Stress Corrosion Cracking

Gauge corner cracking (GCC) is a fatigue-related failure that occurs on the outer edge of a railhead where the wheel flange makes contact. This high-stress contact area is prone to micro-cracks that grow over time under repeated loading.

Why it happens:

• Contact pressure: The wheel–rail interface concentrates stress at the gauge corner.
• Rolling fatigue: Cyclic loading from train wheels causes cracks to form and propagate.
• Friction and slip: Curved tracks and braking create shear forces that worsen damage.
• Material factors: Some rail steels resist wear but are more prone to cracking under stress.
• Environmental exposure: Moisture, debris, and temperature changes accelerate crack growth.

Prevention strategies include:

• Routine ultrasonic or eddy current testing
• Rail grinding to remove early damage
• Friction modifiers to reduce wear
• Upgrading to fatigue-resistant rail materials

Gauge corner cracking is a leading cause of rail degradation and must be proactively managed to maintain track safety and longevity.

The temperature at which stress corrosion cracking (SCC) occurs depends on the material and environment. SCC typically arises within a specific temperature window where tensile stress and corrosion mechanisms are active. Here are common examples by material type:

1. Stainless Steels in Chloride Environments: SCC is most likely between 50°C and 150°C (122°F–302°F), where chloride ions destabilize the passive film on the surface.
2. Carbon Steels in Caustic Solutions: Cracking tends to begin above 60°C (140°F), particularly in concentrated alkaline environments.
3. Brass in Ammonia-Rich Environments: SCC can occur even at room temperature when exposed to ammonia or ammonium salts.
4. High-Strength Alloys (e.g., aluminum, titanium): Susceptibility can span a broad temperature range, depending on alloy composition, microstructure, and environmental exposure.

Understanding the temperature sensitivity of SCC is critical for selecting the right materials and applying preventive strategies in corrosive service environments.

Stress corrosion cracking (SCC) in pipelines is a result of a complex interaction between mechanical stress, environmental conditions, and material properties. Unlike general corrosion, SCC leads to sudden and brittle fracture—even at stress levels well below a material’s yield strength.

Primary Triggers:

1. Persistent Tensile Stress: This may stem from operational loads, residual welding stress, or external factors such as soil shifting. Long-term stress promotes crack initiation and propagation.
2. Aggressive Chemical Exposure: Pipelines exposed to chlorides (in coastal or marine settings), sulfides (in sour service), or acidic gases like CO₂ become more vulnerable to SCC.
3. Material Susceptibility: Some pipeline steels, particularly older high-strength carbon steels or improperly heat-treated stainless steels, are more prone to stress-induced fracture in corrosive environments.

Contributing Conditions:

• Coating Failures: Disbonded or cracked coatings expose bare metal to moisture and contaminants, triggering SCC beneath the surface.
• Soil Chemistry: In underground pipelines, SCC is influenced by factors like pH, microbial activity, and ion concentrations in surrounding soil.
• Elevated Pressure and Temperature: Operating conditions that push temperature or internal pressure to the upper design limits can accelerate crack growth.

Mitigation Strategies:

• Apply cathodic protection to suppress electrochemical reactions at the metal surface.
• Use SCC-resistant alloys tailored for the intended service environment.
• Perform stress-relief treatments on welds and cold-worked sections during fabrication or repair.
• Inspect regularly using ultrasonic or electromagnetic techniques to detect early signs of cracking.

By understanding and proactively addressing these root causes, pipeline operators can prevent SCC-related failures and extend the service life of critical infrastructure.