What is stress corrosion cracking (SCC)?

Short Answer:

Stress corrosion cracking (SCC) is a type of failure in metals where cracks form and grow due to the combined effect of tensile stress and a corrosive environment. Even if the stress is lower than the metal’s strength, SCC can start and spread silently, weakening the material from inside and causing sudden breakage.

SCC is very dangerous because it happens without any visible warning. It mainly affects materials like stainless steel, brass, and aluminum alloys in environments containing chlorides, ammonia, or other chemicals. To prevent SCC, proper material selection, stress reduction, and environmental control are necessary.

Detailed Explanation:

Stress corrosion cracking and its mechanism

In mechanical and structural systems, metals are often exposed to stress and chemicals. When these two conditions come together, it can lead to a serious type of corrosion called stress corrosion cracking (SCC). This problem doesn’t just reduce the surface strength — it causes cracks to grow inside the material, often unnoticed, until the component fails.

SCC is a slow process that can occur over weeks, months, or even years, making it one of the most dangerous and unpredictable failure modes in engineering.

What is stress corrosion cracking (SCC)?

  • SCC is the growth of cracks in a metal due to the combined effect of:
    1. Tensile stress (either from load or internal residual stress), and
    2. A specific corrosive environment (like chloride or ammonia).
  • The metal may seem strong and safe, but under these two conditions, fine cracks start forming and growing over time.
  • The final fracture often occurs suddenly, without much warning, and at stress levels lower than expected.

Conditions required for SCC

SCC only occurs when three main factors are present:

  1. Susceptible material
    • Certain metals like stainless steel, brass, aluminum, and high-strength steels are more prone to SCC.
  2. Tensile stress
    • This can be from external forces or leftover stress from welding, bending, or heat treatment.
  3. Specific corrosive environment
    • Not all environments cause SCC. It depends on the metal:
      • Stainless steel → Chloride-rich water
      • Brass → Ammonia
      • Titanium → Fluoride environments

If any one of these three factors is removed, SCC will not occur.

How SCC starts and spreads

  • Initiation: Microcracks start at surface flaws, scratches, or grain boundaries under tensile stress.
  • Propagation: The corrosive environment attacks the crack tip, making it sharper and longer.
  • Fracture: The crack reaches a critical size, and the part fails suddenly without large plastic deformation.

The cracks usually follow grain boundaries (intergranular) or go through the grains (transgranular), depending on the material and environment.

Why SCC is dangerous

  1. Hard to detect
    • Cracks form inside the material and may not be visible on the surface.
  2. Sudden failure
    • Components can fail without any sign of damage, leading to serious accidents.
  3. Reduces service life
    • Parts that should last for years may fail early if exposed to SCC conditions.
  4. Affects safety-critical components
    • SCC has caused failures in nuclear reactors, aircraft parts, pressure vessels, and bridges.

Examples of SCC in real applications

  • Stainless steel tanks storing chlorine or saltwater.
  • Brass pipes exposed to ammonia in industrial plants.
  • Aluminum aircraft parts under stress near salty air.
  • Welded joints that were not stress-relieved properly.

These failures can lead to explosions, leaks, structural collapse, or serious environmental damage.

How to prevent SCC

  1. Material selection
    • Use metals and alloys that are resistant to the specific environment.
    • For example, use duplex stainless steel instead of regular stainless steel in chloride environments.
  2. Stress relief
    • Use heat treatment after welding or forming to remove internal stresses.
  3. Avoid corrosive environments
    • Remove or reduce exposure to harmful chemicals or moisture.
  4. Protective coatings
    • Apply paints or layers that block the contact between the metal and environment.
  5. Cathodic protection
    • Use sacrificial anodes to protect the main structure from corrosion.
  6. Regular inspection
    • Use non-destructive testing (NDT) to detect hidden cracks early.
Conclusion

Stress corrosion cracking (SCC) is a dangerous type of failure caused by the combined effect of tensile stress and a specific corrosive environment. It leads to the slow growth of internal cracks that often result in sudden and unexpected fracture. SCC is difficult to detect but can be prevented by choosing the right material, reducing stress, and avoiding harmful environments. Understanding SCC is essential for designing safe and durable components in industries like power, chemical, marine, and aerospace.