Intergranular corrosion (IGC) is a critical failure mode in stainless steel plates, particularly in austenitic grades such as 304 and 316. It occurs when chromium carbides precipitate along grain boundaries under certain thermal conditions (typically within the sensitization temperature range of 450–850°C), leading to chromium-depleted zones adjacent to the grain boundaries. These regions become vulnerable to corrosion, resulting in localized attack that can significantly compromise mechanical integrity. Therefore, reliable testing methods for intergranular corrosion are essential to ensure product quality and long-term service performance.

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1. Mechanism of Intergranular Corrosion

The corrosion resistance of stainless steel depends on the presence of a stable chromium-rich passive film. However, when chromium combines with carbon to form carbides (Cr23C6) at grain boundaries, the surrounding areas lose sufficient chromium content (below approximately 12%), making them susceptible to corrosion. This phenomenon is especially critical in welded or improperly heat-treated materials.

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2. International Testing Standards

Several internationally recognized standards are widely used to evaluate intergranular corrosion resistance in stainless steel plates:

• ASTM A262 – Standard Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels
• ISO 3651-2 – Determination of resistance to intergranular corrosion of stainless steels
• EN ISO 3651 – European equivalent standard

Among these, ASTM A262 is the most commonly applied in global trade and industrial inspection.

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3. ASTM A262 Testing Methods

ASTM A262 defines five different practices (A–E), each suitable for specific evaluation purposes:

Practice A – Oxalic Acid Etch Test (Screening Test)

This is a rapid electrolytic etching method used as a preliminary screening tool. The specimen is etched in a 10% oxalic acid solution, and the microstructure is examined under a microscope.

• Result Interpretation:
  • Step structure: acceptable (no sensitization)
  • Dual structure: partially sensitized
  • Ditch structure: unacceptable (susceptible to IGC)

This method is fast and cost-effective but does not quantify corrosion rate.

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Practice B – Ferric Sulfate–Sulfuric Acid Test

This method involves boiling the specimen in a ferric sulfate and sulfuric acid solution for a specified duration. It evaluates weight loss due to corrosion.

• Suitable for: general-purpose testing
• Limitation: less commonly used for modern low-carbon grades

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Practice C – Nitric Acid Test (Huey Test)

The specimen is subjected to multiple boiling cycles in nitric acid, typically five 48-hour periods. Weight loss is measured after each cycle.

• Provides quantitative corrosion rate
• Suitable for evaluating uniform intergranular attack

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Practice E – Copper Sulfate–Sulfuric Acid Test

This is one of the most widely used methods for detecting susceptibility in austenitic stainless steels.

• The specimen is bent after exposure to the test solution
• Cracks along grain boundaries indicate susceptibility

This method is particularly effective for detecting sensitization in welded materials.

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4. ISO 3651 Testing Methods

ISO 3651 includes two main parts:

• ISO 3651-1: For ferritic and austenitic steels using nitric acid
• ISO 3651-2: For austenitic steels using sulfuric acid with copper sulfate

These methods are similar in principle to ASTM standards but are more commonly used in European and international projects.

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5. Testing Procedure and Key Considerations

To ensure accurate and reliable results, the following factors must be strictly controlled:

• Sample Preparation: Surface must be clean, free from scale, oil, and contamination
• Temperature Control: Boiling conditions must be maintained consistently
• Solution Composition: Chemical concentrations must comply with standards
• Testing Duration: Strict adherence to specified time cycles
• Post-Test Evaluation: Includes weight loss measurement, microscopic examination, and bend testing

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6. Applications in Industry

Intergranular corrosion testing is widely applied in industries where stainless steel plates are exposed to aggressive environments, including:

• Petrochemical and chemical processing plants
• Oil and gas pipelines
• Food and pharmaceutical equipment
• Marine and offshore engineering

For these applications, ensuring resistance to intergranular corrosion is essential for safety, reliability, and long service life.

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7. Our Quality Assurance Commitment

As a professional supplier of stainless steel plates, we strictly follow international standards such as ASTM and ISO in our testing procedures. For every batch of stainless steel plates (including 304 and 316 grades), we can provide:

• Mill Test Certificates (MTC)
• Intergranular corrosion test reports (ASTM A262 / ISO 3651)
• Third-party inspection reports upon request (e.g., SGS, BV)

Our advanced laboratory facilities and experienced technical team ensure that all products meet the highest quality requirements before delivery.

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Conclusion

Intergranular corrosion testing is a vital step in ensuring the durability and reliability of stainless steel plates. By applying standardized methods such as ASTM A262 and ISO 3651, manufacturers and suppliers can effectively detect sensitization and prevent potential failures in service.

With strict quality control, advanced testing capabilities, and a strong commitment to customer satisfaction, we provide stainless steel solutions that meet the most demanding international standards.

Contact us today to learn more about our stainless steel plates and testing services.