Resistance to Cracking Failure Under Tension and Corrosion

Stress corrosion cracking (SCC) is a combined mechanical and electrochemical corrosion process that results in cracking of certain materials. It can lead to unexpected sudden brittle failure of normally ductile metals subjected to stress levels well below their yield strength. Internal stresses in a material can be sufficient to initiate an attack of stress corrosion cracking. Stress corrosion cracking is not simply an overlapping of corrosion and mechanical stresses, but can be understood as an autocatalytic, self-accelerating process leading to high metal dissolution rates (anodic reaction). Initially, a small pit is formed and develops into a crack due to applied or residual stress in the material. The crack formation opens up a new active (non-passive) metal surface, which again will corrode very easily. This leads to further crack propagation and again to the exposure of new highly active metal surfaces in the crack.

CONDITIONS TO START STRESS CORROSION CRACKING

Metal dissolution in the crack will advance rapidly until mechanical failure occurs. SCC is a highly specific form of corrosion that occurs only when the following three different requirements are fulfilled at the same time.

• Mechanical (load, stress)
• Material (susceptible alloy – e.g.: austenitic stainless steel)
• Environment (highly corrosive, chlorides, sulfide)

TEST SPECIFICATION FOR STRESS CORROSION CRACKING

The test methods are listed below.

• NACE TM0177 – Laboratory Testing of Metals for Resistance to Sulfide Stress Cracking and Stress Corrosion Cracking in H2S Environments.
• ISO 15156 – Petroleum and Natural Gas Industries – Materials for use in H2S-containing Environments in Oil and Gas Production
• ASTM G36 – Standard Practice for Evaluating Stress Corrosion Cracking Resistance of Metals and Alloys in a Boiling Magnesium Chloride Solutions.