What is stress corrosion? Stress corrosion cracking/Stress cracking: Electrochemical Corrosion

This explains the stress corrosion and the conditions. Three different mechanisms involved in stress corrosion. Explains caustic embrittlement, season cracking, hydrogen embrittlement. Preventive measures of stress corrosion.
Stress Corrosion/Stress Cracking/Stress Corrosion Cracking
You will be able to define stress corrosion.
You will be able to enumerate the conditions and causes for stress corrosion.
You will be able to explain the mechanism of stress corrosion with special cases .
You will be able to suggest the preventive measures for stress corrosion.
Stress corrosion
Stress corrosion occurs due to the tensile stresses (applied stress and residual stress) on the susceptible material in the presence of a specific corrosive environment, resulting in the formation of crack which propagates.
Causes of Stress corrosion
Susceptible metal Specific corrosive environment
Copper alloys NH3 and ammonia compounds
Mild steel Alkalis and nitrates
Stainless steel Chlorides and acid chlorides
Causes of Stress corrosion
Due to stresses caused by heavy machining works in fabricated articles of certain alloys
Rolling
Drawing
Insufficient annealing
Cold or hot bending
Welding
Mechanism
Embrittlement
Caustic embrittlement
Hydrogen embrittlement.
Active Path Dissolution
Film induced cleavage
Caustic embrittlement
in high pressure steam boilers
Carbon and low alloy steels - in the presence of
caustic solutions, nitrates, phosphates, carbonates
Na2CO3(used for water softening) present in boiler feed water decomposes
NaOH flows into hair cracks of boiler walls by capillary action and reacts with iron to form sodium ferroate
Season cracking in copper alloys
Due to high residual stresses caused by cold working in fabricated articles of brass in the presence of ammonia or amines.
Dissolution of brass in ammonia leads to intergranular corrosion
Hydrogen embrittlement
Hydrogen being small, dissolves in all metals and diffuse between metal atoms in the crystal lattice.
Active Path Dissolution
Eg. Precipitation of chromium carbide over stainless steel.
Chromium concentration is low along the grain boundaries (susceptible region) - less passivated (anode) - corrosion occurs
Unstressed region - more passivated (cathode)
Film induced cleavage
Brittle film over ductile metal-Crack induced in the film and propagates into ductile metal -If brittle film is formed due to corrosion process, it repeats till transgranular fracture occurs
Prevention of stress corrosion
Avoid specific chemical environment.
Control operating temperature.
Control stresses while machining.
Choose materials which do not react with the specific chemical environment.