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Stress Corrosion Cracking

What is Stress Corrosion Cracking?

The Basics of Stress Corrosion Cracking

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The Basics of Stress Corrosion Cracking

Stress corrosion cracking (SCC) is a type of localized corrosion that occurs under the combined action of tensile stress and a corrosive environment, leading to the formation of cracks and potential catastrophic failure of the material. SCC is commonly observed in various industries, including oil and gas extraction, nuclear power, aerospace, and chemical processing. Several forms of SCC have been observed and include sulfide stress cracking, anodic stress corrosion cracking, chloride stress corrosion cracking, and hydrogen stress cracking. There are other distinct terms of SCC for particular environment and metal combinations that are susceptible to this mode of attack.
Stress corrosion cracking can be defined as the cracking induced by the combined action of tensile stress, either residual or applied, a corrosive environment, and a susceptible material, resulting in the formation of brittle, often intergranular, cracks. Terms often used to describe SCC include stress-assisted corrosion cracking and environmentally assisted cracking (EAC).

Several factors influence the occurrence and severity of SCC in mechanical systems. Some variables include:

  • Tensile Stress: The magnitude of the applied or residual tensile stress can influence the susceptibility of the material to SCC. Higher stress levels increase the likelihood of crack initiation and propagation.
  • Microstructure: The grain size, grain orientation, and presence of inclusions or precipitates in the material can impact the initiation and propagation of SCC. Certain microstructural features can act as crack initiation sites or impede crack growth.
  • Oxygen Concentration: The concentration of dissolved oxygen in the environment can influence the electrochemical reactions occurring on the material’s surface, affecting the initiation and propagation of SCC.
  • Exposure Cycles: The frequency and duration of exposure cycles to the corrosive environment can impact the extent of SCC. Intermittent exposure may lead to different SCC behavior compared to continuous exposure.

Reducing the likelihood and severity of SCC in machined components is typically accomplished by a combination of mitigation strategies, such as:

  • Environmental Control: SCC is unique as it occurs in specific combinations of metals and environments, typically with aggressive species in the environment being at very small levels. Control the operating environment by minimizing the exposure to aggressive species, reducing temperature fluctuations, and controlling pH levels to reduce the rate of SCC.
  • Post-Weld Heat Treatment: Perform post-weld heat treatments to relieve residual stresses, particularly in weldments, which can be susceptible to SCC due to the combination of tensile stresses and potential metallurgical changes. This is specific to austenitic stainless steels.
  • Non-Destructive Testing (NDT): Implement regular non-destructive testing methods, such as ultrasonic testing, radiography, or eddy current testing, to detect and monitor the presence of SCC in components, allowing for timely repairs or replacements.
  • Proactive Designing: Design mechanical systems and components to minimize stress concentrations, limiting the maximum stress.
  • Permeable Gaps: Eliminate crevices and other areas where aggressive chemicals or deposits can accumulate, which can lead to localized corrosion and accelerate SCC. This is typically done by implementing protective metallic coatings into the component’s initial design phase.

Armoloy's Solution to Corrosion Cracking

Armoloy offers multiple metal surface treatments with varying levels of protection from the common causes of SCC. Offering both broad-spectrum and industry-specific applications, our protective metallic coatings add significant value through increased performance and decreased revenue losses from unplanned maintenance and downtime.

Our protective coatings ensure a thin, precise coat that won’t impact production, but will improve surface hardness and prevent environmental defects. Beyond increasing wear life, Armoloy tailors our metallic coatings based on the specific requirements of your application and industry.

SCC is a dangerous and stealthy corrosion mechanism that can lead to sudden failure in mechanical systems. Understanding the various forms of SCC related to the combinations of metals and operating environments can help reduce the likelihood of the potentially catastrophic failure mode from shutting down your operations at a costly price.

Beyond the Lab: Metal Failures in Narrative Form

Other Metal Failure Modes

Other common metal failures include:

Stress corrosion cracking can also result from, or be a precursor to, other potential metal failures

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