Intergranular corrosion (IGC) testing evaluates the resistance of stainless steels, nickel alloys, and other metals to localized attack along grain boundaries caused by sensitization during welding, heat treatment, or service exposure. Accredited third-party laboratories perform IGC testing in accordance with internationally recognized standards such as ASTM A262, ASTM G28, ASTM A763, ISO 3651, and GB/T 4334 to verify material quality and prevent sudden structural failure. This guide explains the corrosion mechanism, applicable sample types, test practices, evaluation criteria, and typical industrial applications.
Technical Overview
Intergranular corrosion is a localized form of degradation that 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 range of approximately 425°C to 815°C, chromium carbides (Cr23C6) precipitate at the grain boundaries and deplete the surrounding zones of chromium. Because a minimum chromium content of about 10.5% is required to maintain the passive film, these chromium-depleted regions become preferential anodes and dissolve rapidly in corrosive media.
Similar damage can result from the precipitation of intermetallic phases such as sigma phase, chi phase, or chromium nitrides, which also deplete grain-boundary regions of corrosion-resistant alloying elements. In welded components, sensitization appears as weld decay in the heat-affected zone or as knife-line attack in stabilized grades. The danger of intergranular corrosion lies in its insidious nature: components may retain a normal appearance while losing cohesion between grains, resulting in grain dropping, severe loss of strength, and brittle fracture without visible warning.
Intergranular corrosion testing is therefore essential for material qualification, heat treatment verification, weld procedure qualification, and failure analysis. Accelerated laboratory tests use aggressive electrolytes, elevated boiling temperatures, and carefully controlled exposure durations to reveal susceptibility within days rather than years of service.
Testing Samples
Intergranular corrosion tests can be performed on a wide range of alloys and product forms. Specimens are typically cut from production material with full-thickness sections so that the tested surface represents the actual microstructure, including any sensitized zones.
Austenitic Stainless Steels
Grades such as 304, 304L, 316, 316L, 321, and 347 are the most frequently tested materials because of their widespread use and known susceptibility to chromium carbide precipitation. Both solution-annealed and sensitized conditions can be evaluated, and extra-low-carbon or stabilized grades are often tested to confirm their immunity.
Ferritic and Martensitic Stainless Steels
Ferritic grades such as 430, 439, 444, and 446 can sensitize through chromium carbide and nitride precipitation. Testing follows ASTM A763 or the corresponding ISO 3651 procedures designed specifically for ferritic microstructures.
Duplex Stainless Steels
Duplex grades such as 2205 and 2507 are tested to confirm proper phase balance and absence of detrimental intermetallic phases formed during fabrication or incorrect heat treatment.
Nickel-Based Alloys
Alloys such as Alloy 600, 625, 800, 825, and C-276 are evaluated under ASTM G28 to detect sensitization caused by carbide or intermetallic precipitation, particularly after thermal processing.
Welded Joints and Finished Components
Specimens containing welds, heat-affected zones, bent or formed sections, and finished parts such as pipes, tubes, plates, bars, forgings, and castings can all be tested. Surface preparation, including degreasing, pickling, and passivation or fine grinding, is controlled to ensure reproducible results.
Testing Items
Depending on the selected standard and client requirements, the following test items are commonly included in an intergranular corrosion testing program:
- Determination of susceptibility to intergranular corrosion (pass or fail evaluation)
- Corrosion rate calculation based on specimen weight loss, reported in mm/y or mpy
- Bend test evaluation after boiling acid exposure to reveal intergranular cracking
- Metallographic examination of etch structures and measurement of attack depth
- Quantitative assessment of the degree of sensitization using electrochemical reactivation methods
- Verification of stabilizing or solution-annealing heat treatments
- Comparison of base metal, weld metal, and heat-affected zone susceptibility
Testing Methods
Several standardized test practices exist, each with a distinct electrolyte, exposure condition, and evaluation criterion. The choice depends on the alloy type, intended service environment, and the governing specification or purchase order.
ASTM A262 Practice A: Oxalic Acid Etch Test
A rapid screening method in which specimens are electrolytically etched in a 10% oxalic acid solution at 1 A/cm² for approximately 90 seconds. The etched microstructure is examined under a microscope and classified as step, dual, or ditch structure. Step and dual structures indicate acceptable resistance, while a ditch structure requires confirmatory testing by another practice.
ASTM A262 Practice B: Ferric Sulfate–Sulfuric Acid Test
Also known as the Streicher test, specimens are immersed in a boiling 50% sulfuric acid solution containing ferric sulfate for 120 hours. Susceptibility is determined by calculating the corrosion rate from weight loss. This practice is suitable for most austenitic grades, including chromium-bearing grades without molybdenum.
ASTM A262 Practice C: Nitric Acid Test
The Huey test exposes specimens to boiling 65% nitric acid for five consecutive 48-hour periods, with fresh solution each period. Weight loss is measured after each period to calculate corrosion rates. This severe test is particularly effective at detecting sigma phase and other chromium-rich precipitates, making it a standard requirement in nuclear-grade material specifications.
ASTM A262 Practice E: Copper–Copper Sulfate–16% Sulfuric Acid Test
The Strauss test immerses specimens in boiling 16% sulfuric acid with copper sulfate while maintaining contact with metallic copper turnings for 24 hours. After exposure, specimens are bent 180 degrees over a suitable mandrel and examined for intergranular fissures and cracking. It is the most widely specified acceptance test for stainless steel components.
ASTM A262 Practice F: Copper–Copper Sulfate–50% Sulfuric Acid Test
A more aggressive variant of the Strauss test developed for molybdenum-bearing austenitic stainless steels such as 316 and 317. The higher sulfuric acid concentration and 120-hour exposure provide a more discriminating evaluation for these grades.
ASTM G28 for Nickel-Based Alloys
Method A uses boiling ferric sulfate–50% sulfuric acid for 120 hours, while Method B uses a mixed acid of ferric sulfate with hydrofluoric and nitric acids for 24 hours. Corrosion rates calculated from weight loss determine susceptibility to intergranular attack in high-alloy materials.
ASTM A763 for Ferritic Stainless Steels
Practices W, X, Y, and Z employ different electrolytes, including ferric sulfate–sulfuric acid and copper-bearing solutions, to evaluate ferritic grades across a range of sensitization severities, with evaluation by weight loss and bend testing.
ISO 3651 and GB/T 4334 Series
ISO 3651-1 and ISO 3651-2 specify intergranular corrosion tests for ferritic, austenitic, and duplex stainless steels using sulfuric acid–copper sulfate and nitric acid–hydrofluoric acid media. The GB/T 4334 series provides equivalent Chinese national test methods, with methods designated A through E corresponding to different acid solutions and evaluation procedures.
Electrochemical Methods
Double-loop electrochemical potentiokinetic reactivation (DL-EPR), as described in ASTM G108 and related procedures, provides a quantitative measure of the degree of sensitization in a short time, making it valuable for research, screening, and life assessment studies.
Applications
Intergranular corrosion testing supports material acceptance and quality control across many safety-critical industries:
- Chemical and petrochemical plants: reactors, heat exchangers, columns, and piping exposed to acidic process streams
- Oil and gas: offshore platforms, subsea components, refinery equipment, and sour service piping
- Nuclear power: primary circuit piping, reactor internals, and steam generator tubing requiring Huey test qualification
- Pharmaceutical and food processing: sanitary tubing, vessels, and fittings where product purity and corrosion integrity are essential
- Pulp and paper: digesters, bleach plant equipment, and liquor handling systems
- Pressure vessels and boilers: verification of heat treatment compliance per ASME and EN design codes
- Aerospace: evaluation of sensitization in heat-treatable aluminum alloys and stainless steel fasteners
- Failure analysis: confirmation of intergranular attack as a root cause of in-service cracking
FAQ
What is the difference between the Huey test and the Strauss test?
The Huey test (ASTM A262 Practice C) uses boiling 65% nitric acid for five 48-hour cycles and measures weight loss, making it highly sensitive to sigma phase and chromium-depleted regions. The Strauss test (ASTM A262 Practice E) uses boiling copper–copper sulfate–16% sulfuric acid for 24 hours and is evaluated by a 180-degree bend test. The Huey test is more severe and is typically required for nuclear applications, while the Strauss test is the most common general acceptance test.
How should I select the appropriate test standard?
Selection depends on the alloy type, product specification, and service environment. Austenitic stainless steels are usually tested per ASTM A262 or GB/T 4334, nickel alloys per ASTM G28, and ferritic grades per ASTM A763 or ISO 3651. The governing purchase specification or design code generally dictates the required practice and acceptance criteria.
Can welded samples be tested?
Yes. Cross-section specimens containing base metal, weld metal, and the heat-affected zone are commonly tested to evaluate weld decay and knife-line attack. The specimen orientation is selected so that the sensitized zones are fully exposed to the test solution.
What is the typical test duration?
The oxalic acid screening test takes less than one hour of laboratory exposure plus metallographic evaluation. The Strauss test requires a 24-hour boil, the Streicher and G28 tests require 120 hours, and the Huey test requires a total of 240 hours across five solution-change cycles. Including specimen preparation, metallography, and reporting, most projects are completed within one to two weeks.
What are the common acceptance criteria?
Acceptance criteria vary by specification. For weight-loss-based methods, corrosion rates must fall below specified maximum values, for example in the range of a few tenths of a millimeter per year depending on the practice and grade. For bend-test methods, the bent specimen must show no intergranular cracks or fissures at a specified magnification. Metallographic criteria limit the measured depth of intergranular attack.
How can intergranular corrosion be prevented in practice?
Prevention measures include using extra-low-carbon grades such as 304L and 316L, selecting stabilized grades containing titanium or niobium such as 321 and 347, applying proper solution annealing followed by rapid cooling, controlling weld heat input, and using duplex or nickel alloys in severely corrosive environments. Routine intergranular corrosion testing confirms that these measures have been effective.