The Ferric Sulfate–Sulfuric Acid Test, defined as Practice B in ASTM A262, is designed to evaluate the material's susceptibility to intergranular attack (IGA) by measuring its corrosion rate under specific conditions. It involves immersing austenitic stainless steel specimens in a boiling solution of ferric sulfate and sulfuric acid for a duration of 120 hours.
The results of this test reflect susceptibility to intergranular corrosion but are not necessarily indicative of the material's performance in other corrosive environments. The test does not predict resistance to other forms of degradation such as general corrosion, pitting, or stress-corrosion cracking, and its use is therefore limited to the detection of intergranular attack.
Before delving deep into Practice B, let’s have a brief look at intergranular corrosion.
What is Intergranular Corrosion?
Intergranular corrosion (IGA) 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. IGA 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.
In the following sections, we will explore the apparatus, procedure, and applications of the Ferric Sulfate–Sulfuric Acid Test in greater detail.
What is Practice B: Ferric Sulfate–Sulfuric Acid Test ?
The procedure for Practice B is as follows:
A specimen, representative of the material to be tested, is immersed in a boiling solution of ferric sulfate and sulfuric acid for a specified time period (typically 120 hours).
During the test, the material undergoes a controlled exposure to highly corrosive conditions.
At the conclusion of the test, the mass loss of the specimen is measured and converted into a corrosion rate.
This corrosion rate is then compared to a predetermined maximum value to determine whether the material meets the expected resistance to intergranular attack for its grade.
In terms of corrosion potential, the ferric sulfate-sulfuric acid test operates at approximately 0.6 V versus a standard calomel electrode (SCE). This potential is lower than that observed in Practice C (0.75 to 1.0 V) but higher than the values for Practices E and F (approximately 0.1 V). These variations in corrosion potential influence the aggressiveness of each test, with Practice B falling between the others in terms of severity.
By quantifying the material's corrosion rate, this practice provides an objective measure of its susceptibility to intergranular corrosion under aggressive conditions.
Uses of Practice B
The Ferric Sulfate–Sulfuric Acid Test is highly effective in detecting susceptibility to intergranular attack caused by:
Chromium carbide precipitation in unstabilized austenitic stainless steels.
The presence of the sigma phase, which can form in certain grades of stainless steel during high-temperature exposure.
Rapid Screening Test for the Ferric Sulfate–Sulfuric Acid Test
Use of the Oxalic Acid Etch Test
Prior to conducting the Ferric Sulfate–Sulfuric Acid Test, specimens of certain grades of stainless steels (as specified in Table 1 of ASTM A262) may undergo a rapid screening test using the Oxalic Acid Etch Test (Practice A). This pre-screening process involves the preparation, etching, and classification of etch structures as detailed in Practice A. The classification of the etch structures is crucial as it directly influences whether further testing is necessary. The table given below specifies how the evaluations from the Oxalic Acid Etch Test correlate with the requirements of the Ferric Sulfate–Sulfuric Acid Test.
Specimens that exhibit acceptable etch structures in the Oxalic Acid Etch Test are generally free of intergranular attack when subjected to the Ferric Sulfate–Sulfuric Acid Test. Such specimens can be deemed acceptable without undergoing further corrosion testing in Practice B. Conversely, all specimens classified as suspect based on their etch structures must proceed to testing in the Ferric Sulfate–Sulfuric Acid Test for confirmation.
Specimens must undergo heat treatment to ensure that the material's microstructure reflects the conditions it would encounter in service, thereby providing accurate and representative test results.
The rapid screening test explicitly excludes the evaluation of process-affected areas, as defined in Section 21 of ASTM A262. Such areas, including welds, carburized zones, and mechanically deformed regions, may exhibit non-uniform corrosion behavior. The scope of Practice B does not currently address the application of the Ferric Sulfate–Sulfuric Acid Test to these areas, emphasizing the need to focus on base metal characteristics for reliable results.
Apparatus of Ferric Sulfate–Sulfuric Acid Test
The proper selection and setup of equipment are critical for the accurate and reliable execution of the Ferric Sulfate–Sulfuric Acid Test. The following components are required for the apparatus:
Allihn Condenser
The test requires an Allihn condenser with a minimum of four bulbs and a ground glass joint that matches the Erlenmeyer flask.
Substitutions for the Allihn condenser or flask are not permitted. Specifically, cold-finger condensers with standard Erlenmeyer flasks are unsuitable for this test. These alternatives result in:
Lower corrosion rates, potentially caused by vapor loss or increased oxygen content in the solution.
Inaccurate acceptance of materials that should be rejected due to understated corrosion rates.
Erlenmeyer Flask
The test requires a 1-L Erlenmeyer flask equipped with a ground glass joint compatible with the condenser.
The flask’s opening should allow for easy insertion and removal of the specimen. A larger opening is preferable to facilitate the process.
Specimen Support
Glass Cradle: A cradle made of glass, sized to fit both the flask opening and the specimen, is recommended. This cradle should allow for unrestricted flow of the testing solution around the specimen. Such cradles can be produced by a glassblowing shop. Equivalent methods, such as glass hooks or stirrups, may also be used, provided they enable adequate circulation of the testing solution.
Boiling Chips
Boiling chips are essential to prevent bumping during the boiling process, ensuring a smooth and consistent reaction environment.
High Vacuum Silicone Grease
This grease is applied to the ground glass joint to ensure a secure and airtight connection between the flask and the condenser.
Hot Plate
A hot plate capable of maintaining the solution at a continuous boil is required. The heat source must be reliable and adjustable to provide consistent heating throughout the test duration.
Analytical Balance
An analytical balance with a precision of 0.001 g is necessary for accurately weighing the specimens before and after testing. This precision ensures reliable calculation of corrosion rates.
Desiccator
A desiccator is used for storing prepared specimens prior to testing. This step protects the specimens from moisture and contamination, which could affect test results.
During the test, iron oxide deposits may accumulate on the upper portion of the Erlenmeyer flask. To remove these deposits after test completion, boil a 10% hydrochloric acid solution in the flask. This simple cleaning procedure ensures that the equipment remains in optimal condition for future tests.
Reagents and Materials used in Ferric Sulfate–Sulfuric Acid Test
The Ferric Sulfate–Sulfuric Acid Test requires specific reagents to prepare the test solution accurately and ensure reliable results. They are as follows:
Ferric Sulfate Hydrate (Fe?(SO?)?·xH?O): This reagent must have approximately 75% Fe?(SO?)? by mass. Ferric sulfate is a critical additive that establishes and controls the corrosion potential during the test. Substitutions for ferric sulfate are not allowed as they may compromise the accuracy of the test results.
Sulfuric Acid (H?SO?): Reagent-grade sulfuric acid with a concentration of 95.0 to 98.0% by mass is required. This provides the acidic environment necessary for the test.
Ferric Sulfate–Sulfuric Acid Test Solution
The preparation of the test solution involves several critical steps to ensure its proper composition and safety during handling:
Begin by measuring 400.0 mL of Type IV reagent water and pouring it into the Erlenmeyer flask under a ventilated hood to control exposure to acid vapors.
Slowly measure 236.0 mL of reagent-grade sulfuric acid and add it to the water with constant stirring. The acid must be added carefully to avoid rapid heat evolution that can lead to boiling or splashing.
Weigh 25 g of reagent-grade ferric sulfate to the nearest 0.1 g and dissolve it in the sulfuric acid solution.
Add boiling chips to the flask to prevent bumping during the boiling process. Ensure the chips are resistant to the corrosive test solution.
Apply silicone grease to the ground glass joint of the Erlenmeyer flask to create a secure and airtight connection with the condenser.
Cover the flask with an Allihn condenser, ensuring cooling water is circulated through the condenser to manage vapor.
Boil the solution until all ferric sulfate dissolves completely, ensuring uniformity of the test solution. Monitor the process closely to prevent violent boiling, which can lead to spills or loss of acid.
Sampling
Proper sampling is crucial for obtaining representative test results. The focus is on ensuring the samples are free from localized conditions that may affect corrosion properties.
Base Metal Samples: Specimens for the test must be representative of base metal, which does not exhibit conditions such as welding, carburization, nitriding, oxidation, mechanical deformation, or heat-affected zones. These conditions can lead to non-uniform corrosion properties.
Process-Affected Metal: Specimens with process-affected metal, such as welds or mechanically deformed regions, may exhibit corrosion behaviors that differ significantly from base metal. Testing such specimens can lead to unpredictable results as the test averages the mass loss across the entire surface.
If localized conditions are a concern, tests that do not average the mass loss, such as Practice A (Oxalic Acid Etch Test) or Practice E (Copper-Copper Sulfate–16% Sulfuric Acid Test), should be used. In these cases, the specific test details and acceptance criteria must be agreed upon by the purchaser and producer.
Preparation of Test Specimens
Proper preparation of test specimens is essential for obtaining accurate and reliable results in the Ferric Sulfate–Sulfuric Acid Test. The following guidelines outline the steps for preparing specimens:
Heat Treatment
Extra-low carbon and stabilized grades of stainless steel must undergo heat treatment within the range of 650 to 675°C (1200 to 1250°F). This range corresponds to the maximum precipitation of carbides.
The duration of heating and the method of subsequent cooling should be part of a sensitization treatment, which must be agreed upon by the material producer and purchaser. The treatment parameters should align with the desired performance requirements.
Note: A common sensitization treatment involves heating the specimens for 1 hour at 675°C (1250°F).
Specimen Dimensions
Each test specimen should have a total surface area of 5 to 20 cm². This range ensures consistent exposure during testing and accurate measurement of corrosion rates.
Surface Preparation
Where possible, grind all specimen surfaces using CAMI/ANSI 120 grit (FEPA/ISO P120) abrasive paper. This can be performed wet or dry, but when grinding dry, polish slowly to avoid overheating.
Use water as a coolant during wet grinding. Overheating or using abrasive materials with grinding aids should be avoided, as some grinding aids (e.g., fluorides) can interfere with the corrosion test results.
Removal of Oxide Scale and Heat Tint
Completely remove any oxide scale and heat tint formed during heat treatments. Oxide scale that cannot be removed by grinding (e.g., in stamped numbers) should be removed using pickling solutions described in Practice A380/A380M, Table A1.1.
Residual oxide scale must be avoided as it can lead to galvanic action and undesired activation in the test solution, compromising the accuracy of results.
Measurement and Calculation of Exposed Area
Measure the specimens, including the inner surfaces of any holes, to the nearest 0.05 mm (0.001 in.).
Use these measurements to calculate the total exposed surface area of each specimen. Accurate area determination is critical for reliable mass loss and corrosion rate calculations.
Degreasing and Storage
Degrease the specimens thoroughly using nonchlorinated agents, such as soap with lukewarm water or acetone.
Dry the specimens completely and weigh each one to the nearest 0.001 g. Proper degreasing ensures no contamination interferes with the test results.
After preparation, store the specimens in a desiccator to protect them from moisture and contaminants until testing begins.
Procedure for the Ferric Sulfate–Sulfuric Acid Test
The Ferric Sulfate–Sulfuric Acid Test is conducted under controlled conditions to evaluate the susceptibility of austenitic stainless steel to intergranular attack. The procedure includes the following steps:
Preparing the Test Solution
Ensure the test solution is boiling before beginning the immersion test. If not already boiling, heat the solution to bring it to a boil.
Keep the flask covered with the condenser, ensuring that cooling water is flowing, except when inserting or removing specimens. This minimizes vapor loss and prevents changes in solution concentration.
Inserting Specimens
Turn off the heat source and allow the boiling to subside before handling the specimens.
Place the prepared specimens in glass cradles to ensure uniform exposure to the solution.
Uncover the flask and carefully insert the specimens. Replace the condenser immediately after insertion, restore cooling water flow, and turn the heat source back on.
Monitoring Vapor Loss
Mark the liquid level on the flask to monitor vapor loss throughout the test. Significant changes in liquid level indicate acid concentration due to vapor loss, which could invalidate the test. If this occurs, repeat the test with a fresh solution and reground, reweighed specimens.
Immersion Duration
Immerse the specimens in the boiling solution for a total of 120 hours (five days) without interruption. After the immersion period, remove the specimens, rinse them in water or acetone, and dry them thoroughly.
Weigh the specimens after drying and subtract the new weights from their original weights to determine the mass loss.
Intermediate Weighings
While intermediate weighings are not typically necessary, they may be performed if preliminary results are required. Specimens can be removed, weighed, and re-immersed at any time during the 120-hour period.
Solution Maintenance
No changes to the solution are typically required during the test. However, if an unusually high corrosion rate is observed, indicated by a color change in the solution from yellow to green, additional ferric sulfate inhibitor may need to be added.
If the total weight loss of all specimens in a flask exceeds 2 g, add more ferric sulfate to the solution. Ferric sulfate is consumed at a rate of 10 g for every 1 g of dissolved stainless steel, making replenishment critical to maintaining test consistency.
Simultaneous Testing
Multiple specimens may be tested simultaneously, provided there is sufficient space in the flask. The number of specimens is typically limited to three or four, depending on the number of glass cradles that can be accommodated.
This methodical approach ensures accurate evaluation of stainless steel materials, providing valuable insights into their resistance to intergranular corrosion. By following these steps, the test delivers reliable and reproducible results for critical applications.
Calculation and Reporting of Corrosion Rates
The effect of the ferric sulfate-sulfuric acid solution on the material is quantified by determining the weight loss of the specimen. The corrosion rate is expressed in millimeters of penetration per month, calculated using the following formula:
Corrosion Rate Formula
Millimeters per month=7305×W/(A×t×d)
Where:
t: Time of exposure in hours.
A: Surface area of the specimen in square centimeters (cm²).
W: Weight loss of the specimen in grams (g).
d: Density of the material in grams per cubic centimeter (g/cm³).
For chromium-nickel steels: d=7.9?g/cm³.
For chromium-nickel-molybdenum steels: d=8.0?g/cm³.
Conversion Factors for Corrosion Rates
To accommodate various reporting standards and unit preferences, the corrosion rate in millimeters per month can be converted to other units using the following factors:
Inches per month: Multiply by 0.04
Inches per year: Multiply by 0.47
Millimeters per year: Multiply by 12
Mils per year: Multiply by 472
Milligrams per square decimeter per day: Multiply by 1000×density/3
Grams per square meter per hour: Multiply by 1.39×density
Reporting Requirements
The test report must include the corrosion rate in the unit specified or agreed upon by the producer and purchaser.
Include relevant test conditions such as:
Time of exposure (t).
Surface area (A).
Weight loss (W).
Density of the material (d).
Clearly document any deviations or observations, such as anomalies in the solution or unexpected weight loss patterns.
Conclusion
The Ferric Sulfate–Sulfuric Acid Test (Practice B) stands as a cornerstone in evaluating the susceptibility of austenitic stainless steels to intergranular attack (IGA). While Practice B is highly effective in detecting intergranular corrosion, it is not designed to evaluate resistance to other forms of degradation, such as pitting, general corrosion, or stress-corrosion cracking. Its scope is specific, making it an invaluable tool when used in conjunction with other ASTM A262 practices, such as the Oxalic Acid Etch Test (Practice A) for rapid screening.
In industrial applications where corrosion resistance is important, adherence to ASTM A262 ensures that stainless steels perform reliably under challenging conditions.
Do you want to know about the other practices used to assess intergranular corrosion? Check out our blogs here:
