Intergranular Corrosion Sensitivity Testing of Stainless Steel

Intergranular corrosion (IGC) sensitivity testing determines whether stainless steels are susceptible to grain boundary attack caused by chromium carbide precipitation during welding, heat treatment, or service in the sensitization temperature range. Conducted in accordance with ASTM A262, ASTM A763, ISO 3651, and equivalent national standards, tests such as the oxalic acid etch, Huey, Streicher, and Strauss methods allow manufacturers to verify material quality, qualify weldments, and prevent premature failure of critical components. This article explains the metallurgical background, sample requirements, test procedures, acceptance criteria, and typical applications of stainless steel intergranular corrosion testing.

Technical Overview

Intergranular corrosion is a localized form of attack that proceeds along the grain boundaries of a metal while the grains themselves remain largely unaffected. In stainless steels, the dominant cause is sensitization: when an unstabilized austenitic grade such as 304 or 316 is exposed to temperatures between approximately 425 °C and 815 °C—typically during slow cooling from welding or during improper heat treatment—chromium combines with carbon to precipitate chromium-rich carbides (Cr23C6) at the grain boundaries. This leaves a thin, chromium-depleted zone adjacent to the boundary that can no longer maintain a passive film, making it preferentially dissolved in corrosive media.

Sensitized material may show no external signs of deterioration, yet it can suffer grain dropping, loss of toughness, and sudden leaking or fracture under load. Intergranular corrosion sensitivity testing therefore subjects representative specimens to accelerated laboratory exposures that reveal this susceptibility before the material enters service. The most widely applied standards include:

• ASTM A262 – Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels (Practices A, B, C, E, F)
• ASTM A763 – Practices for Detecting Susceptibility to Intergranular Attack in Ferritic Stainless Steels (Practices W, X, Y, Z)
• ISO 3651-1 and ISO 3651-2 – Determination of resistance to intergranular corrosion of stainless steels (Huey test and sulfuric acid/copper sulfate test)
• GB/T 4334 series – Chinese national standards mirroring the international acid-boiling and electrolytic etch methods
• JIS G 0571 to JIS G 0575 – Japanese industrial standards equivalent to the ASTM practices

Testing Samples

Plate, Sheet, and Strip

Flat products are sampled across the full thickness so that both surfaces and the core structure are evaluated. Typical coupon dimensions are approximately 30 mm × 20 mm, with the total exposed surface area generally kept between 5 cm² and 20 cm² to suit the boiling-flask geometry used in the acid immersion tests.

Pipe and Tube

Specimens are cut from tube walls and may be tested as curved segments or flattened sections depending on the specification. For welded pipe, the sample must include the weld bead, the heat-affected zone (HAZ), and adjacent base metal, because the HAZ is the region most vulnerable to sensitization.

Bar, Forging, Wire, and Casting

Solid products are sectioned to expose a representative cross-section. Castings and forgings should be sampled from the thickest section, where slow cooling through the sensitization range is most likely to have occurred during manufacture.

Welded Joints and Fabricated Components

Weldment coupons encompass the full joint configuration, including root passes, cap passes, and HAZs on both sides. Repair welds and dissimilar-metal joints are also tested, since multiple thermal cycles can progressively sensitize the material.

Heat-Treatment Conditions

Specimens are usually tested in the as-received (as-fabricated) condition. When the purchase specification requires it, an additional laboratory sensitizing treatment—commonly 650 °C for 2 hours followed by air cooling—is applied before testing to assess the intrinsic susceptibility of the alloy chemistry. All coupons are ground to a uniform finish (typically 120 to 180 grit), degreased, dried, and weighed to 0.001 g prior to exposure.

Testing Items

A complete intergranular corrosion evaluation may include the following items, selected according to the material grade, product specification, and intended service:

• Electrolytic oxalic acid etch structure classification (step, dual, or ditch structure)
• Weight-loss corrosion rate determination in boiling acid media, expressed in μm/48 h, mdd, or mm/y
• Post-exposure bend testing (typically 180°) to reveal intergranular fissures
• Metallographic examination and measurement of grain boundary attack depth after exposure
• Evaluation of welded joints for attack at the fusion line and heat-affected zone
• Comparison of as-received versus deliberately sensitized specimen behavior
• Acceptance assessment against the applicable product specification and issuance of test reports supporting EN 10204 3.1 or 3.2 certification

Testing Methods

ASTM A262 Practice A – Oxalic Acid Etch Test (Screening)

Coupons are electrolytically etched at a current density of 1 A/cm² for 1.5 minutes in a 10% oxalic acid solution at room temperature, then examined under a microscope at 250× to 500×. The etch structure is classified as step, dual, or ditch. A step or dual structure generally indicates the steel is not sensitized and the lot may be accepted without further testing where the specification permits; a ditch structure indicates susceptibility and requires confirmation by one of the quantitative boiling-acid practices. Practice A is rapid and low-cost but is strictly a screening test.

ASTM A262 Practice B – Ferric Sulfate–Sulfuric Acid Test (Streicher Test)

Specimens are immersed for 120 hours in a boiling solution of 50% sulfuric acid containing 25 g of ferric sulfate. The ferric ions act as a depolarizer, accelerating attack on chromium-depleted boundaries. Corrosion rate is calculated from mass loss using the relation CR = (8.76 × 10⁴ × W) / (A × T × ρ), where W is mass loss in grams, A is exposed area in cm², T is time in hours, and ρ is density in g/cm³. Acceptance limits are set by the product specification, commonly around 1.0 μm/48 h for many austenitic grades.

ASTM A262 Practice C – Nitric Acid Test (Huey Test)

Specimens are exposed to boiling 65% nitric acid for five consecutive 48-hour periods, with fresh solution each period. The average and period-by-period mass loss rates are reported. The Huey test is the most aggressive of the A262 practices and uniquely detects attack associated with sigma phase as well as chromium carbide, which makes it mandatory for alloys intended for nitric acid service. Excessive scatter between periods signals selective grain boundary or intermetallic attack.

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

Practice E immerses specimens for 24 hours in boiling 16% sulfuric acid with copper sulfate, in contact with metallic copper, followed by a 180° bend test and low-magnification inspection for intergranular fissures. Practice F applies a more severe boiling 50% sulfuric acid/copper sulfate solution with copper contact for 120 hours to molybdenum-bearing grades such as 316, 316L, and 317, and is evaluated by mass loss. The Strauss-type environment simulates mildly reducing process media.

Ferritic and Duplex Stainless Steels

Ferritic grades are evaluated under ASTM A763, whose Practices W, X, Y, and Z parallel the oxalic, ferric sulfate–sulfuric, copper–copper sulfate–sulfuric, and nitric acid exposures. For duplex stainless steels, ASTM A923 Method C (boiling ferric sulfate–50% sulfuric acid) is widely used to detect detrimental intermetallic precipitates that degrade corrosion resistance and toughness.

ISO 3651 and Equivalent National Methods

ISO 3651-1 specifies the nitric acid Huey exposure for austenitic and duplex steels, while ISO 3651-2 specifies sulfuric acid/copper sulfate testing, with Method A using metallic copper contact and a shorter exposure and Method B omitting copper and extending the exposure for greater severity. GB/T 4334 and JIS G 0571–0575 implement essentially the same family of methods, including the nitric–hydrofluoric acid test used for molybdenum-bearing and duplex grades; a common sensitizing preconditioning of 650 °C × 2 h may be specified in these systems.

Applications

Intergranular corrosion sensitivity testing is specified wherever welded or thermally processed stainless steel contacts corrosive media, and where sudden grain boundary failure would threaten safety, containment, or product purity:

• Chemical and petrochemical plants: reactors, columns, heat exchangers, and piping handling acids and chlorides
• Nuclear power: primary circuit piping, reactor internals, and reprocessing equipment with strict sensitization control
• Oil and gas: offshore platforms, subsea components, and sour service installations
• Pharmaceutical and biotechnology: sanitary piping, vessels, and clean-in-place systems requiring corrosion-free surfaces
• Food and beverage: processing tanks, brew kettles, and storage vessels
• Pulp and paper: bleaching plant equipment exposed to aggressive oxidizing chemistry
• Material qualification: incoming inspection, weld procedure qualification, and EN 10204 3.1/3.2 certification of plate, pipe, tube, and fittings

FAQ

What causes intergranular corrosion in stainless steel?

The principal cause is sensitization—chromium carbide precipitation at grain boundaries when the steel passes through 425–815 °C, which depletes the adjacent metal of chromium below the roughly 12% needed for passivity. Sigma phase precipitation and grain boundary segregation in high-alloy grades can produce similar susceptibility.

How can sensitization be prevented?

Effective measures include using low-carbon grades (304L, 316L), stabilized grades alloyed with titanium or niobium (321, 347), employing solution annealing (typically 1,040–1,150 °C followed by rapid water quench) to redissolve carbides, and controlling welding heat input with low interpass temperatures.

Which ASTM A262 practice should be selected?

Practice A is a fast screen. Practices B and C are quantitative mass-loss tests, with C being the most severe and required for nitric acid service. Practice E is the least severe and suits mildly reducing environments, while Practice F is intended for molybdenum-bearing grades. The purchase specification or end user normally dictates the choice.

What is the difference between the Huey and Strauss tests?

The Huey test (Practice C) uses boiling 65% nitric acid for 5 × 48 hours and is scored by corrosion rate; it detects both carbide and sigma phase attack. The Strauss test (Practice E) uses boiling sulfuric acid with copper sulfate and metallic copper for 24 hours and is scored by a bend test for fissures; it is less aggressive and simulates reducing media.

How are results judged as pass or fail?

Acceptance depends on the governing specification: etch structure must not be ditch type (Practice A), calculated corrosion rate must not exceed the specified limit (Practices B, C, F), or the bent specimen must show no intergranular cracks at the required magnification (Practice E). Metallographic attack depth limits may also apply for certain products.

How long does testing take?

The oxalic acid etch requires only about one day. Immersion tests require 24 hours (Practice E), 120 hours (Practices B and F), or 240 hours (Practice C) of boiling exposure plus specimen preparation, bending, and metallography, so a complete program typically takes one to two weeks.