Intergranular Attack (IGA) and its Importance
What Is Intergranular Attack?
Intergranular attack (IGA) is a form of localised corrosion that preferentially occurs along the grain boundaries of a metal, while leaving the bulk of the grains relatively unaffected. The result is a network of corrosion damage that follows the grain boundary structure of the alloy, weakening the material’s mechanical integrity without producing obvious surface deterioration visible to the naked eye.
IGA is a critical concern in the stainless steel, nickel alloy, and aluminium alloy industries, particularly in applications involving corrosive chemical environments such as those encountered in the chemical processing, power generation, marine, and nuclear industries.
Causes of Intergranular Attack
Sensitisation in Austenitic Stainless Steel
The most common cause of IGA in stainless steel is sensitisation—a metallurgical phenomenon that occurs when the steel is exposed to temperatures between approximately 425°C and 870°C (the sensitisation range). At these temperatures, chromium migrates to grain boundaries and combines with carbon to form chromium carbide (Cr₂₃C₆) precipitates.
This chromium depletion creates narrow zones along grain boundaries that are low in chromium (below the ~12% threshold needed for passivity) and therefore highly susceptible to corrosion. In a corrosive environment, these depleted zones corrode preferentially, resulting in IGA.
Sources of sensitisation include:
- Slow cooling after welding (heat-affected zone sensitisation)
- Improper heat treatment
- Prolonged service at elevated temperatures
Intergranular Corrosion in Aluminum Alloys
In aluminium alloys (particularly 2xxx and 7xxx series), IGA occurs due to the preferential precipitation of second-phase particles (CuAl₂, MgZn₂) at grain boundaries, creating galvanic couples between the precipitate-enriched boundary and the depleted adjacent zone.
Detection and Testing for IGA
ASTM A262 – Standard Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steel
ASTM A262 defines five practices (A through E) for evaluating IGA susceptibility:
Practice | Test Method | What It Detects |
A | Oxalic acid etch | Screening for sensitised structure |
B | Ferric sulfate–sulfuric acid | General IGA susceptibility |
C | Nitric acid immersion | IGA in high-Si stainless |
D | Nitric–hydrofluoric acid | IGA in Mo-bearing grades |
E | Copper–copper sulfate–sulfuric acid (Strauss) | IGA from sensitisation |
ASTM G28 – Wrought Nickel-Rich Chromium-Bearing Alloys
Standard test methods for detecting susceptibility to IGA in nickel alloys (e.g., Alloy 600, 625, 825).
Metallographic Examination
Optical microscopy of cross-sections reveals intergranular penetration depth and extent. SEM-EDS identifies the chromium-depleted zones and confirms precipitate chemistry.
Prevention of Intergranular Attack
- Low-carbon grades (304L, 316L): Reducing carbon content below 0.03% minimizes carbide precipitation during welding.
- Stabilised grades (321, 347): Addition of titanium or niobium preferentially forms carbides with these elements rather than chromium, preventing sensitisation.
- Solution annealing: Heat treatment above the sensitisation range followed by rapid quenching dissolves chromium carbides and restores corrosion resistance.
- Careful welding procedure: Controlling interpass temperature and heat input minimises HAZ sensitisation.
Conclusion
Intergranular attack is a subtle yet highly damaging form of corrosion that targets grain boundaries, significantly weakening materials without obvious surface signs. Driven primarily by microstructural changes such as sensitisation or precipitate formation, it poses serious risks in critical applications involving stainless steels, nickel alloys, and aluminium alloys.
Through proper material selection, controlled processing, and standardised testing, the risk of IGA can be effectively minimised. Ultimately, understanding and preventing intergranular attack is essential for ensuring long-term structural integrity, corrosion resistance, and safety in demanding industrial environments.
Why Choose Infinita Lab for IGA Testing?
Infinita Lab is a leading provider of intergranular attack testing per ASTM A262, ASTM G28, and related standards. Our nationwide network of accredited corrosion testing laboratories delivers accurate, fast, and well-documented results for stainless steel and nickel alloy qualification.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you. Request a Quote
Frequently Asked Questions (FAQs)
How is IGA different from pitting corrosion? IGA specifically follows grain boundaries, creating interconnected pathways of damage through the microstructure. Pitting corrosion is localized at random surface sites where passive film breakdown occurs. IGA can degrade mechanical properties dramatically with little visible surface damage, while pitting produces discrete surface cavities.
Which ASTM A262 practice is most commonly specified for stainless steel weld qualifications? ASTM A262 Practice E (copper–copper sulfate–sulfuric acid, Strauss test) is the most widely specified for weld procedure qualification and production control of austenitic stainless steels in chemical process equipment and pressure vessels.
Can solution-annealed stainless steel be re-sensitized? Yes. If solution-annealed stainless steel is subsequently heated into the sensitization range (425–870°C) during fabrication (e.g., welding), it can be re-sensitized. Low-carbon or stabilized grades minimize this risk.
What is the weld decay phenomenon? Weld decay refers to IGA that occurs in the heat-affected zone (HAZ) of a weld in sensitized austenitic stainless steel—typically 5–15 mm from the fusion line, where the temperature cycle passed through the sensitization range. It can result in grain boundary dissolution in service.
Is IGA testing destructive? Yes. ASTM A262 tests require specimen preparation, immersion in corrosive test solutions, and either metallographic cross-section examination or bend testing after immersion—all of which are destructive. Sufficient material must be available for specimen preparation.
