February 1, 2025
February 1, 2025
The Copper-Copper Sulfate–16% Sulfuric Acid Test, designated as Practice E in ASTM A262, is a standardized method for assessing the susceptibility of austenitic stainless steels to intergranular corrosion (IGC). This test evaluates the material's response under specific conditions, offering insights into its resistance to localized grain boundary corrosion.
It is important to note that the presence or absence of intergranular corrosion in this test does not necessarily predict the material's performance in other corrosive environments. Furthermore, the test is not designed to assess resistance to other forms of corrosion, such as general corrosion, pitting, or stress-corrosion cracking, limiting its application to intergranular attack evaluations.
Before delving deep into Practice E, let’s have a brief look at intergranular corrosion.
Intergranular corrosion (IGC) is a localized form of material degradation that occurs along the grain boundaries of stainless steels. This phenomenon is typically caused by chromium carbide precipitation or the formation of detrimental phases like sigma during high-temperature exposure or improper heat treatments. The depletion of chromium near grain boundaries reduces the material's corrosion resistance, leaving it susceptible to attack in specific environments. IGC is a critical concern in industries where stainless steels are used, such as chemical processing, power generation, and marine applications, where corrosion resistance is vital for safety and performance.
To evaluate and classify the susceptibility of stainless steels to intergranular corrosion, the ASTM A262 standard outlines a series of test practices. These practices are designed to simulate conditions that reveal intergranular attack and provide insights into the material's resistance. The standard includes multiple practices, such as the Oxalic Acid Etch Test (Practice A), which provides rapid qualitative screening, and Ferric Sulfate–Sulfuric Acid Test (Practice B), which quantitatively measures corrosion rates. Each test targets specific microstructural conditions, enabling engineers to assess material performance and compliance with application-specific requirements.
For more details, visit our detailed article on intergranular corrosion: Intergranular corrosion - A Comprehensive Guide
In the following sections, we will explore the apparatus, procedure, and applications of the Copper-Copper Sulfate–16% Sulfuric Acid Test in greater detail.
Rapid Screening Test: Before subjecting specimens to the Copper-Copper Sulfate–16% Sulfuric Acid Test, a rapid screening test can be conducted using the Oxalic Acid Etch Test (Practice A). This pre-screening provides a quick assessment of the material's etch structure and identifies whether further testing is necessary. The use of the Oxalic Acid Etch Test as a preliminary step enhances efficiency and ensures that only necessary specimens undergo further testing, saving time and resources while maintaining accuracy.
Do you want to know more details about Practice A and how it is conducted? Check out our detailed blog here: Practice A - Oxalic Etch Test
Correlation Between Etch Structures and Corrosion Susceptibility:
Heat Treatment Before Screening: Certain grades of stainless steel may require heat treatment before conducting the rapid screening test. This step ensures that the material's microstructure reflects the conditions it would experience in service, providing accurate and representative test results.
In the Copper-Copper Sulfate–16% Sulfuric Acid Test, a representative sample of austenitic stainless steel is embedded in copper shot or grindings and exposed to a boiling acidified copper sulfate solution for 15 hours. This procedure evaluates the material's susceptibility to intergranular corrosion through a combination of exposure and mechanical stress.
After the 15-hour exposure, the specimen is subjected to bending to reveal any weaknesses. The presence of intergranular cracking or crazing on the bent specimen indicates susceptibility to intergranular attack. This test is highly effective in identifying localized corrosion along grain boundaries caused by improper heat treatments or exposure to corrosive environments.
Suppliers may opt to perform alternative testing methods under specific conditions unless the purchaser explicitly prohibits this in the purchase order. One such alternative is outlined below:
ISO 3651-2, Method A:
When the alternative test procedure is used, this must be explicitly noted in the test report. Clear documentation ensures transparency in the testing process and provides traceability, ensuring the purchaser is aware of the method used to evaluate the material's corrosion resistance.
Allihn Condenser: An Allihn condenser with a minimum of four bulbs and a ground glass joint matching that of the Erlenmeyer flask is a required component for this test. The use of substitutions, such as the cold-finger type of condenser with standard Erlenmeyer flasks, is not permitted. This is because cold-finger condensers can result in lower corrosion rates due to vapor loss or increased oxygen content in the solution, potentially leading to the acceptance of materials that should otherwise be rejected.
1-Liter Erlenmeyer Flask: The apparatus includes a 1-L Erlenmeyer flask with a ground glass joint compatible with the condenser. The opening of the flask limits the size of the specimen, so a larger opening is desirable to accommodate various specimen dimensions effectively.
Glass Cradle: The use of a glass cradle is necessary for supporting the specimen within the flask. These cradles can be sourced from a glassblowing shop and must be appropriately sized to fit through the flask opening along with the specimen. They should also allow for the free flow of the testing solution around the specimen. Alternative specimen supports, such as glass hooks or stirrups, may also be used, provided they ensure unrestricted circulation of the solution around the specimen.
Boiling Chips: They are essential to prevent bumping during the boiling process, ensuring a smooth and controlled reaction environment.
High Vacuum Silicone Grease: The ground glass joint of the flask requires the application of high vacuum silicone grease to create a secure and airtight connection between the flask and the condenser.
Hot Plate: A hot plate capable of providing consistent heat for the continuous boiling of the solution is required. The heat source should be reliable and adjustable to maintain a steady boiling condition throughout the test.
Analytical Balance: An analytical balance with a precision of 0.001 g is necessary for accurately measuring the weight of specimens both before and after the test. This level of precision is critical for calculating corrosion rates reliably.
Desiccator: A desiccator is used for storing prepared specimens before testing. It protects the specimens from moisture and contamination, which could otherwise compromise the accuracy of the test results. This well-defined apparatus ensures that the testing process is conducted under controlled conditions, yielding consistent and reliable results for evaluating material susceptibility to intergranular corrosion.
Specimen Supports: The use of an open glass cradle is recommended for supporting specimens along with copper shot or grindings inside the test flask. This setup ensures proper exposure of the specimen to the test environment while allowing free circulation of the test solution. For larger specimens, such as those made from heavy bar stock, it may be necessary to embed the specimen in a layer of copper shot at the bottom of the flask. In such cases, a copper cradle may also be used as an alternative support to maintain proper contact with the copper shot or grindings and ensure effective test conditions.
Heat Source: A gas or electrically heated hot plate is required to heat the test solution and maintain it at a continuous boiling state throughout the test period. The heat source should provide consistent and uniform heating to avoid fluctuations in the test solution's temperature, ensuring reliable and repeatable test results.
Acidified Copper Sulfate Test Solution: The test solution is prepared by dissolving 100 g of reagent-grade copper sulfate (CuSO4·5H2O) in 700 mL of distilled water. Then, 100 mL of concentrated sulfuric acid (H2SO4, sp gr 1.84) is added, and the mixture is diluted to 1000 mL with distilled water.
This specific composition ensures the solution is optimized for evaluating the susceptibility of stainless steels to intergranular corrosion under controlled conditions.
Copper Addition: The addition of electrolytic-grade copper shot or grindings is essential for the test. Among these, copper shot is preferred due to its ease of handling both before and after the test. A sufficient quantity of copper shot or grindings must be used to cover all surfaces of the test specimen, regardless of whether the specimen is supported in a vented glass cradle or embedded in a layer of copper shot at the bottom of the flask.
The amount of copper used is not critical as long as an excess of metallic copper is present in the system. However, the galvanic coupling between the copper and the test specimen plays a significant role in ensuring effective testing. This coupling promotes a controlled electrochemical interaction that simulates real-world corrosion conditions, allowing for accurate evaluation of the material's susceptibility to intergranular attack.
Reuse of Copper Shot or Grindings: Copper shot or grindings used in the Copper-Copper Sulfate–16% Sulfuric Acid Test can be reused for subsequent tests, provided they are thoroughly cleaned in warm tap water after each test. Proper cleaning ensures that residual contamination does not interfere with subsequent testing, maintaining the integrity of the results.
The preparation of test specimens is a critical step in ensuring accurate and reliable results. The following guidelines outline the procedures for preparing specimens:
The size and specific area from which the specimen is taken, such as the end or middle of a coil or the midway surface and centre, are typically outlined in the agreement between the purchaser and the seller.
The testing apparatus dictates the final size and shape of the test specimen. Specimens should be designed for easy insertion and removal through the neck of the test container. The table given below (adopted from Table 5 of ASTM A262) provides a reference for determining acceptable specimen sizes, taking into account potential restrictions imposed by the testing apparatus.
| Material | Size of Test Specimen | |
Wrought wire or rod: Up to 6 mm (¼ in.) in diameter, incl Over 6 mm (¼ in.) in diameter |
Full diameter by 75 mm (3 in.) (minimum) long
Cylindrical segment 6 mm (¼ in.) thick by 25 mm (1 in.) (maximum) wide by 75 to 125 mm (3 to 5 in.) long - When bending such specimens, the curved surface should be on the outside of the bend | |
Wrought sheet, strip, plates, or flat rolled products: Up to 5 mm (3/16 in.) thick Over 5 mm (3/16 in.) thick |
Full thickness by 9 to 25 mm (3/8 to 1 in.) wide by 75 mm (3 in.) (minimum) long 5 to 13 mm (3/16 to ½ in.) thick by 9 to 25 mm (3/8 to 1 in.) wide by 75 mm (3 in.) (minimum) long. One surface shall be an original surface of the material under test and it shall be on the outside of the bend. Cold-rolled strip or sheets may be tested in the thickness supplied. | |
Tubing Up to 38 mm (1½ in.) in diameter, incl Over 38 mm (1½ in.) in diameter |
Full ring, 25 mm (1 in.) wide A circumferential segment 75 mm (3 in.) (minimum) long cut from a 25 mm (1 in.) wide ring. Specimens from welded tubes over 38 mm (1½ in.) in diameter shall be taken with the weld on the axis of the bend. | |
This table provides standardized dimensions for test specimens based on material type and size, ensuring compatibility with testing apparatus and uniformity in corrosion evaluations.
If specimens are obtained by shearing, the sheared edges must be machined or ground off prior to testing. Grinding should be performed carefully to avoid overheating or burning the edges, as these can affect the test results. A squared edge is desirable for consistent evaluation.
Scale Removal: Any scale present on the specimens should be removed prior to testing.
Degreasing: Each specimen should be degreased using a suitable cleaning solvent such as:
This step ensures the removal of oils, grease, and contaminants that could interfere with the corrosion testing process.
Testing of Austenitic Stainless Steels in As-Received Condition: Austenitic stainless steels in their "as-received" (mill-annealed) condition should inherently meet the requirements of this test without additional treatment. This reflects the material's inherent resistance to intergranular attack under normal manufacturing conditions.
For extra-low-carbon and stabilized grades, specimens are subjected to a sensitizing heat treatment before testing. This heat treatment is conducted in the range of 650 to 675°C (1200 to 1250°F), which corresponds to the range of maximum carbide precipitation.
The volume of the acidified copper sulfate test solution should be sufficient to fully immerse the specimens and ensure a minimum ratio of 8 mL/cm² (50 mL/in²) of specimen surface area. This ensures that the test solution adequately covers the specimens and maintains the appropriate conditions for the evaluation.
As many as three specimens can be tested in the same container. Ideally, all specimens within a flask should belong to the same grade of material, although this is not strictly required as long as the solution volume-to-sample area ratio is maintained.
The test specimens should be immersed in the test solution at ambient temperature. The solution is then brought to a boil and maintained at boiling throughout the duration of the test period. The timing of the test begins when the solution reaches its boiling point.
To minimize bumping of the solution when using glass cradles to support the specimens, placing a small amount of copper shot (approximately eight to ten pieces) at the bottom of the flask is recommended. This helps stabilize the boiling process.
The standard test duration is a minimum of 15 hours unless a longer time is agreed upon between the purchaser and producer. If a test period longer than 15 hours is used, it should be clearly specified in the test report. Fresh test solution is not required for extended test durations of 48 or even 72 hours.
If adherent copper remains on the specimen after the test, it can be removed by briefly immersing the specimen in concentrated nitric acid at room temperature. This step ensures accurate post-test evaluation and preserves the integrity of the results.
The bend test is a critical part of Practice E, designed to evaluate the material's susceptibility to intergranular corrosion after exposure to the acidified copper-copper sulfate-sulfuric acid solution. During this test, the specimen is bent through 180° over a diameter equal to the thickness of the specimen being tested. Under no circumstances should the specimen be bent over a smaller radius or through a greater angle than specified in the relevant product specification.
For materials with low ductility, such as severely cold-worked materials, a 180° bend may be impractical. In such cases, users must notify the testing personnel about the material's highly stressed condition. The maximum achievable angle of bend without causing cracks should be determined using an untested specimen with the same configuration. This determined angle is then applied during the evaluation of specimens exposed to the test solution, ensuring a consistent approach. The angle of bend used in testing must be clearly reported in the final documentation.
Duplicate specimens should be prepared from sheet materials to ensure both sides of the rolled sample are bent. This approach helps detect intergranular attack caused by carburization on one side of the material during the final stages of rolling. Each duplicate specimen must be identified so that both surfaces of the sheet are subjected to the tension side of the bends during testing.
For samples machined from round sections or cast materials, the curved or original surface should remain on the outside of the bend to ensure accurate testing of the relevant surface conditions.
Specimens are typically bent using a vise and a hammer. The bend is initiated with a hammer and completed by bringing the two ends together in the vise. For heavier specimens, specialized bending fixtures or an air or hydraulic press may be required to facilitate bending without damaging the material.
Tubular products require a flattening test as specified in Test Methods and Definitions A370. This ensures the integrity of the test and enables proper evaluation of the material's resistance to intergranular attack.
When testing austenitic stainless steel plates that are 4.76 mm (0.1875 in.) or thicker, specific guidelines apply if agreed upon by the purchaser and producer:
The bend test procedures ensure that the material's intergranular corrosion susceptibility is thoroughly evaluated under controlled conditions, reflecting its real-world application and performance. Properly conducted bend tests provide critical data for assessing the reliability and durability of stainless steels in corrosive environments.
The evaluation of bend specimens is a crucial step in determining the presence of intergranular attack after exposure to the acidified copper-copper sulfate-sulfuric acid test solution. The bent specimen should be examined under low magnification, typically between 5× and 20×. The observation of fissures or cracks on the surface of the specimen is an indication of intergranular attack and warrants further investigation.
If the evaluation results are inconclusive, a metallographic examination should be conducted. This involves analyzing the outer radius of a longitudinal section of the bend specimen under a magnification of 100× to 250×. This detailed examination provides a more definitive assessment of the presence or absence of intergranular attack.
Any cracking that originates at the edge of the specimen should be disregarded during evaluation, as these are likely not related to intergranular attack but rather to specimen preparation or handling. Similarly, surface features such as deformation lines, wrinkles, or "orange peel" texture without accompanying fissures or cracks should also be disregarded as they do not indicate intergranular attack.
In cases where cracks are suspected to arise from poor ductility rather than corrosion, further investigation is required. This can be achieved by bending a similar specimen that has not been exposed to the boiling test solution. A visual comparison between the tested and untested specimens can help clarify whether the observed cracks are due to corrosion or inherent material properties, such as low ductility.
This multi-step evaluation process ensures that conclusions regarding intergranular attack are accurate and based on clear evidence, minimizing the risk of misinterpretation and ensuring reliable results for material performance assessments.
The Copper-Copper Sulfate–16% Sulfuric Acid Test (Practice E) is a critical method for evaluating the susceptibility of austenitic stainless steels to intergranular corrosion (IGC). By simulating service conditions that may lead to localized grain boundary attacks, this test helps engineers, manufacturers, and quality control professionals assess the integrity and corrosion resistance of stainless steel materials.
The combination of chemical exposure and mechanical stress (bend test) ensures a thorough evaluation, making this method particularly valuable in industries.
Interested to know about other methods to assess intergranular corrosion? Check out our blogs here:
