Intergranular Corrosion Testing (IGC)
Intergranular corrosion (IGC) attacks along the grain boundaries of materials like steel, stainless steel, and alloys. It often causes significant structural damage before any surface-level signs appear. Identifying susceptibility to intergranular corrosion early is critical to material qualification, weld validation, and long-term component integrity. At WH Labs, our dedicated team of metallurgists provides intergranular corrosion testing services in accordance with ASTM A262, ASTM G28, and ASTM A923 to help you evaluate your materials and make informed decisions before they go into service.
What is Intergranular Corrosion (IGC)?
Intergranular corrosion targets the grain boundaries of a material while leaving the grains themselves intact. The result is a progressive loss of cohesion between grains, which significantly reduces mechanical strength and ductility, often without visible surface damage. Austenitic stainless steels are especially susceptible, as chromium carbides can form at grain boundaries during heat treatment or welding, locally depleting the chromium content below the threshold required for corrosion resistance.
Why Does Intergranular Corrosion Happen?
Susceptibility to intergranular corrosion is directly linked to the precipitation of chromium carbides at grain boundaries. It is a process triggered by exposure to temperatures between 425°C and 815°C, commonly referred to as the sensitization range. As chromium carbides form, the chromium concentration in the surrounding grain boundary region drops below the 12% threshold required for corrosion resistance, leaving those areas vulnerable to attack.
The presence of other elements, such as nickel and molybdenum, can also influence a material’s resistance to IGC. Nickel can enhance the overall corrosion resistance, while molybdenum can improve resistance to pitting and crevice corrosion, which are other forms of localized corrosion. However, the interplay of these elements during processes like welding and heat treatment can sometimes create conditions favorable for IGC.
How Do We Test for Intergranular Corrosion?
Several standardized test methods exist to evaluate a material’s susceptibility to IGC. The appropriate method depends on the material grade and the corrosion mechanism being assessed:
Huey Test
The Huey test (ASTM A262 Practice C) involves immersing a specimen in a 65% nitric acid solution and boiling it for five consecutive 48-hour periods. After each period, the specimen is removed, cleaned, and weighed. Results are expressed as a corrosion rate in inches per month (ipm), calculated from the cumulative weight loss across all five intervals. This method is particularly effective at detecting sensitization caused by chromium carbide precipitation at grain boundaries. Note that Practice C is only appropriate for molybdenum-free grades such as 304L. Alloys containing molybdenum show artificially high attack rates in nitric acid and should be tested using Practice B or E instead.
Strauss Test
The Strauss test (ASTM A262 Practice E) evaluates intergranular attack susceptibility in austenitic stainless steels by immersing a specimen in a boiling solution of copper sulfate and 16% sulfuric acid for 15 hours. After immersion, the specimen is bent 180° and examined visually for cracking along the grain boundaries. Cracking indicates susceptibility to intergranular attack; no cracking is a pass. Practice E is suitable for all 300 series grades, including low-carbon and stabilized grades.
Selecting the appropriate test method depends on the material grade, its elemental composition, and the specific corrosion mechanism being evaluated.
Other IGC Tests
For aluminum alloys, different tests like the NAMLT (Nitric Acid Mass Loss Test) or the copper sulfate test are used. The NAMLT test measures the mass loss of an aluminum sample after exposure to a nitric acid solution, while the copper sulfate test involves immersing the material in a copper sulfate solution and examining it for signs of corrosion.
How to Choose the Right IGC Test Method?
The right IGC test depends on your material type first. ASTM A262 covers austenitic stainless steels (300 series grades like 304L and 316L). ASTM G28 is used for wrought nickel-based alloys such as Alloy 625 and Alloy 825. For duplex and super duplex stainless steels, ASTM A923 is the correct standard. A262 was not designed for these grades.
Within ASTM A262, start with Practice A (oxalic acid etch) only if you need a fast screening pass. It takes four hours and tells you whether a material is acceptable or needs further testing. It cannot quantify corrosion or reject a material on its own. For a quantitative result, Practice B (Streicher) measures weight loss after 24-120 hours. Practice E (Strauss) is shorter at 15 hours and gives a qualitative result via a bend test. Practices B and E are both suitable for all 300 series grades. Practice F is specifically recommended for cast 316-type alloys.
One critical restriction: do not use Practice C (Huey test) on molybdenum-containing grades such as 316L. Molybdenum causes abnormally high attack rates in nitric acid, producing misleading results. The Huey test is reserved for Mo-free grades like 304L or applications involving direct nitric acid service.
A practical selection summary:
| Material | Recommended standard | Notes |
|---|---|---|
| 304L, 316L (austenitic SS) | ASTM A262 Practice B or E | Practice A first if a quick screening is needed |
| 316L with Mo | ASTM A262 Practice B or E — not C | The Huey test is unsuitable for Mo-bearing grades |
| Cast 316-type | ASTM A262 Practice F | Weight-loss quantification |
| Alloy 625, 825, C-276 (nickel alloys) | ASTM G28 Method A | Method B for highly corrosion-resistant grades |
| Duplex/super duplex (2205, 2507) | ASTM A923 | A262 E is an acceptable secondary option |
Intergranular Corrosion in Different Materials
Different materials have different susceptibilities and test methods:
- 5xxx Series Aluminum: Tested per ASTM G67 (NAMLT), which measures mass loss after immersion in concentrated nitric acid, and ASTM G110, which evaluates IGC susceptibility after sensitizing heat treatment and immersion in a sodium chloride and hydrogen peroxide solution. These tests are standard requirements for aluminum alloys used in marine and aerospace applications.
- Duplex Stainless Steels: Tested per ASTM A923, which covers cast and wrought duplex austenitic/ferritic stainless steels and is used to detect detrimental intermetallic phases. Despite their improved resistance to stress corrosion cracking compared to standard austenitic grades, duplex steels remain susceptible to IGC if incorrectly heat-treated or welded.
Understanding these factors is crucial for preventing problems like stress corrosion cracking, galvanic corrosion, or sour service corrosion in industrial settings. For instance, in the oil and gas industry, materials are often exposed to sour environments containing hydrogen sulfide, which can exacerbate IGC and other forms of corrosion. On the other hand, crevice corrosion testing is critical in industries such as the marine and offshore industry to help engineers choose the correct materials for the equipment, compare grades (304 stainless steel vs 316 vs duplex, etc.), and validate product quality and compliance.
Importance of Intergranular Corrosion Tests
IGC testing serves two critical functions for manufacturers and engineers:
Identifying Weaknesses
By conducting IGC tests, manufacturers can spot potential weak spots in materials. This is vital for industries like aerospace, automotive, and construction, where material failure can have catastrophic consequences. Knowing a material’s weaknesses means you can either choose a better material or modify the existing one to improve its resistance.
For example, in the aerospace industry, manufacturers subject components made from high-strength aluminum alloys to rigorous testing to ensure they won’t fail during flight. Similarly, in the automotive industry, engineers must test parts exposed to road salts and other corrosive substances to guarantee their longevity and safety.
Ensuring Longevity
Understanding how materials degrade due to Intergranular Corrosion helps engineers develop strategies to extend the lifespan of components. This could involve selecting more resistant materials, altering heat treatments, or applying protective coatings. Engineers can also use the data from IGC tests to design components that minimize the risk of corrosion by avoiding sharp corners and crevices where corrosion can initiate.
In the construction industry, ensuring the longevity of materials used in infrastructure projects like bridges and buildings is crucial. By testing for IGC, engineers can select materials that will remain durable and safe for decades, reducing maintenance costs and preventing structural failures.
Types of Intergranular Corrosion
There are different types of intergranular corrosion, including:
- Intergranular Attack (IGA): General attack along grain boundaries. This type of corrosion can weaken the material and lead to fractures under stress.
- Intergranular Stress Corrosion Cracking (IGSCC): Cracks that form along grain boundaries due to stress and corrosion. IGSCC is particularly dangerous because it can cause sudden and catastrophic failures in structures like pipelines and pressure vessels.
- Intergranular Selective Leaching: Removal of one element from the alloy, weakening the grain boundaries. An example of selective leaching is the dezincification of brass, where zinc is preferentially removed, leaving behind a porous, weakened structure.
Knowing the specific type helps engineers take targeted actions to prevent damage. For instance, by understanding the conditions that lead to IGSCC, engineers can design systems to minimize residual stresses and avoid environments that promote corrosion.
Intergranular Corrosion Test Methods
These ASTM standards define methods developed by industry experts to achieve consistent intergranular corrosion testing results.
ASTM A262 Test Method
ASTM 262 is a standard method for intergranular corrosion testing in stainless steels. It helps detect potential weaknesses that could lead to failure. By using this method, engineers can catch problems early and save resources by preventing future corrosion issues.
This test method includes several practices, such as Practice A (Oxalic Acid Etch Test), Practice B (Streicher Test), Practice C (Huey Test), and Practice E (Strauss Test). Each practice is designed to assess different aspects of IGC susceptibility and provides valuable information about the material’s behavior under corrosive conditions.
ASTM G28 Test Method
Used primarily for nickel-rich and nickel-chromium alloys, this test exposes materials to harsh environments to assess their resistance to IGC. It’s crucial for industries where corrosion resistance is key, like chemical processing and power generation.
The ASTM G28 test method includes two practices: Practice A (Ferric Sulfate-Sulfuric Acid Test) and Practice B (Copper-Copper Sulfate-50% Sulfuric Acid Test), which is a more aggressive test than Method B. These practices help identify materials that may suffer from IGC and guide the selection of suitable materials for demanding applications.
How to Interpret Intergranular Corrosion Test Results
Understanding your IGC test report goes beyond a simple pass or fail. The way results are read depends entirely on which test method was used, and knowing what to look for can save you from making the wrong material decision.
Reading the IGC Test Report
Practice A (oxalic acid etch) results are read visually under a microscope, where the structure is classified as acceptable or suspect. A suspect result does not mean rejection. It means a quantitative follow-up test is required. For weight-loss methods, such as Practice B (Streicher) and Practice F, results are expressed as a corrosion rate in mm/yr. Importantly, ASTM A262 does not define a universal pass/fail threshold. The acceptable rate is agreed upon between producer and purchaser based on the intended service environment. On the other hand, Practice E (Strauss) is evaluated after bending the specimen 180°: cracking along grain boundaries indicates susceptibility; no cracking is a pass.
Factors Affecting the IGC Test Results
A failed result does not automatically mean a scrapped batch. In many cases, it points to sensitization during welding or incorrect heat treatment, both correctable upstream issues. Testing early in the production cycle rather than post-fabrication is always the more cost-effective approach. If your material certificate shows a corrosion rate without agreed acceptance criteria, that number alone is not sufficient to make a material decision. Our metallurgists at WH Labs can help you interpret results in the context of your specific application and service environment.
Conclusion
Intergranular corrosion testing is a critical part of materials science. It helps ensure that materials will withstand their intended use without failing unexpectedly. By understanding and testing IGC, we can build safer, longer-lasting products. At our ISO/IEC 17025:2017 and ISO 9001:2015-accredited laboratory, we provide expert guidance on implementing these preventive measures based on the intergranular corrosion testing results. If you need help selecting the right test method, interpreting results, or qualifying a material for a specific service environment, contact our team at WH Labs to discuss your requirements.
