5 Key Characteristics Distinguishes Intergranular Stress Corrosion Cracking (IGSCC)

What is Intergranular Stress Corrosion Cracking (IGSCC)?

Intergranular Stress Corrosion Cracking (IGSCC) is a form of material degradation that occurs when specific environmental, material, and stress conditions align. It primarily affects metals such as austenitic stainless steels and alloys that become sensitized where grain boundaries lose corrosion resistance due to phenomena like chromium carbide precipitation. IGSCC is characterized by crack propagation along these weakened grain boundaries, often resulting in brittle failure.

This mechanism is particularly concerning in industrial systems operating under high stresses or exposed to aggressive environments, such as chlorides or sulfur compounds. Left undetected, IGSCC can lead to catastrophic failures in pipelines, reactors, and other critical components. Understanding its causes and prevention is essential for maintaining asset integrity and operational reliability.

How to Relate Detected Cracks to the IGSCC Mechanism

When evaluating detected cracks in a material, linking their characteristics to the intergranular stress corrosion cracking (IGSCC) mechanism requires careful observation, testing, and analysis.

The following steps highlight how to establish this relationship:

1. Crack Morphology

• Intergranular Path: IGSCC is characterized by cracks propagating along the grain boundaries of the material rather than through the grains.
• Brittle Appearance: Fracture surfaces typically appear jagged and brittle under microscopic examination, with minimal plastic deformation.
• Direction of Cracking: In cylindrical components (e.g., pipes), IGSCC cracks often form in the hoop direction due to the high tensile stresses in that orientation.

2. Stress Correlation

• Stress Concentration Zones: Cracks initiating at areas with high tensile residual or operational stresses (e.g., welds, structural lugs) strongly suggest stress corrosion cracking.
• Evidence of Tensile Stress: Finite Element Analysis (FEA) or stress modelling can confirm stress concentrations that align with the crack's location and orientation.

3. Metallographic Analysis

• Intergranular Cracking: Cross-sectional metallographic examination under a microscope confirms whether the cracks propagate along the grain boundaries.
• Sensitization Evidence: Use tests like "ASTM A262 Method A" to detect chromium carbide precipitation along grain boundaries, indicative of sensitization. Sensitization reduces grain boundary corrosion resistance, making them susceptible to attack.

4. Environmental Analysis

• Corrosive Media: Analyse the operating environment for factors that promote IGSCC: Chlorides (e.g., seawater, cleaning solutions).Sulfur compounds (e.g., acid rain or industrial pollutants).Oxidizing agents (e.g., dissolved oxygen).
• Deposit Analysis: Perform Energy-Dispersive X-ray Spectroscopy (EDS) on fracture surfaces to identify corrosive species like sulfur, chlorine, or oxygen.

5. Fracture Surface Analysis

• Brittle Fracture Features: Use Scanning Electron Microscopy (SEM) to observe the crack surface. IGSCC surfaces typically show: Intergranular fracture features. Absence of ductile dimples or plastic deformation.
• Elemental Analysis: Sulfur, chloride, or other corrosive elements detected on the fracture surface link environmental exposure to cracking.

Case Study Example

The failure of a methanator reactor fabricated with 304H stainless steel was investigated following the detection of leakage near its structural lugs. The reactor exhibited two distinct cracks running in the hoop direction, with a brittle fracture appearance. Subsequent cross-sectional examination revealed additional cracks initiating from the outer diameter (OD) and propagating to the inner diameter (ID) along the grain boundaries. Metallurgical tests confirmed severe sensitization in the affected regions. Further, sulfur deposits on the intergranular fracture surface indicated the influence of external corrosive agents, most likely acid rain. This study identifies the root cause of failure as intergranular stress corrosion cracking (IGSCC), exacerbated by inadequate design considerations for downtime corrosion protection in an open-air environment.

Summary of the 304H methanator failure:

1. Cracks initiated at high-stress zones near structural lugs.
2. Metallography confirmed sensitization.
3. EDS analysis revealed sulfur on the fracture surface.
4. Environmental analysis linked acid rain as the corrosive medium.
5. Combined, these findings confirmed IGSCC as the primary failure mechanism.

By systematically combining these observations, detected cracks can be conclusively related to the IGSCC mechanism.