January 25, 2025
January 25, 2025
The Nitric Acid Test, designated as Practice C in ASTM A262, is a standardized procedure used to measure the susceptibility of austenitic stainless steels to intergranular corrosion (IGC). This test involves immersing the material in a boiling solution of nitric acid and assessing its resistance to corrosion under these specific conditions. While highly effective for detecting IGC, the presence or absence of intergranular attack in this test does not serve as an indicator of the material’s performance in other corrosive environments. Additionally, the test is not predictive of resistance to general corrosion, pitting, or stress-corrosion cracking.
Before delving deep into Practice C, 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, Ferric Sulfate–Sulfuric Acid Test (Practice B), Nitric Acid Test (Practice C) etc 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, check out our detailed article on intergranular corrosion.
In the following sections, we will explore the apparatus, procedure, and applications of the Nitric Acid Test in greater detail.
The Nitric Acid Test involves the following key steps:
The Nitric Acid Test is particularly effective in detecting susceptibility to rapid intergranular attack caused by chromium carbide precipitation. This form of degradation typically occurs when stainless steels are exposed to temperatures that promote the precipitation of chromium carbides at grain boundaries, leading to localized depletion of chromium and reduced corrosion resistance.
One of the distinguishing features of Practice C is its high corrosion potential, reported to range between 0.75 and 1.0 volts versus a standard calomel electrode (SCE). This potential is higher than that of other ASTM A262 practices, such as:
The higher corrosion potential reflects the severely oxidizing conditions of the nitric acid solution, making Practice C one of the most aggressive tests for detecting intergranular attack. Due to these conditions, it is most appropriate to invoke this test when the material is intended for applications involving nitric acid service.
Before conducting the Nitric Acid Test, specimens of specific stainless steel grades as listed in the table given below (adopted from Table 2 of ASTM A262) can undergo a rapid screening test using the Oxalic Acid Etch Test (Practice A). This screening process evaluates the etch structures of the specimens, which directly influence whether further testing in the nitric acid test is necessary.
| Grade | Acceptable Etch Structures | Suspect Etch Structures |
| AISI 304 | Step, dual, end grain I | Ditch, end grain II |
| AISI 304L | Step, dual, end grain I | Ditch, end grain II |
| ACI CF-8 | Step, dual, isolated ferrite pools | Ditch, interdendritic ditches |
| ACI CF-3 | Step, dual, isolated ferrite pools | Ditch, interdendritic ditches |
Prior to the etch test, the material must be heat-treated according to the guidelines mentioned later on. This step ensures the material's microstructure accurately reflects the conditions it would experience in service, improving the reliability of the screening process.
The test methodology accounts for two primary classes of specimens: Base metal and Process-affected metal.
Base Metal : Base metal refers to materials free from non-uniform conditions that could alter their corrosion properties, ensuring a more consistent and predictable testing outcome.
Process-Affected Metal : Process-affected metal includes specimens with localized conditions that may impact corrosion properties in a non-uniform manner.
The rapid screening test does not evaluate process-affected areas, such as welds, carburized regions, or heat-affected zones. The application of the Nitric Acid Test to process-affected areas is outside the scope of Practice C, emphasizing the test’s focus on base metal characteristics.
Specimens exhibiting acceptable etch structures in the Oxalic Acid Etch Test are typically free of intergranular attack when subjected to the Nitric Acid Test. These specimens can be considered acceptable without undergoing further testing in Practice C. Conversely, specimens with suspect etch structures must proceed to the Nitric Acid Test to confirm their corrosion resistance.
Container
The test requires a 1-L Erlenmeyer flask equipped with a cold finger-type condenser, as illustrated in Figure 9 of ASTM A262. This setup ensures proper containment and minimizes vapor loss during the boiling nitric acid test, maintaining the integrity of the solution and test conditions.
Specimen Supports
Specimens are supported in the test container using glass hooks, stirrups, or cradles designed to:
Heater
A heating device capable of maintaining the test solution at a consistent boiling point throughout the test is required. An electrically heated hot plate is typically used for this purpose, ensuring uniform heat distribution and controlled boiling conditions.
Analytical Balance
An analytical balance capable of weighing to the nearest 0.001 g is necessary for accurately measuring the weight of the specimens before and after testing. This precision is essential for determining mass loss and calculating the corrosion rate.
Desiccator
A desiccator is required for storing prepared specimens prior to testing. This prevents contamination and protects the specimens from moisture or other environmental factors that could affect test results.
Nitric Acid Test Solution
The test solution must be 65.0 ± 0.2% nitric acid by weight, determined by chemical analysis. This ensures the solution meets the specified concentration for accurate and consistent testing.The nitric acid used must conform to the American Chemical Society Specifications for Reagent Chemicals. Additional requirements for this test method are outlined in Table 3 of ASTM A262, ensuring the reagent meets high-purity standards. The solution is prepared by adding reagent-grade nitric acid (HNO3) to reagent water at a ratio of 108 mL of reagent water per liter of reagent nitric acid. Proper mixing and measurement are critical to achieving the required concentration.
Handling nitric acid requires strict safety measures:
Only base metal samples should be obtained and prepared for the Nitric Acid Test. These are free from localized conditions such as welding, carburization, nitriding, oxidation, or mechanical deformation, which can impact corrosion properties. Specimens containing process-affected metal, such as welds or heat-affected zones, exhibit non-uniform corrosion behavior. The mass loss rate for such specimens may differ significantly from that of base metal, leading to unpredictable test results. Process-affected metal is excluded from Practice C, and alternative tests like Practice A (Oxalic Acid Etch Test) or Practice E (Copper-Copper Sulfate–Sulfuric Acid Test) should be considered when localized conditions are a concern.
Specimens must be prepared with care to ensure accurate results:
Unless otherwise specified by the purchaser, the producer has discretion over the procedures for obtaining representative base metal samples, removing specimens, and determining the number of specimens to be tested. Any agreements between the producer and purchaser should detail the testing requirements and acceptance criteria.
Extra-low carbon and stabilized grades of stainless steel must be heat treated at temperatures ranging from 650 to 675°C (1200 to 1250°F), which corresponds to the range of maximum carbide precipitation. The duration of the heat treatment and the method of subsequent cooling should align with a sensitizing treatment agreed upon by the material producer and purchaser. This treatment directly affects the material’s corrosion resistance and must be carefully planned.
For bar, wire, or tubular products, the proportion of exposed cross-sectional area may influence results, as these surfaces are prone to end grain attack in nitric acid. Specimens should maintain a low proportion of end grain unless such surfaces are expected to be exposed in service involving nitric acid. In such cases, the proportion of end grain should be kept high to simulate real-world conditions.
Specimens should have their surfaces ground using CAMI/ANSI 120 grit (FEPA/ISO P120) paper-backed abrasive material. This can be conducted either wet or dry:
Avoid Abrasives with Grinding Aids: Some grinding aids contain fluorides that can interfere with the corrosion test results, leading to inaccuracies.
All traces of oxide scale and heat tint formed during heat treatment must be completely removed. For areas where grinding cannot effectively remove these layers (e.g., stamped numbers), pickling solutions as described in Practice A380/A380M, Table A1.1, may be used. Proper removal ensures consistent exposure of the base material during the test, avoiding galvanic effects caused by residual oxide.
The specimen, including the inner surfaces of any holes, should be measured to the nearest 0.05 mm (0.001 in.). Use these measurements to calculate the total exposed surface area in square centimeters (cm²). Accurate area measurement is essential for reliable corrosion rate calculations.
Degrease the specimen thoroughly using nonchlorinated agents, such as soap and lukewarm water or acetone. This step ensures the removal of any surface contaminants that could interfere with the test. Once degreased, dry the specimens completely and weigh each one to the nearest 0.001 g. Proper cleaning and weighing are critical for obtaining accurate pre-test data.
Store the prepared specimens in a desiccator to protect them from moisture and contamination until the test is ready to be performed.
Specimen Quantity for Testing
For standard testing, one specimen per material or lot is typically sufficient. However, in cases of dispute, it is recommended to test at least two specimens for verification purposes. This ensures the reliability and reproducibility of the test results.
This meticulous preparation process ensures the accuracy and consistency of the Nitric Acid Test, providing a reliable evaluation of austenitic stainless steels' resistance to intergranular corrosion. Properly prepared specimens allow for precise and reproducible testing.
A sufficient quantity of nitric acid test solution must be used to ensure complete immersion of the specimens and maintain a solution volume of at least 20 mL/cm² (125 mL/in²) of specimen surface area. Typically, a volume of about 600 mL is used per test.
Each specimen should ideally be tested in a separate container to prevent interaction between specimens. However:
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Excessive corrosion of one specimen can accelerate the corrosion of other specimens in the same container. This is often detectable by changes in the color of the test solution. To avoid this issue, it is advisable to separate specimens that appear prone to excessive corrosion during the test. A record should be maintained indicating which specimens were tested together.
The standard test consists of five boiling periods, each lasting 48 hours. A fresh test solution is used for each boiling period to ensure consistent conditions.
Alternatively, a modified test schedule may be used if agreed upon by the purchaser. One 48-hour period and two 96-hour periods (in any order) can replace the standard five 48-hour periods.
Calculation
The effect of the acid on the material is calculated by determining the loss of weight of the
specimen after each test period and for the total of the test periods. The corrosion rate for each
specimen for each test period, and for the total of the test periods is calculated using the equation given below:
Millimeters per month=7305×W/(A×t×d)
Where:
The Nitric Acid Test (Practice C) plays a pivotal role in evaluating the susceptibility of austenitic stainless steels to intergranular corrosion (IGC). With its rigorous methodology, including the use of boiling nitric acid under highly oxidizing conditions, this test provides a reliable and precise measure of a material's resistance to IGC caused by chromium carbide precipitation or detrimental phases. Its high corrosion potential makes it particularly valuable for applications involving exposure to nitric acid environments, where resistance to intergranular attack is critical.
Through a combination of pre-screening using the Oxalic Acid Etch Test, meticulous specimen preparation, and careful execution of testing protocols, Practice C ensures reliable results that guide material selection and quality assurance. While the test is effective in detecting intergranular corrosion, it does not predict resistance to other forms of degradation, such as general corrosion, pitting, or stress-corrosion cracking. As such, its application is highly specialized but essential for ensuring the long-term performance and safety of stainless steels in demanding environments.
By adhering to the outlined procedures and using the insights provided by Practice C, industries can confidently assess the integrity of materials, enhance product reliability, and meet stringent performance standards in corrosion-prone environments.
