Intergranular corrosion (IGC) severity assessment is a specialized materials testing process that quantifies how susceptible a metal—typically stainless steel, nickel alloy, or aluminum alloy—is to preferential attack along its grain boundaries caused by sensitization during welding, heat treatment, or high-temperature service. Accredited third-party laboratories perform these evaluations using internationally recognized standards such as ASTM A262, ASTM G28, ASTM G67, ISO 3651, JIS G 0571–0575, and GB/T 4334, providing objective pass/fail conclusions and corrosion-rate data for material acceptance and failure analysis. This guide explains the testing samples, evaluated parameters, standardized methods, industrial applications, and frequently asked questions related to intergranular corrosion assessment.
Technical Overview
Intergranular corrosion (IGC) is a localized degradation mechanism in which corrosive attack concentrates along the grain boundaries of a polycrystalline metal while the grain interiors remain largely intact. Because grain boundaries often carry precipitates, segregation, and strained lattice regions with different electrochemical behavior from the matrix, they can dissolve preferentially in aggressive media, causing grains to detach from the surface, a phenomenon known as grain dropping.
The most common cause is sensitization of austenitic stainless steels. When these alloys are heated within approximately 425 to 815 °C—for example, in the heat-affected zone of a weld, during slow cooling after annealing, or through prolonged service exposure—chromium-rich carbides (M23C6) precipitate at grain boundaries and deplete the adjacent zones of chromium below the level required to maintain a stable passive film. Similar sensitization can result from sigma-phase precipitation, chromium nitride formation in ferritic and duplex grades, or galvanic micro-cells between boundary precipitates and the matrix in certain aluminum alloys.
The danger of IGC lies in its stealth and severity: components may show little external evidence of damage while losing internal cohesion, cross-sectional strength, and ductility, ultimately failing suddenly in a brittle manner. A rigorous severity assessment therefore combines immersion corrosion testing, weight-loss measurement, bend-ductility evaluation, and metallographic examination to establish whether a material, weld, or heat treatment meets specification requirements.
- Determination of sensitization susceptibility and microstructural classification
- Quantification of corrosion rate by gravimetric weight loss
- Evaluation of bend ductility and intergranular cracking after exposure
- Measurement of maximum intergranular penetration depth by cross-sectional microscopy
- Assessment of grain dropping and boundary ditching severity
- Comparison of results against product specification acceptance limits
Testing Samples
Austenitic Stainless Steels
Commonly assessed grades include 304, 304L, 316, 316L, 321, 347, 310, 904L, and 6Mo super-austenitic alloys. Samples are prepared from plate, sheet, strip, pipe and tube, bar, forgings, castings, and fasteners, as well as from welded joints containing the weld metal, fusion line, and heat-affected zone.
Ferritic, Martensitic, and Duplex Stainless Steels
Ferritic grades such as 409, 430, and 444, together with duplex grades such as S32205 and S32750, are evaluated under practices such as ASTM A763 (Practices W, X, Y, Z) and ISO 3651-2. Where acceptance specifications require worst-case evaluation, coupons may first be given an artificial sensitizing heat treatment, for example at 650 °C for a defined hold time.
Nickel-Based and Corrosion-Resistant Alloys
Alloys such as Alloy 600, 625, 800H, 825, and C-276 are tested according to ASTM G28 Method A (ferric sulfate–sulfuric acid) or Method B (mixed nitric–hydrofluoric acid), which target sensitization from carbides, intermetallic phases, or grain-boundary segregation characteristic of high-alloy systems.
Aluminum Alloys
Sensitized 5xxx-series alloys (such as 5083, 5059, and 5456) used in marine structures are evaluated with the ASTM G67 nitric acid mass loss test (NAMLT), which measures the extent of grain-boundary beta-phase (Mg2Al3) precipitation. Etch screening of 2xxx and 7xxx alloys can be performed per ASTM G110.
Specimen Preparation Requirements
Coupons are cut to the dimensions specified by the applicable standard, commonly around 30 mm × 20 mm or sized from the actual product section. Surfaces are ground to a uniform finish, typically 120 to 600 grit, marked for identification, degreased and dried, then weighed to 0.001 g with the exposed area and material density recorded for corrosion-rate calculation.
Testing Items
Intergranular corrosion severity assessment typically covers the following measurable items, each of which is reported against the acceptance criteria of the governing material specification:
- Sensitization susceptibility classification (step, dual, or ditch structure)
- Corrosion rate by weight loss, expressed in µm/48 h, mdd, mm/y, or mpy
- Bend test result (90° or 180° bend) with evaluation for intergranular fissuring
- Maximum depth of intergranular attack measured on a polished cross-section
- Presence and extent of grain dropping or grain-boundary ditching
- NAMLT mass-loss rating (mg/cm²) for 5xxx aluminum alloys
- Corrosion-rate ratio between sensitized and annealed companion specimens for mixed-acid tests
- Conclusion of pass or fail against ASTM, ISO, JIS, EN, or GB specification limits
Testing Methods
Oxalic Acid Etch Screening Test (ASTM A262 Practice A; GB/T 4334.1; JIS G 0571)
A small coupon is electrolytically etched in 10% oxalic acid at a current density of approximately 1 A/cm² for about 90 seconds. Under optical microscopy at 250× to 500×, the etch structure is classified as a step structure (no continuous carbide network, acceptable), a dual structure (acceptable for certain practices), or a ditch structure (indicative of susceptibility, requiring a confirmatory immersion test). The method is rapid and economical, making it ideal for screening large production lots.
Ferric Sulfate–Sulfuric Acid Test (ASTM A262 Practice B; ASTM G28; ISO 3651-2; GB/T 4334.2; JIS G 0572)
Specimens are exposed to a boiling 50% sulfuric acid solution containing ferric sulfate for 24 to 120 hours depending on the standard and alloy. The ferric ions maintain a strongly oxidizing potential that selectively attacks chromium-depleted boundaries. Severity is judged from the weight-loss corrosion rate and from metallographic evidence of intergranular penetration; ISO 3651-2, for example, sets a common acceptance limit of 1.5 µm per 48 hours for many austenitic and duplex grades.
Boiling Nitric Acid (Huey) Test (ASTM A262 Practice C; GB/T 4334.3; JIS G 0573)
Coupons undergo five consecutive 48-hour boiling periods in fresh 65% nitric acid, with the solution renewed after each period. This severe test reveals sensitization from carbides as well as from sigma and other nitric-attacked phases. A progressively increasing corrosion rate across the five periods is a diagnostic signature of advancing intergranular attack and grain dropping, and results are compared with maximum rate limits defined in the product specification.
Nitric–Hydrofluoric Acid Test (GB/T 4334.4)
Designed for molybdenum-bearing austenitic stainless steels such as 316 and 316L, this mixed-acid immersion at approximately 70 °C evaluates susceptibility through the ratio of corrosion rates between the as-delivered specimen and a laboratory-sensitized companion coupon. A ratio within the specified limit indicates acceptable resistance.
Copper–Copper Sulfate–Sulfuric Acid (Strauss) Test (ASTM A262 Practices E and F; ISO 3651-2; GB/T 4334.5; JIS G 0575)
Specimens are boiled in 16% sulfuric acid with copper sulfate while in metallic contact with copper turnings, which depress the solution potential and target chromium-depleted zones, typically for 15 to 72 hours per the governing standard. Practice F uses 50% sulfuric acid to provide more aggressive conditions for specific grades. After exposure, the coupon is bent 180° over a mandrel with a diameter related to the specimen thickness and examined at 10× to 20× magnification; intergranular cracks on the tension surface constitute failure.
Post-Exposure Evaluation and Severity Grading
Corrosion rates are calculated from the formula CR = (K × W)/(A × T × D), where W is mass loss, A is exposed area, T is exposure time, D is density, and K is a unit-conversion constant. Metallographic cross-sections are then examined to measure the maximum depth of intergranular penetration and to grade the degree of attack—commonly classified as absent, slight, moderate, or severe (with continuous grain dropping). For aluminum alloys, NAMLT mass-loss categories ranging from below 2 mg/cm² (not susceptible) to above 50 mg/cm² (severely susceptible) provide a recognized severity scale.
Applications
Intergranular corrosion severity assessment supports quality control, materials qualification, and root-cause failure investigation across many safety-critical industries:
- Chemical and petrochemical processing: reactors, columns, heat exchangers, and piping exposed to hot acids and chlorides
- Oil and gas: clad plate, downhole tubulars, umbilicals, and refinery equipment operating in sensitizing temperature ranges
- Nuclear power: primary circuit piping and components where neutron fluence and thermal aging can accelerate sensitization
- Pharmaceutical and food processing: hygienic tubing and vessels requiring verified resistance after fabrication welding
- Marine and shipbuilding: sensitized 5xxx aluminum hull and superstructure panels assessed by NAMLT ratings
- Aerospace: aluminum and specialty alloy structural elements with stringent toughness requirements
- Power generation: flue gas desulfurization systems and boiler components exposed to condensing acidic media
- Welding procedure qualification, incoming material inspection, and forensic analysis of service failures
FAQ
What is intergranular corrosion and why is it dangerous?
Intergranular corrosion is selective attack along grain boundaries that undermines the cohesion between grains. Because the damage develops beneath an apparently sound surface, affected components can lose strength and ductility without visible warning and may fail suddenly in a brittle manner, which is why quantitative assessment before service is essential.
What causes sensitization in stainless steels?
Sensitization occurs when chromium carbides precipitate at grain boundaries during exposure to roughly 425–815 °C, depleting the surrounding metal of chromium and destroying its passive film locally. Welding heat-affected zones, slow cooling after heat treatment, and long-term high-temperature service are the most frequent causes; sigma phase and nitride precipitation produce similar effects in ferritic and duplex grades.
How is the severity of intergranular corrosion quantified?
Severity is quantified through several complementary metrics: the weight-loss corrosion rate during immersion testing, the classification of etch structures as step, dual, or ditch, the presence or absence of cracking after a standardized bend test, and the maximum penetration depth measured metallographically. Each metric is compared with the acceptance limits of the applicable ASTM, ISO, EN, JIS, or GB specification.
Which test method should be selected for a given material?
As a general guide, austenitic stainless steels are screened by ASTM A262 Practice A and confirmed by Practice B, C, E, or F depending on the grade and service medium; molybdenum-bearing grades may require the nitric–hydrofluoric test of GB/T 4334.4; nickel alloys follow ASTM G28; ferritic and duplex grades are covered by ASTM A763 or ISO 3651-2; and 5xxx aluminum alloys are assessed by ASTM G67. Accredited third-party laboratories can recommend the most discriminating method for the alloy and application.
What sample preparation and quantity are required?
Typical immersion coupons measure about 30 mm × 20 mm and are ground to a uniform finish, degreased, dried, and weighed. In practice, laboratories request two to three replicate coupons per condition, plus, for weldments, a sample that includes the complete weld cross-section; some specifications additionally require a sensitized reference coupon or a set of as-annealed companion specimens.
Can welded or as-fabricated components be tested?
Yes. Testing weldments is one of the most common applications, since the heat-affected zone is where sensitization typically occurs. Coupons are sectioned so the weld, fusion line, and heat-affected zone are fully exposed to the test solution, and both the bend-test and metallographic evaluations focus on these regions.
How long does an assessment take?
Screening by the oxalic acid etch test can be completed within one to two working days. Full immersion tests require their nominal exposure periods—24 to 120 hours for ferric sulfate testing, 240 hours across five boiling periods for the Huey test, and up to several days for Strauss-type tests—followed by bend testing and metallography, so complete programs typically take from three to ten working days depending on the standard selected.