Corrosion handbook
06/21 9
1.2.4 Environmentally induced cracking
Stress corrosion cracking (SCC)
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. The stress in a material can be either
applied (external) or residual (internal) and sufficient to initiate an attack of stress
corrosion cracking.
Stress corrosion cracking is not simply an overlapping of corrosion and
mechanical stresses. Instead, it is 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 once
more to the exposure of new highly active metal surfaces in the crack. 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 (see Fig. 9)
• Material (e.g. generally alloyed metals)
• Stresses (applied or residual)
• Corrosive Environment
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DSSOLHGRU
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HJJHQHUDOO\
DOOR\HGPHWDOV
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Fig. 8: SCC of a bracket made of the
material 1.4301 (A2, 304) after approx.
4 years of use in an indoor swimming
pool.
Fig. 9: Factors required for the occurrence of stress corrosion cracking.
It is well known that certain grades of austenitic stainless steel can suffer stress
corrosion cracking in harsh environments such as indoor swimming pools. In
most of these cases, corrosion is initiated by chlorides attacking the passive layer.
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