Intergranular Corrosion Testing: An Overview of ASTM A 262

Understanding and mitigating corrosion is important for ensuring the robustness and dependability of metals. Corrosion can take many distinct forms. Because intergranular corrosion erodes metal from the inside, it is very tricky. It often leaves the external appearance seemingly intact. This introduction explores the testing procedures for intergranular corrosion and explores the phenomena itself. It particularly focuses on the ASTM A 262 standard.

Introduction

Metal grain boundaries are the subject of intergranular corrosion. They are more prone to deterioration than the individual grains. This form of corrosion can arise due to variations in chemical composition or microstructural characteristics. Despite its subtle exterior manifestations, intergranular corrosion significantly compromises the mechanical strength and integrity of the metal. It is predominantly observed in austenitic stainless steels. If treated to incorrect heat treatments or exposed to corrosive environments, other alloys may also become fragile.

When corrosive chemicals target the grain boundaries preferentially, intergranular corrosion starts. Precipitates like carbides or nitrides are frequently present to aid in this selective degradation. They can form at these boundaries during specific heat treatment processes. Chromium carbides in stainless steel precipitate out of the material when heated to certain temperatures. It results in a depletion of chromium at grain boundaries. Such areas become more prone to corrosion as a result.

Because of the possible risks associated with intergranular corrosion, materials scientists and engineers need to employ stringent testing procedures to detect and quantify it. The ASTM A 262 standard provides a structure for these evaluations. It offers a range of tests to determine the degree to which stainless steels and other alloys are vulnerable to intergranular assault. These examinations vary in complexity from straightforward etch tests with oxalic acid to intricate procedures using boiling sulfuric and ferric acid solutions.

Pros can have a deeper understanding of how metals corrode by following the ASTM A 262 standard. They can also create plans to lessen its impacts. It will improve the longevity, use, and safety of metal components in a variety of applications. The purpose of this article is to provide further detail on the intergranular corrosion testing procedures. It highlights how important ASTM A 262 is to preserving the durability of metal buildings in harsh settings.

Importance of ASTM A 262

This standard testing method helps identify which austenitic stainless steels are more prone to intergranular corrosion. Their creation and maintenance are under the purview of the American Society for Testing and Materials. This standard offers a series of practice tests that have similar corrosive environments to be able to evaluate the resistance of stainless steels to intergranular attack. The tests are important for industries where material failure can mean a catastrophic outcome. Some prime examples are: aerospace, nuclear, and chemical processing sectors.

Read more: Intergranular Corrosion (IGC) Testing Services

Understanding the Practices

The ASTM A262 includes several distinct practices (A through F), each tailored to identify intergranular corrosion under different conditions:

• Practice A (called Oxalic Acid Etch Test): This method is a rapid screening one which is used to detect how susceptible stainless steel grades are to intergranular attack. It is useful for checking the effects of heat treatment. It also helps in detecting the presence of chromium carbide precipitates along the grain boundaries of metals.
• Practice B (called The Streicher Test): This method employs a boiling ferric sulfate-sulfuric acid solution to know the material’s resistance to corrosion in a more aggressive environment. It is particularly suitable for detecting susceptibility to intergranular corrosion in chromium-depleted zones near grain boundaries.
• Practice C (called The Huey Test): This method involves boiling the test specimen in a strong nitric acid solution, making it well-suited for detecting intergranular attack in materials exposed to nitric acid or similar corrosive environments. It is used for materials intended for use in chemical processing environments.
• Practice E (called The Strauss Test): This method utilizes a copper sulfate-sulfuric acid solution to test the material’s resistance to intergranular corrosion. It is done in a controlled setting, which simulates exposure to certain corrosive agents.
• Practice F (called Copper-Copper Sulfate-Sulfuric Acid Test): It examines how vulnerable “as-received” stainless steel is to intergranular attack. The material’s intrinsic resistance to corrosion is disclosed because no prior heat treatment is used.

The Role of Sensitization

Sensitization is essential to ascertain whether stainless steels are prone to intergranular corrosion. Grain boundaries become precipitated with chromium carbides when materials are heated to temperatures between approximately 500°C and 800°C. This is the actual situation. The steel becomes more vulnerable to corrosion at the grain boundaries as a result of this chromium loss. Testing can be used to determine which materials have become sensitized and are vulnerable to intergranular corrosion.

Niobium and Chromium

Metals like niobium and chromium are frequently added to stainless steels to increase their resistance to corrosion. Niobium is used to generate stable carbides. It prevents chromium depletion near grain boundaries. Meanwhile, chromium is essential for forming a passive layer on the steel surface. It offers protection against corrosion. These test methods help evaluate the effectiveness of these alloying elements in preventing intergranular corrosion.

Practical Applications and Industry Relevance

The test is a tool for material selection and quality control. It also acts as a guideline for material processing as well as heat treatment practices. By identifying how susceptible the materials are to intergranular corrosion, manufacturers can make well-informed choices. They can decide about material specifications, heat treatment procedures, and the suitability of materials for specific environments. This makes sure that, going forward, reliable and safe components are used in critical applications, such as infrastructure and medical devices.

Conclusion

Intergranular corrosion really threatens the integrity of stainless steels and other alloys. The test methods provide a framework to check how susceptible the materials are to this form of corrosion. It helps industries to safeguard against material failure. Through meticulous testing and evaluation, this standard helps ensure that materials deployed in critical applications remain robust and reliable. Additionally, preserving the safety and functionality of essential infrastructure and technologies is important. Material scientists and engineers play a role in advancing material performance and durability. They do it by understanding and applying the principles of this test. Eventually, it contributes to the development of safer, more resilient products and systems.

Through the ASTM A262 test, Infinita Lab offers a rigorous approach to identify and mitigate risks, and make sure that materials meet the highest standards of durability and performance. By working alongside Infinita Lab, businesses can confidently contribute to a better built world. 

VIdeo 01: Intergranular Corrosion testing II NDT II Pipe inspection and testing