highly alloyed and molybdenum-bearing stainless steels (ASTM A262 Practice F, ISO 3651-2 Method B).
• Nickel alloy testing in ferric sulfate–sulfuric or mixed-acid media (ASTM G28 Methods A and B).

Metallographic Examination and Evaluation

After exposure, the specimens are cleaned and examined. Evaluation includes visual and low-magnification inspection for grain dropping, followed by cross-sectional microscopy on a calibrated metallographic microscope. The examiner measures the maximum and average depth of intergranular penetration using a calibrated eyepiece reticle or digital image analysis, documents the microstructure with photomicrographs, and records any localized end-grain attack. Where required, scanning electron microscopy can confirm the morphology of attack and identify grain boundary precipitates. The results are compared against the acceptance limits of the governing standard or product specification, which may specify a maximum permissible penetration depth, a maximum corrosion rate, or both.

Complementary Bend Test

For immersion practices such as the Strauss test, the standards permit acceptance by bend testing, in which the coupon is bent through a specified angle and the tension surface is inspected for intergranular cracking. Metallographic cross-section examination is the preferred alternative when specimens are too thick, too small, or too complex to bend, or when a quantitative record of attack depth is contractually required.

Applications

Intergranular corrosion metallographic examination is specified worldwide for material qualification, fabrication control, and failure analysis. Typical applications include:

• Welding procedure qualification and heat-affected zone sensitization assessment for stainless steel fabrications.
• Pressure vessels, boilers, heat exchangers, and process piping in chemical, petrochemical, pharmaceutical, and fertilizer plants.
• Nuclear power components and auxiliary systems where grain boundary integrity is safety-critical.
• Offshore and marine equipment, condenser tubing, and desalination plant hardware.
• Incoming material certification and independent verification of plate, pipe, fitting, and forging suppliers.
• Quality control for low-carbon and stabilized stainless steel grades, including verification that delivery heat treatment has fully dissolved carbides.
• Failure analysis of components that cracked in service, distinguishing intergranular corrosion from stress corrosion cracking or other mechanisms.
• Research and development programs for new alloys, heat treatments, and welding consumables.

FAQ

What is intergranular corrosion, and why is it dangerous?

Intergranular corrosion is localized attack concentrated at grain boundaries that leaves the bulk grains largely untouched. Because the boundaries provide structural cohesion, selective dissolution of this network causes grain dropping, loss of load-bearing cross section, and brittle-appearing cracking, often with minimal external evidence before failure. Metallographic examination is therefore essential for detecting it before components enter service or after a suspected service incident.

What causes sensitization in stainless steel?

Sensitization occurs when a chromium-bearing steel is exposed to approximately 425 °C to 815 °C long enough for chromium carbides to precipitate at grain boundaries, depleting nearby chromium below the level needed for passivity. Welding, slow cooling through the critical range, and improper heat treatment are the most common causes. Remedies include using low-carbon (L) grades, stabilized grades containing titanium or niobium, controlled heat input during welding, and post-weld solution annealing.

What is the difference between the oxalic acid etch test and the boiling acid tests?

The oxalic acid etch test is a rapid electrolytic screening method that classifies the grain boundary microstructure as step, dual, or ditch structure within a short laboratory session; it is non-quantitative and cannot be used alone for rejection. The boiling acid tests are longer confirmatory immersions that produce measurable attack or weight loss, from which susceptibility is quantified through penetration depth, bend performance, or corrosion rate.

How are results evaluated and what are the acceptance criteria?

Screening results are judged by structure class, with ditch structures triggering confirmatory testing. Immersion test results are judged against the limits of the governing product specification, which may specify a maximum depth of intergranular penetration, an allowable corrosion rate, or the absence of cracks after bending. The laboratory reports measured depths, structure classifications, corrosion rates where applicable, and conformity statements supported by photomicrographs.

Why choose metallographic examination instead of a bend test?

Metallographic examination provides a quantitative, permanent record of attack depth, works on thick, welded, or geometrically complex sections that cannot be bent, and eliminates ambiguity between intergranular attack and mechanical cracking. Most international standards, including ISO 3651, formally permit metallographic cross-section evaluation as an acceptance route equivalent to bend testing.

Can welded joints and finished components be tested?

Yes. Full weld cross sections are routinely mounted so that weld metal, fusion line, heat-affected zone, and base metal are examined together, allowing the sensitized zone to be located and measured. Samples can also be taken from finished components, tube spools, and failed parts, provided sampling does not compromise component integrity.

How long does the examination take?

The oxalic acid etch screening examination is typically completed within one working day. Confirmatory boiling tests require the full exposure duration defined by the standard, for example approximately 15 to 24 hours for Strauss-type tests, about 120 hours for ferric sulfate–sulfuric acid tests, and 240 hours in five periods for the Huey test, followed by metallography and reporting. Overall turnaround is usually within one to two weeks depending on the specified practice.

Which standards should be specified for a project?

The choice depends on the alloy system and the contracting specification. ASTM A262 or ISO 3651 or GB/T 4334 govern stainless steels, ASTM A763 covers ferritic grades, ASTM G28 applies to nickel alloys, and ASTM G110 covers aluminum alloys. Because the various practices within each standard detect different forms of sensitization, laboratories should be consulted when translating between ASTM, ISO, GB, and JIS requirements to confirm technical equivalence for the grade and service environment involved.