Intergranular Corrosion Testing of Stainless Steel According to Chinese National Standard GB/T 4334

Intergranular corrosion (IGC) testing of stainless steel in accordance with the Chinese national standard GB/T 4334 is the essential procedure for verifying that austenitic and duplex stainless steels resist grain-boundary attack caused by sensitization. This guide explains the metallurgical background, specimen requirements, the five standard test methods (Methods A through E), acceptance criteria, typical industrial applications, and answers to frequently asked questions about GB/T 4334 intergranular corrosion testing.

Technical Overview

Intergranular corrosion is a localized degradation mechanism in which the grain boundaries of a stainless steel are preferentially attacked while the grain interiors remain largely unaffected. In severe cases, the boundaries are fully consumed, the alloy disintegrates into individual grains, and the component loses virtually all of its strength and toughness even though its surface shows little visible damage. Because the failure is subsurface and sudden, IGC is considered one of the most dangerous forms of localized corrosion in stainless steel equipment.

The root cause is sensitization. When a stainless steel is exposed to the sensitizing temperature range of approximately 450 °C to 850 °C — for example, during slow cooling from solution annealing, during stress-relief heat treatment, or in the heat-affected zone of a weld — chromium combines with carbon at the grain boundaries and precipitates as chromium-rich carbides (Cr23C6). The thin zones adjacent to the boundaries become depleted in chromium to below the ~12 percent level required for passivation, and these chromium-depleted regions are selectively dissolved in corrosive media. Sigma phase and chromium nitride precipitation can produce similar effects, particularly in molybdenum-bearing and duplex grades.

In China, the governing national standard is GB/T 4334-2020, "Test methods for intergranular corrosion of stainless steels," which consolidated the earlier five separate parts (GB/T 4334.1 through GB/T 4334.5-2008) into a single document. The standard specifies five laboratory methods, designated Method A through Method E, broadly equivalent to Practices A through E of ASTM A262 and aligned with the principles of ISO 3651-1 and ISO 3651-2. The standard applies mainly to austenitic and austenitic-ferritic (duplex) stainless steels in the form of plates, strips, tubes, bars, forgings, castings, and welded joints.

• Method A — 10% oxalic acid etch test (rapid electrochemical screening)
• Method B — 50% sulfuric acid–ferric sulfate test (boiling, mass-loss based)
• Method C — 65% nitric acid test, Huey type (boiling, mass-loss based)
• Method D — nitric–hydrofluoric acid test (for molybdenum-bearing grades)
• Method E — 16% sulfuric acid–copper sulfate test with copper chips, Strauss type (boiling, followed by bend testing)

Testing Samples

Specimen Selection and Dimensions

Specimens must be representative of the delivered condition of the material and are normally taken from the same heat, product form, and heat-treatment lot as the component being qualified. Typical test coupons provide a total exposed surface area of roughly 10 to 30 cm², depending on the method and product form. For welded components, the coupon must include base metal, weld metal, and the heat-affected zone so that sensitization produced by the welding thermal cycle is captured. The number of specimens is specified by the method or the product standard, and three parallel specimens are commonly tested for statistical reliability.

Surface Preparation

All surfaces are ground to a uniform finish, typically 120 to 180 grit or finer, to remove scale, decarburized layer, and cold-worked material. Sharp edges are rounded to avoid preferential edge attack. Specimens are then degreased, cleaned, rinsed, dried, measured for surface area, and weighed to the nearest 1 mg before exposure. Accurate weighing and area measurement are essential because most methods are evaluated by gravimetric corrosion rate.

Sensitization Heat Treatment

When the product standard, drawing, or purchase agreement requires the material to be evaluated in the sensitized condition — common practice for stabilized and extra-low-carbon grades such as 06Cr18Ni11Ti (321), 022Cr19Ni10 (304L), and 022Cr17Ni12Mo2 (316L) — specimens receive a sensitizing heat treatment before testing, classically 650 °C ± 10 °C for 2 hours followed by air cooling. Materials intended for service in the as-delivered (solution-annealed) condition are tested without additional heat treatment.

Testing Items

A GB/T 4334 test campaign typically covers the following evaluation items, depending on the method selected:

• Electrolytic etch-structure classification: stepped, ditch, mixed, or end-grain-pit structures under a metallurgical microscope (Method A)
• Mass-loss corrosion rate determination, expressed in g/(m²·h) or mm/a, calculated from weight loss, exposed area, exposure time, and density (Methods B, C, and D)
• Corrosion-rate ratio between the test specimen and a laboratory-sensitized reference specimen (Method D)
• Post-exposure bend testing and crack/fissure inspection of the tension surface at low magnification (Method E)
• Microstructural confirmation of intergranular attack on cross-sectioned specimens where results are marginal or disputed
• Reporting of material grade, heat number, product form, heat-treatment condition, test method, solution composition, exposure duration, and results

Testing Methods

Method A — 10% Oxalic Acid Etch Test

Method A is a rapid screening test. The polished specimen is made the anode and etched electrolytically in a 10% oxalic acid solution at a current density of 1 A/cm² for 1.5 minutes at room temperature. The etched surface is examined at 200× to 500× magnification and classified as a stepped structure, ditch structure, mixed (dual) structure, or end-grain pits. Stepped and end-grain-pit structures indicate the absence of continuous grain-boundary carbide networks and the specimen passes; a ditch structure indicates susceptibility, and the material must be verified by one of the full boiling tests (Method B, C, D, or E) before acceptance or rejection.

Method B — 50% Sulfuric Acid–Ferric Sulfate Test

The specimen is immersed in boiling 50% sulfuric acid solution containing dissolved ferric sulfate, which maintains a controlled oxidizing potential that selectively attacks chromium-depleted boundaries. Exposure is continuous for 120 hours in a glass flask fitted with a reflux condenser. The corrosion rate is calculated from mass loss using the relationship CR (mm/a) = 8,760 × Δm / (A × t × ρ), where Δm is the mass loss in grams, A the area in cm², t the time in hours, and ρ the density in g/cm³. Acceptance limits are defined by the applicable product standard or by agreement.

Method C — 65% Nitric Acid (Huey) Test

Method C is the most aggressive of the mass-loss methods and is specified where the steel will contact strongly oxidizing media such as nitric acid. Specimens are exposed to boiling 65% nitric acid for five consecutive 48-hour periods (240 hours total), with fresh acid for each period. The average corrosion rate over the five periods is reported; rising rates across successive periods indicate progressive grain-boundary or intergranular-type attack, including that caused by sigma phase as well as carbides. This method is widely used for nuclear-grade and nitric-acid-service materials.

Method D — Nitric–Hydrofluoric Acid Test

Method D is intended for molybdenum-bearing austenitic steels such as 316 and 317 types, for which the oxalic acid and Strauss tests are less discriminating. Specimens are immersed in a solution of 10% nitric acid plus 3% hydrofluoric acid at 70 °C for two 2-hour periods with solution renewal. Evaluation is by comparison: the corrosion rate of the test specimen is compared against that of a laboratory-sensitized reference specimen from the same heat, and the material is judged acceptable when the rate ratio does not exceed the limit agreed (typically not more than about 1.5).

Method E — 16% Sulfuric Acid–Copper Sulfate Test (Strauss Type)

Method E is the most widely specified acceptance test for detecting chromium carbide precipitation. Specimens are immersed in boiling 16% sulfuric acid solution containing copper sulfate, in contact with copper chips or turnings that hold the redox potential in the range where only chromium-depleted boundaries dissolve. Exposure is 16 hours of continuous boiling (24 hours where specified). After exposure, the specimen is bent through 180° around a mandrel appropriate to its thickness, and the tension surface is examined at up to 10× magnification. The presence of intergranular cracks or fissures constitutes failure; a crack-free bend indicates resistance to intergranular corrosion.

Evaluation and Acceptance Criteria

Acceptance is method-dependent. Method A relies on etch-structure classification; Methods B and C rely on corrosion-rate limits stated in the product standard; Method D relies on the rate ratio to the sensitized reference; and Method E relies on the absence of cracking after bending. Where a dispute arises, or a result is marginal, metallographic cross-section examination of the exposed specimen is used to confirm whether the attack is genuinely intergranular.

Applications

GB/T 4334 intergranular corrosion testing is mandatory or contractually required across many sectors of Chinese industry, including:

• Pressure vessels, boilers, and heat exchangers designed under GB/T 150 and TSG special equipment regulations
• Petrochemical and chemical process plants: reactors, columns, piping, and valves in corrosive service
• Nuclear power components, where the Huey-type Method C controls grain-boundary quality of primary-circuit materials
• Food, beverage, and pharmaceutical equipment built from sanitary stainless steel tubing and fittings
• Shipbuilding, marine engineering, and offshore structures exposed to chloride-bearing environments
• Mill certification and incoming inspection of plates, tubes, pipes, bars, forgings, castings, flanges, and fasteners
• Welding procedure qualification and welder performance testing, where HAZ sensitization is assessed on cross-weld coupons
• Failure analysis, sensitization troubleshooting, and heat-treatment optimization in research and development

FAQ

Which Chinese national standard covers intergranular corrosion testing of stainless steel?

The current standard is GB/T 4334-2020, which unified the five former parts GB/T 4334.1 through GB/T 4334.5-2008. Its five methods correspond closely to Practices A through E of ASTM A262, and it is technically aligned with ISO 3651-1 and ISO 3651-2, facilitating international acceptance of test results.

How do I choose between Methods A, B, C, D, and E?

Selection is dictated by the steel grade, the intended service environment, and the product standard. Method A is a fast screen applicable to most austenitic grades. Method E is the general acceptance test for detecting chromium carbide precipitation, including in welded joints. Method B suits austenitic grades in moderately oxidizing media. Method C is chosen for nitric acid service and where sigma-phase attack must be revealed. Method D is reserved for molybdenum-bearing steels such as 316 and 317. When in doubt, follow the product standard, the drawing, or agree the method with the purchaser.

Which stainless steels are most susceptible to intergranular corrosion?

Conventional carbon grades such as 12Cr18Ni9 (302) and 06Cr19Ni10 (304) are most at risk after sensitization between about 450 °C and 850 °C. Susceptibility is mitigated by using extra-low-carbon grades (304L, 316L), stabilized grades with titanium or niobium additions (321, 347), solution annealing after fabrication, and controlled low-heat-input welding practices.

How is failure defined in a GB/T 4334 test?

Failure is defined as a ditch structure in Method A; a corrosion rate exceeding the limit in the product standard for Methods B and C; a rate ratio above the agreed threshold in Method D; or any intergranular crack or fissure on the bent tension surface in Method E. Acceptance limits for mass-loss methods are always those stated in the relevant product standard or the purchase agreement.

Do low-carbon and stabilized grades require sensitization before testing?

Only when specified. These grades are normally tested in the as-delivered condition, but where the service history or the specification requires it — for example, material that will be welded or heated in service — the specimens are given the standard sensitizing treatment of 650 °C for 2 hours with air cooling before the boiling test is performed.

How long does a GB/T 4334 test take?

Method A takes under an hour including microscopy. Method E requires roughly two days for the 16-hour boil plus bend examination. Method B requires about six days including the 120-hour boil and weighing. Method C is the longest, typically around twelve working days for the five 48-hour periods plus specimen preparation and evaluation. Turnaround should also account for machining, sensitization treatment, and reporting.