Intergranular Corrosion Rating and Testing of Stainless Steel: Methods, Standards, and Evaluation Cr

Intergranular corrosion (IGC) testing is a critical quality control procedure for evaluating the sensitization resistance of stainless steels used in corrosive environments. This article provides a comprehensive technical overview of intergranular corrosion rating experiments, covering the sensitization mechanism, sample preparation, internationally recognized test methods such as ASTM A262 and GB/T 4334, evaluation criteria, and practical applications across industries.

Technical Overview

Intergranular corrosion (IGC) is a localized form of attack that preferentially occurs along the grain boundaries of a metal, leading to a dramatic loss of mechanical strength and cohesion with little visible evidence of overall material loss. In stainless steels, intergranular corrosion is most commonly caused by sensitization, a metallurgical phenomenon that takes place when the material is exposed to temperatures between approximately 425 °C and 815 °C during welding, slow cooling from solution annealing, or improper heat treatment.

The sensitization mechanism involves the precipitation of chromium-rich carbides (primarily Cr23C6) at grain boundaries. Because carbon diffuses more rapidly than chromium at these temperatures, chromium is depleted in a narrow zone adjacent to the grain boundary. When the local chromium content falls below approximately 12%, the passive film in these zones can no longer be maintained, making the grain boundary regions anodic relative to the grain interiors. In aggressive media, these chromium-depleted zones dissolve preferentially, and the attack propagates along the grain boundaries until grains are undermined and dislodged.

An intergranular corrosion rating experiment is designed to reveal this susceptibility in a controlled laboratory environment. The experiment consists of subjecting prepared specimens to a specific corrosive medium under standardized conditions, followed by evaluation through either metallographic examination of the etched microstructure, measurement of the corrosion rate, or a bend test to detect intergranular fissures. The result is a formal rating or classification that determines whether the material meets specification requirements for the intended service environment.

• Sensitization temperature range: approximately 425–815 °C
• Root cause: chromium carbide precipitation and chromium depletion at grain boundaries
• Prevention strategies: low-carbon (L) grades, stabilized grades with titanium or niobium, solution annealing, and controlled welding procedures
• Primary standards: ASTM A262, ASTM A763, GB/T 4334, ISO 3651, JIS G0571–G0575

Testing Samples

Proper sample selection and preparation are essential for obtaining representative and reproducible results. Specimens must reflect the actual metallurgical condition of the product, including its thermal history, surface condition, and grain orientation.

Austenitic Stainless Steels

The most frequently tested materials include grades 304, 304L, 316, 316L, 321, and 347. These steels are the primary focus of ASTM A262 and GB/T 4334 series standards. Sensitized austenitic grades exhibit classic ditch structures after etching, while properly solution-annealed material shows step structures. Stabilized grades such as 321 and 347 contain titanium or niobium additions that tie up carbon as stable carbides and suppress chromium carbide formation.

Ferritic and Martensitic Stainless Steels

Ferritic grades such as 430, 444, and 446 are tested in accordance with ASTM A763, which specifies practices W, X, Y, and Z. Ferritic steels can become sensitized during welding or cooling from elevated temperatures because of chromium carbide and nitride precipitation, and their evaluation requires dedicated test procedures.

Duplex Stainless Steels

Duplex grades such as 2205 and 2507 may be evaluated using corrosion tests adapted for austenitic-ferritic microstructures. Their duplex structure generally improves resistance to intergranular attack, but improper heat treatment can still produce detrimental phases and grain boundary precipitation.

Welded and Heat-Treated Components

Specimens are commonly extracted across weld seams, heat-affected zones, and base metal to characterize susceptibility introduced by fabrication. Sample preparation typically involves cutting to standardized dimensions (for example, 10 mm × 20 mm to 30 mm squares or full-thickness cross sections), grinding to a fine surface finish, degreasing with solvents such as acetone or alcohol, drying, and weighing to the nearest 0.001 g when mass-loss evaluation is required.

Testing Items

Depending on the standard and practice selected, an intergranular corrosion rating experiment may include the following test items and measured parameters:

• Etch structure classification: step structure, dual structure, ditch structure, end-grain pits, or island structure determined by metallographic examination after electrolytic etching
• Corrosion rate determination: calculated from mass loss after exposure to boiling test solutions, typically expressed in milligrams per square decimeter per day (mdd) or millimeters per year (mm/y)
• Bend test evaluation: 180-degree bending of corroded specimens followed by low-magnification inspection for intergranular cracks and fissures
• Metallographic cross-section examination: measurement of intergranular attack depth and morphology after exposure testing
• Weight-loss comparison: ratio of corrosion rates between two solution exposures or between heats to detect selective attack
• Acceptance verification: pass/fail determination against the limits specified by the purchase order, material specification, or applicable design code

Testing Methods

Several standardized practices exist, each tailored to specific alloy families and service environments. The most widely applied system is ASTM A262 for austenitic stainless steels, which parallels the GB/T 4334 series used in China.

Oxalic Acid Etch Test (ASTM A262 Practice A / GB/T 4334 Method A)

This rapid screening test involves electrolytic etching of a polished specimen in a 10% oxalic acid solution at a current density of approximately 1 A/cm² for about 90 seconds. The etched microstructure is then examined at 250× to 500× magnification and classified into step, ditch, dual, end-grain pits, or island structures. A step structure, in which grain boundaries are attacked uniformly without preferential ditching, indicates an unsensitized, acceptable condition. A ditch structure, in which grain boundaries are deeply attacked and may completely surround grains, indicates sensitization and requires confirmation by an additional quantitative test. This method is a destructive screening test and cannot be used alone for final acceptance.

Ferric Sulfate–Sulfuric Acid Test (ASTM A262 Practice B / GB/T 4334 Method B)

Specimens are immersed for 120 hours in a boiling solution of 50% sulfuric acid containing ferric sulfate. The oxidizing ferric ions maintain the attack on chromium-depleted grain boundary regions. The corrosion rate is calculated from mass loss and specimen surface area, and materials are rated acceptable if the rate falls below the specified limit, commonly on the order of 1.2 µm per 120 hours for certain specifications.

Boiling Nitric Acid (Huey) Test (ASTM A262 Practice C / GB/T 4334 Method C)

The Huey test immerses specimens in boiling 65% nitric acid for five consecutive 48-hour periods, each with fresh solution. It is the most severe of the A262 practices and is applied to materials intended for nitric acid service. Results are reported as the average corrosion rate from the five periods. The test also detects other sensitization-related phenomena such as sigma phase attack and grain dropping.

Nitric–Hydrofluoric Acid Test (ASTM A262 Practice D)

This method applies specifically to molybdenum-bearing austenitic grades such as 316 and 317. Specimens undergo two consecutive 4-hour exposures in a 10% nitric acid–3% hydrofluoric acid solution at 70 °C. Because molybdenum-rich regions are preferentially attacked in this medium, the test evaluates the ratio of corrosion rates between the material as-supplied and after a sensitizing heat treatment.

Copper–Copper Sulfate–Sulfuric Acid (Strauss) Test (ASTM A262 Practice E / GB/T 4334 Method E)

In the Strauss test, specimens are exposed for a minimum of 24 hours in a boiling 16% sulfuric acid solution containing copper sulfate and metallic copper chips in contact with the specimen. After exposure, each specimen is bent 180 degrees around a mandrel with a diameter equal to the specimen thickness. The bent surface is inspected at 10× to 20× magnification for cracks or fissures caused by intergranular attack. The presence of such fissures constitutes failure; the absence of cracking indicates the material is not sensitized to a degree detectable by this test.

Evaluation and Rating Criteria

The final rating combines the test results with the applicable acceptance standard. For etch-structure screening, the classification of step, dual, or ditch structure directly reflects the degree of sensitization, with step structure rated as fully acceptable and ditch structure rated as sensitized. For immersion tests, the corrosion rate is compared against specification limits, and for bend tests, the verdict is binary based on the presence of intergranular fissures. Reporting should include the material grade, heat number, heat treatment condition, test practice used, exposure conditions, calculated corrosion rates, etch structure classification, and the final rating or pass/fail conclusion.

Applications

Intergranular corrosion rating experiments are performed across a broad range of industries where the structural integrity of stainless steel depends on sensitization resistance:

• Chemical and petrochemical processing: pressure vessels, reactors, heat exchangers, and piping exposed to organic acids, sulfur-bearing media, and process condensates
• Nuclear power: components fabricated from austenitic stainless steels where sensitization during welding or service can compromise safety margins
• Oil and gas: offshore platforms, subsea equipment, and refinery units requiring documented corrosion qualification of welded fabrications
• Pharmaceutical and food processing: sanitary piping, vessels, and equipment where intergranular attack can cause both integrity loss and product contamination
• Pulp and paper: digesters, bleaching washers, and recovery systems handling aggressive chloride- and sulfur-containing liquors
• Material qualification and research: incoming material inspection, welding procedure qualification, heat treatment validation, and evaluation of new alloy compositions

FAQ

What is the difference between the oxalic acid etch test and the immersion tests?

The oxalic acid etch test is a rapid, destructive screening method that classifies the etched microstructure within minutes and indicates whether sensitization is present. Immersion tests such as the ferric sulfate and Huey tests are quantitative acceptance tests that measure the actual corrosion rate over extended exposure periods and provide the definitive pass/fail verdict required by many specifications.

Can low-carbon or stabilized stainless steels still fail an intergranular corrosion test?

Yes. Although L grades and stabilized grades are designed to resist sensitization, improper welding practices, extended exposure in the sensitization temperature range, multiple repair welds, or incorrect stabilization heat treatment can still produce chromium carbide precipitation or harmful secondary phases that cause test failure.

Which test method should be selected for a given application?

Method selection depends on the alloy family and the service environment. Practice A is used for screening austenitic grades, Practice C is required for nitric acid service, Practice D applies to molybdenum-bearing grades, Practice E is a general sensitization test, and ferritic grades require ASTM A763 practices. The governing product specification or purchase order usually dictates the required method.

How is the corrosion rate calculated in immersion tests?

The corrosion rate is determined from the specimen mass loss divided by the total exposed surface area and the exposure time, and the result is converted into standard units such as mdd, mm/y, or mils per year. Multiple exposure periods are averaged when the standard requires them, such as the five 48-hour periods of the Huey test.

How long does a complete intergranular corrosion rating experiment take?

The oxalic acid screening test can be completed within one working day, including specimen preparation and metallographic examination. Quantitative immersion tests require from 24 hours for the Strauss test up to 240 hours or more for the Huey test, so the full experiment, including sample preparation, exposure, bend testing, and reporting, typically spans one to two weeks.

What does a ditch structure in the etch test mean for material acceptance?

A ditch structure indicates that grains are either partially or completely surrounded by precipitated carbides, which is evidence of sensitization. Materials exhibiting ditch structures are not automatically rejected but must be further evaluated by an applicable quantitative immersion test before a final rating and acceptance decision can be made.