Intergranular corrosion (IGC) testing of Hastelloy nickel-based alloys is a critical quality-control step for verifying resistance to grain-boundary attack in aggressive chemical environments. This technical guide explains the governing ASTM G28 test methods, sample preparation, evaluation criteria, and acceptance practices used by accredited third-party laboratories and material manufacturers worldwide.
Technical Overview
Hastelloy is a family of nickel-based, corrosion-resistant alloys that includes the Ni-Cr-Mo grades such as C-276 (UNS N10276), C-22 (UNS N06022), C-4 (UNS N06455), and C-2000 (UNS N06200), as well as the Ni-Mo grades such as B-2 (UNS N10665) and B-3 (UNS N10675). These alloys are engineered to withstand severely corrosive media, including hot sulfuric acid, hydrochloric acid, wet chlorine gas, and chloride-bearing process streams. However, when the material is exposed to unfavorable thermal cycles—typically in the range of approximately 500 to 1000 degrees Celsius during welding, hot forming, improper annealing, or slow cooling—carbides (such as M6C and M23C6) and intermetallic phases (such as mu, sigma, and P phase) can precipitate at grain boundaries. These precipitates deplete the adjacent zones of chromium and molybdenum, creating preferential attack paths known as sensitization.
Intergranular corrosion proceeds along the weakened grain boundaries and can cause grains to drop out of the surface entirely, leading to severe loss of ductility, leaking, or structural failure with very little measurable overall mass loss. Because the damage is hidden beneath a visually intact surface, accelerated laboratory testing is essential. Intergranular corrosion testing of Hastelloy alloys is therefore specified in the relevant ASTM product specifications, such as ASTM B575 (plate, sheet, and strip), ASTM B574 (rod and bar), ASTM B564 (forgings), ASTM B622 (seamless pipe and tube), B619 (welded pipe), and B626 (welded tube), and is routinely performed by accredited third-party laboratories for material qualification, welding procedure qualification, and failure analysis.
Testing Samples
Plate, Sheet, and Strip
Flat-rolled products are the most commonly submitted sample type. Coupons are cut to the dimensions prescribed by ASTM G28 (nominally 50 mm by 25 mm), with the full product thickness retained whenever possible so that the through-thickness microstructure, including any banding or segregation, is evaluated.
Pipes, Tubes, and Bars
For tubular products, arc-shaped segments are cut from the pipe or tube wall; for bars, rods, and forgings, coupons are taken transverse to the working direction so that elongated grain structures and grain-boundary precipitates are oriented across the exposed face.
Weldments and Weld Overlays
Weld test coupons containing the base metal, heat-affected zone (HAZ), and weld metal are tested to verify that the welding procedure does not sensitize the HAZ. Weld overlay cladding samples are also tested to check dilution zones and bond-line integrity after corrosive exposure.
Material Condition States
Samples may be tested in the as-received (solution-annealed) condition, in the as-welded condition, or after a sensitizing heat treatment specified by the purchaser or the applicable specification (for example, a one-hour treatment at an agreed temperature followed by air cooling). Testing both annealed and sensitized material allows the degree of sensitization susceptibility to be quantified.
Testing Items
Intergranular corrosion testing of Hastelloy alloys typically includes the following test items and reported results:
- Mass-loss corrosion rate after boiling-acid exposure, reported in mm/y or mils per month
- Post-exposure bend testing (typically 180 degrees) to reveal intergranular fissuring or cracking
- Metallographic examination of cross-sections to measure the maximum depth of grain-boundary penetration
- Comparison of corrosion rates and attack morphology against the acceptance limits in the product specification
- Weld HAZ susceptibility evaluation for welded coupons and overlay samples
- Verification of proper solution annealing and absence of deleterious secondary phases
Testing Methods
ASTM G28 Method A: Ferric Sulfate–Sulfuric Acid Test
Method A is the baseline procedure specified for wrought, nickel-rich, chromium-bearing alloys. Specimens are immersed in a boiling solution of 50 weight-percent sulfuric acid containing dissolved ferric sulfate (25 grams of ferric sulfate per 600 mL of solution) in a flask fitted with a reflux condenser. The exposure duration is 24 hours for most grades, while extended 120-hour exposures are used for the most resistant Ni-Cr-Mo grades such as C-276 so that measurable attack develops. Method A is highly sensitive to grain-boundary carbide precipitation.
ASTM G28 Method B: Mixed Acid–Oxidizing Salt Test
Method B was developed for the highly alloyed Ni-Cr-Mo materials, including C-22, C-276, alloy 59, and alloy 686, whose resistance is so high that Method A alone may not discriminate between properly annealed and mildly sensitized material. The boiling solution contains approximately 11.5% sulfuric acid, 1.3% hydrochloric acid, 1% ferric chloride, and 1% cupric chloride, with a 24-hour exposure. Method B is particularly effective at revealing susceptibility caused by intermetallic phases and segregation at grain boundaries.
Specimen Preparation and Test Procedure
Coupons are surface-ground to a uniform finish (typically 120 grit or finer), degreased, rinsed, dried, and weighed to the nearest 0.1 mg. Each specimen is tested in fresh solution at a controlled solution-volume-to-surface-area ratio, with one coupon per flask to prevent galvanic interaction. The solution is maintained at continuous boiling for the full test duration, after which specimens are cleaned, dried, re-weighed, and examined visually and metallographically.
Evaluation and Acceptance Criteria
The corrosion rate is calculated from the mass loss using the relationship CR = (87600 x W) / (A x T x D), where W is the mass loss in grams, A is the exposed surface area in square centimeters, T is the exposure time in hours, and D is the alloy density in g/cm3, giving the rate in mm/y. The calculated rate is compared with the maximum allowable value stated in the applicable ASTM product specification or purchase order. In addition, specimens are subjected to a 180-degree bend test; any cracks, fissures, or grain-dropping on the tension surface indicates intergranular attack. Cross-sectional metallography quantifies the depth of grain-boundary penetration, and the morphology (discrete pitting versus continuous ditching along boundaries) is reported. For context, stainless steels are evaluated under analogous procedures in ASTM A262, ISO 3651-2, and GB/T 4334, but nickel alloys such as Hastelloy must be tested exclusively under ASTM G28 chemistry, which is appropriate for their high alloy content.
Applications
Intergranular corrosion testing of Hastelloy alloys supports material qualification and quality release across a wide range of demanding industries, including:
- Chemical and petrochemical processing: reactors, columns, heat exchangers, and gasketed vessels handling hot mineral acids
- Flue gas desulfurization (FGD) systems and waste incineration scrubbers exposed to chloride-rich acidic condensates
- Pulp and paper bleach plants, including chlorine dioxide washing equipment
- Pharmaceutical and fine chemical manufacturing requiring ultra-clean corrosion-resistant surfaces
- Oil and gas sour service equipment, complementing qualification under NACE MR0175 / ISO 15156
- Seawater cooling, desalination, and hypochlorite handling systems
- Welding procedure and welder performance qualification for fabrications in all of the above services
FAQ
Which standard governs intergranular corrosion testing of Hastelloy alloys?
ASTM G28 is the primary standard, providing Method A (boiling ferric sulfate–50% sulfuric acid) and Method B (boiling mixed acid with ferric and cupric chlorides). Acceptance limits are defined in the applicable ASTM product specifications such as B575, B574, B564, B619, B622, and B626, or by the purchaser's specification.
What is the difference between ASTM G28 Method A and Method B?
Method A uses a boiling ferric sulfate–sulfuric acid solution and is the general acceptance test for chromium-bearing nickel alloys, with 24-hour (or 120-hour for C-276) exposures. Method B uses a more aggressive mixed acid containing ferric and cupric chlorides for 24 hours and is designed to detect subtler sensitization, particularly intermetallic-phase-related susceptibility, in the most highly alloyed Ni-Cr-Mo grades.
Do welded samples need to be tested?
Yes. The heat-affected zone of a weld is the region most vulnerable to sensitization. Testing full-thickness welded coupons, including base metal, HAZ, and weld metal, confirms that the welding heat input and post-weld condition do not compromise intergranular corrosion resistance.
How is a failed result identified?
A failure is indicated by a corrosion rate exceeding the specification limit, by cracking or fissuring observed after the 180-degree bend test, or by metallographic evidence of continuous grain-boundary ditching or grain dropping beyond the acceptance depth.
How should samples be submitted for testing?
Coupons should be cut to approximately 50 mm by 25 mm with material identification, product form, heat number, material condition (annealed, as-welded, or sensitized), and the applicable specification clearly stated. Accredited third-party laboratories can also perform sample cutting, sensitizing heat treatment, and full evaluation as part of a complete test program.