When stainless steels (such as 304 and 316L) are exposed to specific environments, grain boundaries are preferentially corroded, causing sudden fracture of the material without obvious deformation. This phenomenon is known as intergranular corrosion (IGC). To evaluate the intergranular corrosion resistance of stainless steels, the national standard specifies several test methods, among which the GB/T 4334 series is the most widely used. The following is a systematic explanation of the test methods for intergranular corrosion of stainless steels.
I. Mechanism of Intergranular Corrosion (Brief Principle)
Intergranular corrosion mainly occurs when stainless steel is heated and held in the temperature range of 450℃ ~ 850℃ (e.g., in the heat-affected zone during welding). Chromium (Cr) at grain boundaries combines with carbon to precipitate chromium carbide (Cr₂₃C₆). This results in a chromium-depleted zone near grain boundaries, where corrosion resistance is lost, while the grain interior remains corrosion-resistant. Corrosion propagates along grain boundaries like a spiderweb, eventually leading to brittle fracture.
II. Five Commonly Used Test Methods (in accordance with GB/T 4334)
Different methods are selected for different types of stainless steels and service environments.
1. Method A: Copper Sulfate Test (CuSO₄ + H₂SO₄ + Cu)
Applicable materials: Austenitic stainless steels (304, 316, etc.), austenitic-ferritic duplex stainless steels.Principle: The specimen is boiled in a copper sulfate‑sulfuric acid solution with copper chips added to prevent copper precipitation on the specimen surface.Evaluation: After testing, bend the specimen by 180° or 90° and inspect the bent surface for cracks.Features: A classic method, mainly used to assess sensitization in welded heat-affected zones.
2. Method B: Sulfuric Acid‑Ferric Sulfate Test (H₂SO₄ + Fe₂(SO₄)₃)
Applicable materials: Austenitic stainless steels, ferritic stainless steels, duplex stainless steels.Principle: Immerse the specimen in boiling sulfuric acid‑ferric sulfate solution for a specified period.Evaluation: Determine the corrosion rate (weight loss method) or perform a bending test.Features: Suitable for evaluating intergranular corrosion resistance in strong oxidizing acids.
3. Method C: 65% Nitric Acid Test (HNO₃)
Applicable materials: Austenitic stainless steels, ferritic stainless steels.Principle: Immerse the specimen in boiling 65% nitric acid solution.Evaluation: Measure the corrosion rate, usually over multiple immersion periods.Features: Primarily used for intergranular corrosion testing of ferritic stainless steels, and for nitric acid resistance of austenitic stainless steels.
4. Method D: Nitric Acid‑Hydrofluoric Acid Test (HNO₃ + HF)
Applicable materials: Austenitic stainless steels, duplex stainless steels.Principle: Immerse the specimen in nitric acid‑hydrofluoric acid solution at room temperature.Evaluation: Measure the corrosion rate.Features: Hydrofluoric acid dissolves the passive film and accelerates grain boundary attack; especially effective for duplex stainless steels.
5. Method E: Sulfuric Acid‑Copper Sulfate Test with Copper Chips
(Similar to Method A but with a different solution)Applicable materials: Mainly for intergranular corrosion testing of welded joints of austenitic stainless steels.Features: More stringent than Method A; sometimes used for specific high‑end materials.
III. General Test Procedure
The basic flow is similar for all methods:
1. Sampling
Cut specimens from steel plates, pipes or welded joints.Note: The specimen surface must be smooth and free of work-hardened layers (usually pickled or electropolished).
2. Sensitization Treatment (optional)
To detect potential intergranular corrosion susceptibility, heat the specimen at 650℃ for a specified time (e.g., 1 hour) to artificially induce sensitization.For welded components, test specimens are taken directly from the as-welded heat-affected zone.
3. Immersion Corrosion Test
Place the specimen in the specified corrosive solution and immerse it at the temperature and duration specified in the standard (usually boiling).
4. Cleaning and Drying
Remove the specimen, clean off corrosion products, and dry.
5. Result Evaluation
- Bend test: Bend the specimen and inspect for cracks under magnification.
- Weight loss method: Weigh the specimen before and after testing, then calculate the corrosion rate in g/(m²·h).
