Intergranular Corrosion Sensitivity Assessment: Standards, Test Methods, and Evaluation Criteria

Intergranular corrosion (IGC) sensitivity assessment is a critical materials qualification procedure used to evaluate the susceptibility of stainless steels, nickel-based alloys, and aluminum alloys to grain-boundary attack following welding, heat treatment, or extended high-temperature service. Performed in accredited third-party laboratories in accordance with recognized standards such as ASTM A262, ASTM G28, ASTM G67, ISO 3651, and GB/T 4334, the assessment combines accelerated immersion testing, bend testing, and metallographic examination to verify microstructural integrity, confirm heat treatment quality, and demonstrate product compliance. This article outlines the metallurgical background, applicable sample types, testing items, standard test methods, and typical industrial applications of intergranular corrosion sensitivity evaluation.

Technical Overview

Intergranular corrosion is a localized form of attack that occurs preferentially at grain boundaries or in the immediately adjacent zones, while the grain interiors remain 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-rich carbides (typically M23C6) precipitate at grain boundaries and deplete the surrounding matrix of chromium. If the chromium content in these depleted zones falls below the level required to maintain a passive film (roughly 12 percent), the grain boundaries become electrochemically active and dissolve rapidly in corrosive media.

Similar grain-boundary phenomena govern IGC susceptibility in other alloy families. In duplex and ferritic stainless steels, precipitation of chromium carbides, nitrides, or sigma phase can create vulnerable boundary regions. In nickel-based alloys, carbide precipitation and secondary phase formation during thermal processing may reduce boundary corrosion resistance. In 5XXX series aluminum alloys with high magnesium content, continuous grain-boundary precipitates lead to selective attack, while in heat-treatable 2XXX, 6XXX, and 7XXX aluminum alloys, galvanic effects between boundary precipitates and the depleted matrix drive intergranular attack.

Because sensitized material can fail suddenly with little outward warning, losing ductility and cohesion along grain boundaries, a formal sensitivity assessment provides essential assurance in the following areas:

• Verification that low-carbon (L grades) or stabilized (Ti- or Nb-bearing) stainless steels resist sensitization.
• Validation of solution annealing, quenching, and stress-relief heat treatments.
• Qualification of welding procedures and evaluation of heat-affected zones.
• Incoming material inspection and release testing against product specifications.
• Fitness-for-service evaluation of components exposed to sensitizing temperatures in service.

Testing Samples

Austenitic and Duplex Stainless Steels

Specimens are typically prepared from grades such as 304, 304L, 316, 316L, 321, 347, 310, S31803/S32205 duplex, and super duplex S32750. Samples may be taken from plate, sheet, strip, pipe, tube, bar, forgings, castings, or welded joints, and are usually cut to convenient rectangular sections with the full product thickness retained where bend testing is required.

Ferritic and Martensitic Stainless Steels

Ferritic grades such as 430, 446, and 409 are evaluated using test practices designed for chromium-bearing ferritic alloys, since their sensitization behavior differs from that of austenitic grades.

Nickel-Based Alloys

Wrought chromium-bearing nickel alloys such as Alloy 600, 625, 800/800H, 825, and C-276 are tested to detect susceptibility to intergranular attack caused by carbide precipitation or grain-boundary segregation during fabrication and service.

Aluminum Alloys

5XXX series alloys (for example 5083, 5086, 5456) are assessed for sensitization after thermal exposure, while heat-treatable 2XXX, 6XXX, and 7XXX series alloys are evaluated for grain-boundary attack resulting from improper aging or quench practices.

Sample Preparation Requirements

Test coupons are generally ground to a uniform, clean surface finish (typically 120 grit or finer), degreased, rinsed, dried, and weighed to 0.001 g with dimensions recorded for area calculation. Where specified by the purchase order or product specification, a sensitizing heat treatment (for example, 650 degrees Celsius for a defined hold time followed by rapid cooling) is applied before testing to simulate a worst-case thermal history. Weldment specimens are oriented so that the weld metal, heat-affected zone, and base metal are all exposed to the test environment.

Testing Items

Depending on the alloy, product form, and governing specification, the assessment may include the following testing items:

• Oxalic acid etch screening and classification of grain-boundary etch structures (step, dual, or ditch).
• Boiling ferric sulfate and sulfuric acid immersion test with mass-loss corrosion rate determination.
• Boiling nitric acid (Huey) test with corrosion rate measured over successive 48-hour periods.
• Copper and copper sulfate in sulfuric acid immersion test followed by a 180-degree bend ductility test.
• Immersed copper-contact test in 50 percent sulfuric acid for highly sensitive conditions.
• Nickel-alloy immersion testing with mass loss per unit area and corrosion rate calculation.
• Nitric acid mass loss testing (NAMLT) of 5XXX aluminum alloys reported in mg/cm2.
• Sodium chloride and hydrogen peroxide immersion testing of heat-treatable aluminum alloys with metallographic cross-section examination.
• Measurement of intergranular attack depth and grain-dropping severity on metallographic sections.
• Corrosion rate reporting in metric or imperial units (for example, micrometers per period or mils per month) against specification acceptance limits.

Testing Methods

Oxalic Acid Etch Screening Test (ASTM A262 Practice A)

A small coupon is anodically etched at a current density of 1 A/cm2 for 1.5 minutes in 10 percent oxalic acid at room temperature, then examined microscopically. The etch structure is classified as step, dual, or ditch. Materials showing step or dual structures are considered acceptable for certain grades without further boiling tests, while ditch structures require full immersion confirmation testing. Equivalent procedures are given in GB/T 4334 and JIS G 0571.

Ferric Sulfate and Sulfuric Acid Test (ASTM A262 Practice B, ASTM G28 Method A)

Specimens are exposed for 120 hours (ASTM A262 Practice B) or 24 hours (ASTM G28 Method A for nickel alloys) in a boiling solution of 50 percent sulfuric acid containing ferric sulfate. The test is conducted under reflux in a flask fitted with a condenser to maintain constant concentration. Susceptibility is quantified by the mass-loss corrosion rate, with acceptance limits defined by the applicable material specification. This practice is particularly sensitive to chromium depletion from carbide precipitation.

Nitric Acid Huey Test (ASTM A262 Practice C)

Coupons are immersed in boiling 65 percent nitric acid for successive 48-hour periods, typically five periods totaling 240 hours, with fresh solution each period. The Huey test detects susceptibility associated with both chromium carbide precipitation and sigma phase, making it the preferred method for alloys intended for nitric acid service. Corrosion rates from each period are calculated and compared with specification maxima.

Copper and Copper Sulfate in Sulfuric Acid Test (ASTM A262 Practices E and F)

In the widely used Strauss-type test (Practice E), specimens are placed in contact with metallic copper and boiled for 24 hours in a solution of 16 percent sulfuric acid with copper sulfate. Practice F uses a 50 percent sulfuric acid solution with copper sulfate and copper contact for 120 hours, providing a more severe test suited to evaluating stabilized and extra-low-carbon grades. After exposure, the specimens are bent 180 degrees around a mandrel and examined at low magnification; the presence of fissures or cracks indicates intergranular attack. Equivalent methods appear in ISO 3651-2, GB/T 4334, and JIS G 0575.

Tests for Ferritic Stainless Steels (ASTM A763)

Dedicated practices within ASTM A763 cover the detection of intergranular attack in ferritic stainless steels, using boiling acid immersion with mass-loss or bend-test evaluation appropriate to the grade and carbon-nitrogen content.

Tests for Aluminum Alloys (ASTM G67 and ASTM G110)

ASTM G67 (NAMLT) exposes 5XXX series aluminum coupons to concentrated nitric acid at 30 degrees Celsius for 24 hours and reports mass loss in mg/cm2, with higher losses indicating sensitized, continuously precipitated grain boundaries. ASTM G110 immerses heat-treatable aluminum alloys in a sodium chloride and hydrogen peroxide solution, after which cross sections are examined metallographically to measure the depth and continuity of intergranular attack.

Post-Exposure Evaluation and Reporting

All results are compiled into a formal test report including material identification, heat treatment condition, test standard and practice used, exposure duration, calculated corrosion rates or mass losses, bend test results, and metallographic observations. Results are compared against the acceptance criteria of the governing product specification, purchase order, or applicable codes such as ASTM/ASME, EN, ISO, or GB standards.

Applications

Intergranular corrosion sensitivity assessment is applied across industries where grain-boundary integrity is essential to safety and reliability:

• Nuclear power: qualification of piping, reactor internals, and heat exchanger tubing exposed to high-temperature water chemistry.
• Chemical and petrochemical processing: corrosion rate verification for reactors, columns, and piping handling nitric or sulfuric acid media.
• Oil and gas: evaluation of duplex and austenitic stainless components, clad plates, and weldments in sour and chloride-containing environments.
• Pharmaceutical and food and beverage: certification of sanitary stainless steel surfaces and welded process lines where grain dropping cannot be tolerated.
• Marine and offshore engineering: acceptance testing of sensitization-prone aluminum and stainless structures after fabrication heat input.
• Aerospace: qualification of heat-treatable aluminum alloys after forming, welding, or long-term thermal exposure.
• Fabrication quality control: routine verification of solution annealing, stabilization, and post-weld heat treatment procedures.

FAQ

What causes intergranular corrosion sensitivity in stainless steel?

Sensitivity is caused primarily by sensitization, the precipitation of chromium-rich carbides at grain boundaries when the steel is held within roughly 425 to 815 degrees Celsius, which depletes chromium adjacent to the boundaries and destroys the passive film locally. Welding heat input, slow cooling from annealing, and elevated-temperature service are common triggers.

Which test should be selected for 304 or 316 stainless steel?

The choice depends on the intended service and specification. The oxalic acid etch test (ASTM A262 Practice A) is an economical screening step, while the ferric sulfate and sulfuric acid test (Practice B), the Huey nitric acid test (Practice C), or the copper and copper sulfate sulfuric acid test with bend testing (Practice E) provide full quantitative confirmation.

Can the oxalic acid etch test replace boiling acid immersion tests?

No. It is a screening method only. A step or dual etch structure may allow certain grades to be accepted without further testing, but any ditch structure requires a complete boiling immersion test for a definitive result.

What sample sizes and quantities are required?

Typical coupons measure approximately 25 to 50 mm in length by 10 to 30 mm in width with the full product thickness retained for bend testing. Duplicate or triplicate specimens are recommended, and additional sets are required if a sensitizing heat treatment must be evaluated alongside the as-supplied condition.

How long does a complete assessment take?

Screening by oxalic acid etching can be completed within one to two working days including metallography. Full immersion tests require from 24 hours (nickel alloys) up to 240 hours (five-period Huey test) of boiling exposure, plus preparation, post-test cleaning, bend testing, and reporting, so a typical turnaround is one to two weeks.

How can intergranular corrosion susceptibility be prevented?

Effective measures include using extra-low-carbon (L) grades, selecting titanium- or niobium-stabilized grades such as 321 or 347, performing proper solution annealing followed by rapid cooling, controlling welding heat input and interpass temperature, and applying post-weld solution annealing where feasible.