February 2, 2025
February 2, 2025
Practice F outlines the procedure for conducting the boiling copper–copper sulfate–50% sulfuric acid test, which evaluates the susceptibility of austenitic stainless steels to intergranular attack. The test provides a method to measure corrosion resistance under specific conditions, helping determine material integrity for applications requiring resistance to intergranular corrosion.
The results obtained from this test do not necessarily indicate the material’s performance in other corrosive environments. The test is designed to detect intergranular attack and does not predict resistance to general corrosion, pitting, or stress-corrosion cracking. Therefore, it is critical to consider additional corrosion tests if broader corrosion performance is required.
Before delving deep into Practice F, let’s briefly 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), Copper-Copper Sulfate-50% Sulfuric Acid (Practice F) etc. which quantitatively measures corrosion rates. Each test targets specific microstructural conditions, enabling engineers to assess material performance and compliance with application-specific requirements.
In the following sections, we will explore the apparatus, procedure, and applications of the Copper-Copper Sulfate-50% Sulfuric Acid Test in greater detail.
The copper–copper sulfate–50% sulfuric acid test involves immersing a specimen of the material under evaluation into a boiling solution of copper sulfate and sulfuric acid for a specified duration. Alongside the specimen, a piece of copper is also introduced into the solution. The role of this copper piece is to maintain a constant corrosion potential, ensuring that the test conditions remain stable and repeatable.
At the conclusion of the test, the mass loss of the specimen is measured. This loss is then converted into a corrosion rate, which is subsequently compared to a specified maximum allowable value for the particular grade of stainless steel being tested. If the measured corrosion rate falls within the acceptable range, the material is deemed resistant to intergranular attack. Otherwise, it may be considered susceptible, warranting further evaluation or alternative material selection.
The copper–copper sulfate–sulfuric acid test is specifically designed to detect intergranular attack due to chromium carbide precipitation. This phenomenon is particularly relevant to unstabilized cast austenitic stainless steels and some wrought stainless steel grades, where carbide precipitation can occur at grain boundaries during exposure to elevated temperatures.
One of the limitations of this test is that it does not effectively detect susceptibility to intergranular attack associated with sigma phase formation. Sigma phase is a brittle intermetallic compound that can develop in stainless steels after prolonged exposure to elevated temperatures, particularly in duplex and high-chromium austenitic grades. As a result, if sigma phase-induced intergranular attack is a concern, additional testing methods should be employed.
The corrosion potential of this test has been reported as approximately 0.1 V, which is significantly lower than that of other intergranular corrosion tests. For comparison:
The lower corrosion potential suggests that Practice F is a relatively mild test in comparison to other intergranular corrosion tests. It is particularly useful for applications where carbide precipitation is a primary concern, but additional evaluations may be needed to assess the full corrosion resistance of the material in different environmental conditions.
By carefully applying Practice F, engineers and material specialists can determine whether a given stainless steel exhibits acceptable resistance to intergranular attack, allowing them to make informed decisions about material selection.
Preliminary Screening: Before conducting the copper–copper sulfate–50% sulfuric acid test, certain grades of stainless steel, as specified in Table 6, may undergo a rapid screening test. This preliminary evaluation follows the procedures outlined in Practice A, the Oxalic Acid Etch Test for Classification of Etch Structures in Austenitic Stainless Steels. The preparation, etching, and classification of etch structures are described in detail within the referenced practice. The role of these etch structure evaluations in relation to the copper–copper sulfate–50% sulfuric acid test is given below:
| Grade | Acceptable Etch Structures | Suspect Etch Structures |
| CF-3M | Step, dual, isolated ferrite | Ditch, interdendritic ditches |
| CF-8M | Step, dual, isolated ferrite | Ditch, interdendritic ditches |
Heat Treatment Prior to Testing: To ensure accurate results, the material must undergo heat treatment at 650 to 675°C (1200 to 1250°F), before performing the etch test. This step is crucial in stabilizing the microstructure of the stainless steel and ensuring that the etch response is representative of the bulk material rather than transient process-related effects.
The test methodology accounts for two primary classes of specimens: Base metal and Process-affected metal.
When performing the etch test, it is essential to disregard "process-affected" areas. These regions may exhibit microstructural anomalies that are not indicative of the inherent corrosion resistance of the material.
Interpretation of Etch Structures: Corrosion test specimens that exhibit acceptable etch structures in the Oxalic Acid Etch Test can be expected to remain essentially free from intergranular corrosion when subjected to the copper–copper sulfate–50% sulfuric acid test. Consequently, these specimens are considered acceptable without further testing in the copper–copper sulfate–50% sulfuric acid environment.
However, specimens that display suspect etch structures must undergo additional evaluation using the copper–copper sulfate–50% sulfuric acid test. This secondary testing ensures that any potential susceptibility to intergranular corrosion is identified, allowing for a comprehensive assessment of the material’s resistance to corrosive degradation.
No substitutions for the designated condenser or flask are permitted. In particular, the cold-finger type of condenser with standard Erlenmeyer flasks is not allowed. The use of this alternative setup has been shown to produce lower corrosion rates than those obtained using the Allihn type of condenser. This discrepancy may result from vapor loss, increased oxygen content in the solution, or a combination of both factors. To maintain accuracy and comparability across tests, strict compliance with the prescribed apparatus is required.
Cupric Sulfate Pentahydrate (CuSO4·5H2O): The primary reagent, cupric sulfate pentahydrate, should contain approximately 64% cupric sulfate (CuSO4) by mass. This compound plays a critical role in establishing and controlling the corrosion potential. Since any variation in this reagent could impact test results, substitutions are strictly prohibited.
Sulfuric Acid (H2SO4): The sulfuric acid used in this test must have a concentration ranging from 95.0% to 98.0% by mass. Maintaining this specified concentration is essential for ensuring the intended chemical environment during testing.
Copper Metal Requirements: A piece of copper metal, approximately 3 mm × 20 mm × 40 mm (? in. × ¾ in. × 1½ in.), with a bright, clean finish, is required for the test. Alternatively, an equivalent surface area of copper shot or chips may be used.
Before use, the copper must be washed, degreased, and thoroughly dried to remove any contaminants that could affect test outcomes. To clean corrosion products from the copper surface, a brief rinse in a 5% sulfuric acid (H2SO4) solution is recommended. This step ensures the integrity of the copper material, allowing it to perform its intended role in the corrosion testing process.
To ensure the reliability and accuracy of the test, a 600 mL test solution must be prepared following a controlled procedure. Proper safety measures should be observed at all times during preparation, as the handling of concentrated sulfuric acid presents significant risks.
When handling sulfuric acid, it is critical to use protective equipment, including a face shield, rubber gloves, and an acid-resistant apron. The entire preparation process must be conducted under a fume hood to minimize exposure to fumes and potential splashes.
It has been reported that, under certain conditions, violent boiling may occur, potentially resulting in hazardous acid spills. To mitigate this risk, it is essential to ensure that the sulfuric acid concentration remains stable and that an adequate number of boiling chips are present throughout the heating process. Proper control of temperature and reaction conditions is critical in preventing uncontrolled reactions.
By following these precise preparation steps, the copper–copper sulfate–50% sulfuric acid test solution can be reliably prepared for subsequent corrosion testing.
The selection and preparation of test specimens play a crucial role in ensuring accurate and representative corrosion test results. For this procedure, only base metal samples should be obtained and prepared, as variations in material conditions can significantly impact corrosion behavior.
In corrosion testing, two distinct classes of specimens must be considered: base metal and process-affected metal. Process-affected metal includes areas that have undergone transformations capable of altering corrosion properties in a non-uniform manner. These conditions may arise from welding, carburization, nitriding, oxidation, mechanical deformation, or exposure to heat. In contrast, base metal is free from such localized effects and provides a uniform material condition suitable for reliable evaluation.
The testing procedure outlined in Practice F involves immersing the entire specimen in the test solution and calculating the corrosion rate based on the total mass loss across the entire specimen surface. Since process-affected metal alters corrosion behavior in localized regions, its presence within a specimen can introduce variability, leading to unpredictable results. For this reason, specimens containing process-affected regions should not be used for standard corrosion rate calculations.
Given that mass loss rates in process-affected areas differ from those of base metal, an alternative testing approach may be required if localized corrosion resistance is a concern. In such cases, tests that do not rely on averaging mass loss over the entire specimen surface—such as the Oxalic Acid Etch Test (Practice A) or the Copper–Copper Sulfate–16% Sulfuric Acid Test (Practice E)—should be considered. The selection of the appropriate test method and the acceptance criteria should be determined through an agreement between the purchaser and the producer.
Unless otherwise specified by the purchaser, the producer has the discretion to determine the procedures for obtaining representative base metal samples, removing test specimens, and selecting the number of specimens required for testing. This flexibility allows for adjustments based on material characteristics, production processes, and specific testing requirements while ensuring compliance with established corrosion evaluation standards.
Proper preparation of test specimens is essential for obtaining reliable and reproducible corrosion test results. The preparation process involves heat treatment, surface preparation, and pre-test measurements to ensure consistency and accuracy in evaluating material performance.
Extra-low carbon and stabilized grades of stainless steel must be heat-treated before testing. This treatment involves heating the material to a temperature range of 650 to 675°C (1200 to 1250°F), which corresponds to the maximum carbide precipitation range. The duration of heating and the subsequent cooling method used for this sensitization treatment should be determined through mutual agreement between the material producer and the purchaser. The most commonly used sensitization treatment is heating for one hour at 675°C (1250°F).
Each test specimen should have a total surface area between 5 and 20 cm² to ensure a sufficient exposure area for corrosion evaluation. Where feasible, all specimen surfaces should be ground using CAMI/ANSI 120 [FEPA/ISO P120] paper-backed, wet or dry, closed-coated abrasive paper, with water as a coolant. If abrasive paper is used in dry conditions, polishing should be performed slowly to prevent overheating. Abrasives that contain grinding aids must be avoided, as some grinding aids contain fluorides that could alter the measured corrosion rate.
To eliminate any residual oxide scale or heat tint formed during heat treatment, specimens should be thoroughly cleaned. If mechanical grinding does not remove all scale, such as in stamped numbers or intricate features, an appropriate pickling solution from ASTM Practice A380/A380M (Table A1.1) may be used. Residual oxide scale can cause galvanic action, leading to unintended activation of the material in the test solution, which may affect the test results.
Dimensional accuracy is critical in corrosion testing. Each specimen, including any inner surfaces of holes, should be measured to the nearest 0.05 mm (0.001 in.), and the total exposed surface area should be calculated accordingly.
Before testing, the specimens must be degreased using non-chlorinated cleaning agents, such as soap and lukewarm water or acetone. Once degreased, the specimens should be thoroughly dried and weighed to the nearest 0.001 g. To prevent contamination or oxidation before testing, the specimens should be stored in a desiccator until the corrosion test is performed.
To ensure precise and reproducible corrosion test results, the procedure must be carried out under controlled conditions with strict adherence to each step.
Before immersing the test specimen, the solution must be brought to a steady boiling state. If it is not already boiling, apply heat until the solution reaches the required temperature. Throughout the test, the flask should remain covered with a condenser, with cooling water flowing continuously, except when inserting or removing specimens. This prevents vapor loss and maintains the solution's concentration.
Once the solution has reached boiling, turn off the heat source and allow the boiling to subside. The prepared test specimen should then be placed in a second glass cradle to ensure proper suspension in the test solution. After uncovering the flask, carefully insert the specimen, ensuring minimal disturbance to the solution. Immediately replace the condenser, restore the cooling water flow, and turn the heat source back on to resume boiling.
To monitor potential vapor loss, mark the liquid level on the flask. Any significant decrease in liquid level indicates vapor loss, which could result in acid concentration changes. If an appreciable change in level is observed, the test should be repeated with a fresh solution and a newly reground specimen to maintain accuracy.
The specimen should remain immersed in the test solution for 120 hours. After the immersion period, remove the specimen and rinse it thoroughly with water and acetone to eliminate any residual solution. Once dried, any adherent copper deposits on the specimen’s surface may be removed by briefly immersing the specimen in concentrated nitric acid at room temperature.
Following cleaning, weigh the specimen and determine the mass loss by subtracting the final weight from the original weight. Intermediate weighings are generally not required, and the test can proceed without interruption. However, if preliminary results are needed, the specimen may be removed at any time for weighing.
No changes to the solution are required throughout the 120-hour test period. The test conditions should remain consistent to ensure reliable corrosion rate measurements.
The corrosion resistance of the material is evaluated by measuring the weight loss of the test specimen after exposure to the test solution. This weight loss serves as the basis for calculating the material’s corrosion rate.
The corrosion rate should be expressed in terms of millimeters of penetration per month, providing a standardized metric for assessing material degradation under the specified test conditions. The calculation should account for the total surface area of the specimen to ensure accuracy and comparability.
All test results should be documented in a detailed report, including the initial and final specimen weights, the total exposure time, and the calculated corrosion rate. Any observations related to specimen appearance, surface changes, or unexpected anomalies should also be recorded to provide a comprehensive evaluation of material performance.
The corrosion rate is expressed in millimeters of penetration per month, calculated using the following formula:
Millimeters per month=7305×W/(A×t×d)
Where:
The Copper–Copper Sulfate–50% Sulfuric Acid Test (Practice F) is a well-established method for evaluating the susceptibility of austenitic stainless steels to intergranular corrosion. By immersing specimens in a controlled corrosive environment, this test provides valuable insight into the material's resistance to chromium carbide precipitation-induced attack. The calculated corrosion rate serves as a quantitative measure, helping engineers and material specialists determine the suitability of a material for critical applications.
While Practice F is highly effective for detecting intergranular attack in certain stainless steel grades, it has limitations. Specifically, it is not effective in identifying susceptibility to intergranular corrosion caused by sigma phase formation. Additionally, it does not predict general corrosion, pitting, or stress-corrosion cracking behaviour, necessitating complementary testing for a comprehensive corrosion assessment.
Ultimately, Practice F remains a crucial tool in material selection and quality assurance, ensuring that stainless steels meet the necessary corrosion resistance requirements before deployment in industries such as chemical processing, power generation, and marine applications
