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Different Types of Corrosion
- Recognition, Mechanisms & Prevention

Environmental Cracking
 

Recognition
   

What is environmental cracking? Environmental cracking refers to the brittle fracture of a normally ductile material in which the corrosive effect of the environment is a causative factor. Environmental cracking is a broad term that includes corrosion fatigue, high-temperature hydrogen attack, hydrogen blistering, hydrogen embrittlement, liquid metal embrittlement, season cracking, caustic embrittlement, stress-corrosion cracking, stepwise cracking, sulfide corrosion cracking and sulfide stress cracking.

Historically, "season cracking" was first used to describe the cracking phenomenon of brass cartridges in the presence of ammonia; "caustic embrittlement" was used to describe the cracking of riveted mild steel boiler plates. The two terms were collectively known as "stress corrosion cracking" which was in turn replaced by "environment-sensitive cracking" or "environmental cracking". This broad term reflects the role of environment plays in the cracking process.
  Mechanisms
   

What causes environmental cracking? Environmental cracking results from the conjoint action of three components: (1) a susceptible material; (2) a specific chemical species (environment) and (3) tensile stress. For example, copper and its alloys are susceptible to ammonia compounds, mild steels are susceptible to alkalis and stainless steels are susceptible to chlorides.

There is no unified mechanism for environmental cracking in the literature. Various models have been proposed which include the following:

  • Adsorption model: specific chemical species adsorbs on the crack surface and lowers the fracture stress.

  • Film rupture model: stress ruptures the passive film locally and sets up an active-passive cell. Newly formed passive film is ruptured again under stress and the cycle continues until failure.

  • Pre-existing active path model: Pre-existing path such as grain boundaries where intermetallics and compounds are formed.

  • Embrittlement model: Hydrogen embrittlement is a major mechanism of environemntal cracking for steels and other alloys such as titanium. Hydrogen atoms diffuse to the crack tip and embrittle the metal.
  Prevention
    How to prevent environmental cracking? Environmental cracking can be prevented through:
  • Control of stress level (residual or load) and hardness.
  • Avoid the chemical species that causes environmental cracking.
  • Use of materials known not to crack in the specified environment.
  • Control temperature and or potential
  For more details
    More details on environmental cracking are included in the following corrosion courses which you can take as in-house training courses, online courses or distance learning courses:

Corrosion and Its Prevention (5-day module)
 Corrosion and Its Prevention (2-day module)
 Corrosion, Metallurgy, Failure Analysis and Prevention (3 days)
 Marine Corrosion, Causes and Prevention (2 days)
 Materials Selection and Corrosion (2 days)
 Stainless Steels and Alloys: Why They Resist Corrosion and How They Fail (2 days)
     
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