Stress corrosion cracking
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Transcript of Stress corrosion cracking
JABIN MATHEW BENJAMIN13MY04
M.E. INDUSTRIAL METALLURGY
STRESS CORROSION CRACKING
(SCC)
Flixborough explosion, UK : June 1974
28 fatalities as well as the near complete destruction of the NYPRO plant in north Lincolnshire by blast and then fire
The failure of a bypass assembly introduced into a train of six cyclohexane oxidation reactors after one of the reactors was removed owing to the development of a leak.
Resulting from the presence of nitrates that had contaminated river water being used to cool a leaking flange.
Mild steel exposed to hot nitrate solution and to stresses which in extreme cases may be well below the yield strength, and may remain in the structure from the fabrication procedure or may derive from operating stresses developed intergranular cracks.
WHAT IS SCC?
Failure of metal resulting from joint action of Stress Chemical attack
Initiation and propagation of cracks without any outside evidence of corrosion
Sudden and unpredictable failure: Catastrophic
Requirements for SCC
Susceptible metal
Specific environmentTensile stress
METAL-ENVIRONMENT FOR SCC
Cu alloys in ammoniacal solutionAustenitic Steels in oxide and chloride
environment at high temperature
Carbon steels in hydroxide and nitrate environment
Very high temperature water is aggressive to wide variety of materials
Stresses!!
SCC is caused by Static stresses
Residual : Cold working, welding, heat treatment, machining
Thermal stresses Applied stresses Build-up of corrosion products in confined spaces
Compressive residual stress Used to prevent or delay onset of SCC
Stress Corrosion Cracking
Crack produced by SCC in an aircraft part
SCC sites
SCC initiates at any pre-existing flaws
Surface discontinuities Grooves, cracks
Corrosion pits
Grain boundaries Intergranular cracking: sensitization of impurities at
grain boundaries making them very reactive
Mechanisms of SCC
Pre existing active path mechanism Suggests that there are pre existing paths that are
susceptible to anodic dissolution For example
A grain boundary precipitate anodic to the grain boundary Segregated impurities that behave anodic.
Area adjacent to depleted grain boundary will be preferentially attacked.
Creates localized galvanic cells, segregate being generally anodic
Predominant : SCC governed by electrochemical factors than stress.
Grain boundary precipitation
E.g.: formation of chromium carbide on the grain boundary and depletion of Cr adjacent to grain boundary in stainless steels.
Precipitation of carbide in Ni alloys and Mg2Si in Al alloys.
Grain boundary segregation
Impurities, such as P, S, C and Si, segregate at the grain boundary and cracking
Mechanisms of SCC
Strain generated active path mechanism Strain is the controlling influence Initially a passivating film is formed on the metal surface
Film is ruptured by plasticstrain, exposing bare metal.
Film forms again andprocess continues
Crack propagates by successive dissolution of crack tip in the process
Emergence of slip steps also considered to cause film rupture
Formed brass product with SCC caused cracks
Inside of Cu pipe showing crack
Intergranular cracks in Cu
PREVENTION
Choosing a material that is not susceptible to SCC in the service environment
Residual stresses can be relieved by stress-relief annealing
Similarly shot-peening or grit-blasting tend to introduce a surface compressive stress.
Neutralizing the environment whenever possible, e.g. reducing chloride ion concentration.
Reference
Zaki Ahmad, Principles of corrosion engineering and corrosion control, Sept 2006, Elsevier science and technology books.
www.corrosion-doctors.org/forms-scc
THANK YOU