Simulated intergranular corrosion (IGC) testing is an accelerated laboratory method used to determine the susceptibility of stainless steels, nickel alloys, and aluminum alloys to grain-boundary attack caused by sensitization. Conducted in accordance with internationally recognized standards such as ASTM A262, ASTM G28, and ISO 3651, these tests provide essential data for material qualification, welding procedure validation, and failure prevention in petrochemical, nuclear, marine, and chemical processing industries. This article explains the underlying metallurgical mechanisms, applicable sample types, standard test practices, evaluation criteria, and typical industrial applications of IGC simulation testing.
Technical Overview
Intergranular corrosion (IGC) is a localized form of attack that proceeds along the grain boundaries of a metal while the grains themselves remain largely unaffected. In chromium-bearing alloys such as austenitic stainless steel, the most common cause is sensitization: when the material is heated in the range of approximately 450 to 850 degrees Celsius, for example during welding, slow cooling after solution annealing, or stress-relieving heat treatment, chromium carbides precipitate at the grain boundaries and deplete the surrounding zones of chromium. Because passive-film stability requires a chromium content above roughly 12 percent, the depleted zones become anodic relative to the grain interiors and dissolve preferentially in corrosive media.
Similar grain-boundary phenomena occur in ferritic and duplex stainless steels, in nickel-based alloys (chromium carbides, sigma phase, and molybdenum-rich precipitates), and in 5xxx-series aluminum alloys (continuous beta-phase precipitation along grain boundaries). The resulting damage, including grain dropping, loss of cohesion, and cracking under stress, can severely degrade load-bearing capacity with little visible evidence on the exposed surface.
Simulated IGC testing reproduces and accelerates this damage in the laboratory. Coupons are first subjected, where required, to a controlled sensitization heat treatment that simulates worst-case thermal excursions, and then immersed in boiling corrosive media specified by recognized standards. Damage that might take years to develop in service is generated within 24 to 240 hours, allowing susceptibility to be quantified by mass-loss corrosion rate, bend ductility, or metallographic measurement of attack depth.
Testing Samples
Austenitic Stainless Steels
The most frequently tested materials include grades 304, 304L, 316, 316L, 317L, 321, 347, and 310, together with their cast and low-carbon variants as well as super-austenitic grades. Both solution-annealed and deliberately sensitized conditions can be evaluated.
Duplex and Ferritic Stainless Steels
Duplex grades such as 2205 and 2507, and ferritic grades such as 430, 439, and 444, are tested to verify resistance to chromium carbide, chromium nitride, and sigma-phase precipitation, particularly after welding or improper heat treatment.
Nickel-Based Alloys
Chromium-bearing nickel alloys such as Alloy 600, Alloy 625, Alloy 800/800H, and Alloy 825 are evaluated for grain-boundary attack in ferric sulfate-sulfuric acid and mixed acid-oxidizing salt media.
Aluminum Alloys
Aluminum-magnesium alloys of the 5xxx series, such as 5052, 5083, and 5456, especially in sensitized tempers after prolonged heat exposure, are assessed for intergranular susceptibility using nitric acid mass-loss testing.
Product Forms and Specimen Preparation
- Plate, sheet, and strip coupons with a defined surface area
- Pipe and tube segments, both seamless and welded, including the full weld cross-section
- Bars, forgings, castings, and weld overlay deposits
- Welded joints that incorporate base metal, heat-affected zone, and weld metal in a single coupon
- Coupons ground to a uniform finish, degreased, dried, and weighed to 0.001 g before exposure
- Sensitization heat treatment, commonly 650 to 700 degrees Celsius for 30 minutes to 2 hours followed by controlled cooling, applied when specified by the governing product specification or purchase order
Testing Items
Depending on the alloy system, product specification, and intended service environment, a simulated IGC test program typically covers the following evaluation items:
- Susceptibility screening by electrolytic oxalic acid etch with microstructural classification into step, dual, or ditch structures
- Corrosion-rate determination in boiling ferric sulfate-50% sulfuric acid solution
- Corrosion-rate determination in boiling 65% nitric acid (Huey test) over five consecutive 48-hour periods
- Bend-test evaluation after exposure in boiling copper-copper sulfate-sulfuric acid media (Strauss-type tests)
- Mixed acid-oxidizing salt immersion for highly alloyed nickel-based materials
- Nitric acid mass-loss testing of 5xxx-series aluminum alloys
- Post-exposure metallographic examination to measure attack depth and confirm grain dropping
- Calculation of corrosion rates and pass/fail assessment against grade-specific acceptance criteria
Testing Methods
Simulated IGC testing is performed using well-established standardized practices, most of which combine a defined corrosive medium, a fixed exposure duration, and a prescribed evaluation procedure.
Oxalic Acid Etch Screening (ASTM A262 Practice A, GB/T 4334 Method A)
Coupons are electrolytically etched at room temperature in 10% oxalic acid, typically at about 1 A/cm2 for roughly 90 seconds, and the microstructure is classified as step, dual, or ditch. Step and dual structures qualify certain grades as resistant without further testing, whereas a ditch structure mandates a full immersion test. This rapid screen is widely used for incoming material inspection.
Ferric Sulfate-Sulfuric Acid Test (ASTM A262 Practice B, ASTM G28 Method A)
Coupons are immersed for 120 hours (24 hours for many nickel alloys) in boiling ferric sulfate-50% sulfuric acid, a strongly oxidizing solution that selectively attacks chromium-depleted zones. Susceptibility is quantified by the mass-loss corrosion rate, which is compared with grade-specific limits.
Nitric Acid Test (ASTM A262 Practice C, Huey Test)
Specimens are exposed to boiling 65% nitric acid for five consecutive 48-hour periods, with fresh solution introduced each period. The test detects not only chromium carbide precipitation but also sigma phase and other nitric-acid-sensitive phases; acceptance is based on the average and maximum corrosion rates across the periods.
Copper-Copper Sulfate-Sulfuric Acid Test (ASTM A262 Practices E and F, ISO 3651-2)
Coupons are boiled for 24 hours in 16% sulfuric acid containing copper sulfate while in metallic contact with copper shot (Practice E, the Strauss test), or for 120 hours in the 50% sulfuric acid version (Practice F) developed for molybdenum-bearing grades. After exposure, specimens are bent 180 degrees over a mandrel of specified diameter and examined at 10 to 20 times magnification; fissures or cracks perpendicular to the bend axis indicate intergranular attack.
Ferritic Steel and Nickel Alloy Testing (ASTM A763, ASTM G28 Method B)
ASTM A763 Practices W, X, Y, and Z apply ferric sulfate-sulfuric acid, copper-copper sulfate-sulfuric acid, and nitric acid media to ferritic stainless steels. ASTM G28 Method B employs a boiling mixed sulfuric-hydrochloric acid solution containing ferric and cupric chlorides, which is suitable for highly alloyed nickel materials that show adequate resistance in Method A.
International and National Equivalents
GB/T 4334 Methods A through E mirror the oxalic acid etch, ferric sulfate-sulfuric acid, nitric acid, and copper-copper sulfate-sulfuric acid practices for stainless steels, while GB/T 15260 covers nickel alloys and GB/T 7998 covers aluminum alloys. ISO 3651-1 and ISO 3651-2 specify nitric acid and sulfuric-acid-based tests, together with sensitization treatments and acceptance criteria aligned with European material standards.
Aluminum Alloy Testing (ASTM G67)
The nitric acid mass-loss test immerses aluminum-magnesium alloy coupons in concentrated nitric acid at 30 degrees Celsius for 24 hours. The mass loss per unit area, expressed in mg/cm2, correlates with the continuity of beta-phase grain-boundary precipitation and the likelihood of intergranular or exfoliation attack in marine service.
Evaluation and Acceptance Criteria
- Corrosion rate calculated from mass loss, exposed surface area, alloy density, and exposure time, compared with grade-specific limits in the product specification
- 180-degree bend test with visual and low-magnification inspection for intergranular fissures
- Metallographic cross-sections to measure maximum attack depth and confirm the intergranular character of the damage
- Electrolytic etch structure classification for rapid screening purposes
Applications
Simulated IGC testing supports material selection, product acceptance, and integrity management across a wide range of industries:
- Petrochemical and chemical processing: qualification of reactors, heat exchangers, columns, and piping exposed to hot acids and process chemicals
- Nuclear power: Huey-test qualification of steam generator tubing, reactor internals, and reprocessing equipment
- Oil and gas: offshore platforms, subsea components, and clad plates, including sour-service material qualification
- Pharmaceutical, food, and beverage: sanitary tubing, vessels, and weldments requiring high corrosion margins
- Pulp and paper: bleach plant washers, digesters, and piping in chloride- and chlorine-bearing media
- Marine and shipbuilding: sensitized aluminum-magnesium superstructures and hull structures
- Aerospace: high-strength aluminum structural components subject to long-term thermal exposure
- Welding procedure qualification and detection of heat-affected-zone sensitization in critical fabrications
- Supplier quality assurance, incoming inspection, failure analysis, and research into stabilized or low-carbon alloy grades
FAQ
What is the difference between sensitization and intergranular corrosion?
Sensitization is the metallurgical condition, namely chromium carbide precipitation and chromium depletion at grain boundaries, produced by thermal exposure. Intergranular corrosion is the resulting material degradation that occurs when a sensitized alloy is attacked by a corrosive environment along those weakened boundaries.
How do I choose the right test method?
Selection depends on the alloy system and the intended service environment. Oxalic etch screening is used for rapid sorting, the Strauss test simulates mildly oxidizing environments, the Huey test is applied where nitric acid service or sigma-phase sensitivity is a concern, and ferric sulfate-sulfuric acid testing suits general qualification. The governing material specification normally prescribes the required practice and acceptance limits.
Why is a sensitization heat treatment applied before testing?
For low-carbon or stabilized grades delivered in a resistant condition, a sensitizing treatment deliberately reproduces the most damaging thermal excursion the material might experience during fabrication or service. Testing after such treatment demonstrates intrinsic resistance rather than merely the as-delivered condition.
How are results judged, by mass loss or by cracking?
Both approaches are used. Mass-loss-based practices report a corrosion rate that must fall below a grade-specific limit, while bend-based practices require the specimen to survive a 180-degree bend without fissures. Metallographic attack-depth measurement provides supplementary confirmation when results are marginal.
How long does a simulated IGC test take?
Oxalic acid screening takes less than an hour once a specimen is prepared. Full immersion tests range from 24 hours (Strauss and G28 tests) through 120 hours (Practices B and F) to 240 hours for the five-period Huey test, plus specimen preparation, sensitization treatment where required, and post-test evaluation, so the total laboratory time is typically a few days to two weeks.
Can welded joints be tested?
Yes. Coupons are cut so that base metal, heat-affected zone, and weld metal are all exposed to the test solution, and bend tests are oriented across the weld. This is standard practice for welding procedure qualification and for detecting heat-affected-zone sensitization in service-critical fabrications.
Where should simulated IGC testing be performed?
Testing should be carried out in accredited third-party laboratories or qualified in-house facilities equipped with controlled boiling solutions, calibrated balances, certified reagents, and metallographic capability, ensuring that results are traceable to the referenced standards and acceptable to customers, classification societies, and regulators.