stress corrosion cracking
DESCRIPTION
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• Stress corrosion cracking is a failure mechanism that is caused by environment, susceptible material, and tensile stress. Temperature is a significant environmental factor affecting cracking.
“OR” A structure under static tensile stress, much below the yield
stress, in contact with corrosive environment may fail due to SCC. Three conditions must be present simultaneously to produce SCC: a critical corrosive environment, a susceptible alloy and some component of tensile stress.
Now the phenomena of “SCC” is shown in the following figures
Cont’d
• SCC leads to the formation of a crack which would not have developed by the action of the stress or environment alone. It is the unexpected sudden failure of normally ductile metals subjected to a tensile stress in a corrosive environment.• The required tensile stresses may be in the form of directly
applied stresses or in the form of residual stresses.
Causes of SCC
• There are three main causes for SCC as given follows:
• Corrosive Environments• Tensile Stresses Above Threshold• A Susceptible Material
Cont’d• Common sources of tensile stresses are:1: External stresses caused by
• Centrifugal forces• Temperature variation
2: Residual stresses due to• Forming• Heat treatment• Welding• Machining• Grinding
Cont’d
3: Concentration of stresses at the root of pre-existing cracks or notches
Stages Involved In SCC
• The process of SCC consists of three stages:• Crack Initiation• Crack Propagation• Brittle Fracture
Crack Initiation
• SCC is initiated by stress concentrations at defects on the material surface. The defect may be an existing material defect. The defect may also be a result of pitting corrosion, crevice corrosion, intergranular corrosion or local galvanic corrosion.
Crack Propagation
• The passivation film at the tip of the crack is broken due to plastic deformation. Pure and normally very active metal is exposed and will be attacked by corrosion. Grow rate will be a combination of corrosion and cracking. The crack starts to grow when the stress concentration at the end of the crack (KI) exceeds the threshold stress intensity factor for stress corrosion cracking (KISCC).
Brittle Fracture
• When the stress concentration at the end of the crack exceeds the critical stress intensity factor (KI > KIC), there will be a rapid, unstable brittle fracture.
Examples• Many materials, particularly high strength materials, are
susceptible to stress corrosion cracking when exposed to a specific environment. For example, cold worked brass, which is found in ammunition cartridges, is susceptible to stress corrosion cracking when exposed to an environment containing ammonia. In chloride containing environments, titanium alloys, aluminum alloys, and high strength stainless steels are susceptible and specific alloys, which are resistant to stress corrosion cracking, should be used. The stresses required to initiate and propagate cracking are often low and many failures occur due to residual stresses rather than applied stress.
Ways Of Propagation Of SCC
• SCC can proceed in either of two ways : • Intergranular Stress Corrosion Cracking• Transgranular Stress Corrosion Cracking
Intergranular SCC• The cracks which propagate along the grain boundaries
called as intergranular stress corrosion cracking as shown in the following figures.
Transgranular SCC
• The cracks which run through the individual grains called as transgranular stress corrosion cracking as shown in the following figures.
Identification Of SCC
• Visual identification prior to failure is difficult due to the typical tightness of stress-corrosion cracks. A low-power hand lens will greatly aid determination. Detection of cracks can also be enhanced with ultrasonic, radiographic, or acoustic emission techniques.
Preventive Measures• There are a number of different ways to control SCC. The method
used depends on the application and may involve changing the mechanical, metallurgical and environmental conditions.
• Mechanical• Avoid stress concentrators• Relieve fabrication stresses• Introduce surface compressive stresses• Reduce operating stresses
• Metallurgical• Change alloy composition• Change alloy structure• Use metallic conversion coating
• Environmental• Add inhibitor• Modify temperature
