Intergranular corrosion macro examination is a standardized post-corrosion evaluation used to assess the susceptibility of stainless steels, nickel alloys, and aluminum alloys to grain-boundary attack. By combining boiling acid immersion tests with macroscopic evaluation techniques such as bend testing, sound testing, visual inspection, and mass-loss measurement, this method provides a reliable, cost-effective quality control screen for sensitized materials. This article explains the technical principles, applicable standards, sample requirements, testing procedures, and acceptance criteria of intergranular corrosion macro examination.
Technical Overview
Intergranular corrosion (IGC) is a localized form of corrosion that preferentially attacks the grain boundaries of a metal while leaving the grain interiors largely unaffected. In austenitic stainless steels, the most common cause is sensitization: when the material is heated within the critical temperature range of approximately 425 to 815 degrees Celsius, chromium carbides precipitate at grain boundaries, depleting the adjacent zones of chromium and destroying the passive film locally. Similar grain-boundary phenomena occur in ferritic stainless steels, nickel-based alloys, and magnesium-containing aluminum alloys.
Intergranular corrosion macro examination refers to the macroscopic evaluation stage that follows a standardized corrosion immersion test. After the specimen has been exposed to a boiling acidic medium, its condition is judged by methods that do not require high-magnification microscopy, principally the bend test, the sound (ringing) test, visual examination for grain dropping, and measurement of mass loss. These techniques reveal whether the grain boundaries have been attacked severely enough to compromise mechanical integrity, which is the practical engineering concern in pressure vessels, piping, and welded structures.
The advantage of macro examination is speed and simplicity: a specimen that fails the bend test or has lost its metallic ring is clearly unacceptable, and the result directly reflects service risk. Where results are ambiguous, metallographic (microscopic) verification may be performed as a supplementary step, but the macro examination remains the primary acceptance criterion in most product specifications.
Testing Samples
Intergranular corrosion macro examination is applicable to a wide range of corrosion-resistant alloys and product forms. Specimens are normally taken from the delivered product condition or from sensitized test coupons, and welded joints are frequently included because the heat-affected zone is where sensitization is most likely to occur.
Austenitic Stainless Steels
Grades such as 304, 304L, 316, 316L, 317, 321, and 347 in the form of plate, sheet, strip, bar, wire, pipe, tube, forgings, and castings. Stabilized and low-carbon grades are tested to verify that sensitization during welding or heat treatment has been avoided.
Ferritic and Duplex Stainless Steels
Ferritic grades such as 430, 444, and 446, and duplex grades such as 2205 and 2507, which require modified test media and longer exposure times because of their different sensitization mechanisms.
Nickel-Based Alloys
Chromium-bearing wrought nickel alloys such as alloy 600, 625, 800, 825, and C-276, evaluated for grain-boundary precipitation of chromium carbides and sigma phase.
Aluminum Alloys
Magnesium-containing 5XXX series alloys and certain 2XXX and 7XXX series alloys, tested for intergranular or exfoliation susceptibility after improper thermal treatment.
Weldments and Heat-Treated Coupons
Full-thickness cross-weld specimens, heat-affected zone coupons, and laboratory-sensitized samples used for welding procedure qualification, material certification, and research studies.
Testing Items
Typical test items evaluated during intergranular corrosion macro examination include:
- Bend test assessment: detection of cracks or fissures on the tension surface after bending the corroded specimen through a specified angle, typically 180 degrees.
- Visual and macroscopic surface examination: identification of grain dropping, rough or matte surfaces, and localized boundary attack visible to the unaided eye or at low magnification.
- Sound (ringing) test: evaluation of the acoustic response of the specimen after impact, where a dull sound indicates severe intergranular attack and loss of cohesion between grains.
- Mass loss measurement: determination of the corrosion rate from the weight loss per unit area after exposure, used mainly for nitric acid and ferric sulfate type tests.
- Weld and heat-affected zone evaluation: assessment of whether sensitization has occurred in welded or thermally affected regions of the specimen.
- Supplementary cross-sectional verification: low-magnification or metallographic confirmation when bend test results are ambiguous or when surface defects must be distinguished from genuine intergranular cracks.
Testing Methods
Governing Standards
The most widely applied standards include ASTM A262 for austenitic stainless steels, ASTM A763 for ferritic stainless steels, ASTM G28 for wrought nickel-rich chromium-bearing alloys, and ASTM G67 for 5XXX series aluminum alloys. International and regional equivalents include ISO 3651-1 and ISO 3651-2, the GB/T 4334 series, and the JIS G 0571 to G 0575 series. Accredited third-party laboratories select the specific practice according to the alloy, product form, and specification referenced by the purchaser.
Specimen Preparation
Specimens are cut to the dimensions prescribed by the relevant standard, commonly in the range of 80 to 100 mm in length and about 20 mm in width, with thickness up to the maximum permitted for bending. Test surfaces are ground to a uniform finish, typically 120 grit or finer, sharp edges are rounded, and the specimens are degreased, rinsed, dried, and weighed to the required precision before exposure.
Boiling Acid Immersion Tests
The specimen is immersed fully in the boiling test solution for the specified duration. Common practices include:
- Copper-copper sulfate-16 percent sulfuric acid test (Strauss type, ASTM A262 Practice E and ISO 3651 method A): boiling exposure for 24 hours, or 20 hours in ISO 3651, with the specimen in metallic contact with copper shot or turnings to accelerate chromium-depleted boundary attack.
- Nitric acid test (Huey type, ASTM A262 Practice C): exposure to boiling 65 percent nitric acid in five consecutive 48-hour periods, evaluated by corrosion rate calculated from mass loss.
- Ferric sulfate-sulfuric acid test (ASTM A262 Practice B, ASTM G28 Method A): a 120-hour boiling exposure for stainless steels or a 24-hour exposure for nickel alloys, assessed by mass loss.
- Stronger sulfuric acid variants (ASTM A262 Practice F and ASTM A763 practices): used for molybdenum-bearing and ferritic grades that are less reactive in the standard media.
Bend Test Evaluation
After immersion, the specimen is bent through 180 degrees, or through the angle specified by the product specification, around a mandrel of appropriate diameter, typically related to the specimen thickness. The convex tension surface is then examined with the unaided eye and at low magnification, commonly about 10x. The presence of cracks or fissures perpendicular to the bending direction indicates intergranular attack. Isolated scratches, edge tearing, or surface laps that do not follow the bend pattern must not be mistaken for corrosion cracks; in doubtful cases, a cross section is prepared for metallographic confirmation.
Sound Test and Visual Judgment
For heavily attacked specimens, a simple sound test may be applied: the specimen is struck or dropped from a fixed height onto a hard surface. A clear metallic ring indicates retained grain cohesion, whereas a dull thud indicates that intergranular corrosion has progressed to the point of grain separation. Visual examination also identifies grain dropping, in which individual grains fall out and leave a rough, pitted, or sugary appearance on the surface.
Acceptance Criteria
Acceptance is normally defined by the referenced specification. Typical criteria require that the specimen show no intergranular cracks after the specified bend, that the calculated corrosion rate not exceed the limit agreed between manufacturer and purchaser, and that no visible grain dropping be present. Materials failing the macro examination are rejected, subjected to corrective heat treatment such as solution annealing and retested, or verified by metallographic means where the result is borderline.
Applications
Intergranular corrosion macro examination is used throughout industry wherever sensitization could compromise equipment integrity:
- Chemical and petrochemical plants: qualification of stainless steel pressure vessels, reactors, heat exchangers, and piping exposed to corrosive media.
- Oil and gas industry: acceptance testing of corrosion-resistant alloy tubing, casings, claddings, and weld overlays in sour and acidic service.
- Nuclear power generation: evaluation of reactor internals, primary circuit piping, and welded components where grain-boundary attack is a safety-relevant degradation mechanism.
- Food, beverage, and pharmaceutical equipment: verification that welded austenitic stainless steel surfaces remain free of sensitization for hygiene and corrosion resistance.
- Shipbuilding and marine engineering: testing of duplex and super duplex stainless steel structural and piping systems exposed to seawater.
- Welding procedure and welder qualification: confirmation that selected filler metals, heat input, and post-weld treatments avoid heat-affected zone sensitization.
- Incoming material inspection and product certification: routine quality control of mill products supplied to ASTM, EN, GB, or JIS specifications.
- Research and failure analysis: study of sensitization kinetics, evaluation of stabilizing elements, and root-cause investigation of service failures.
FAQ
What is the difference between macro examination and micro examination in intergranular corrosion testing?
Macro examination evaluates the specimen after the corrosion test by methods such as bending, sound testing, visual inspection, and mass loss, without high-magnification microscopy. Micro examination, such as the oxalic acid etch screening test, requires polished and etched metallographic specimens examined under a microscope to classify etch structures. Macro examination is the usual acceptance method; micro methods are used for screening or for confirming ambiguous macro results.
How long does a typical test take?
The immersion itself commonly takes 24 hours for the copper-copper sulfate-sulfuric acid test, 120 hours for the ferric sulfate-sulfuric acid test on stainless steels, and 240 hours for the five-period nitric acid test. Including specimen preparation, bend testing, evaluation, and reporting, the complete procedure at an accredited third-party laboratory typically requires several working days, and longer for multi-period tests.
Can a bend test crack always be attributed to intergranular corrosion?
No. Surface defects such as laps, rolling seams, scratches, or weld discontinuities can also produce cracks during bending. Standards require that only cracks characteristic of intergranular attack, typically fissures distributed across the bent surface, be counted as failures. When doubt exists, a cross section of the bent region is examined metallographically to confirm whether grain boundaries were actually attacked.
Which standard should be selected for a given material?
Selection depends on the alloy system and the purchasing specification. Austenitic stainless steels are usually tested to ASTM A262 or ISO 3651, ferritic stainless steels to ASTM A763, nickel alloys to ASTM G28, and 5XXX aluminum alloys to ASTM G67. The specific practice within each standard is chosen based on the grade, the expected sensitization mechanism, and the severity of the intended service environment.
Can a failed material be retested or rescued?
In many cases yes. If sensitization is caused by improper heat treatment, a full solution anneal can redissolve chromium carbides, after which the material can be resampled and retested. However, materials with intrinsic metallurgical defects or excessive grain-boundary precipitation from repeated welding may remain unacceptable, and the applicable specification governs whether retesting is permitted.