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Hydrogen Induced CrackingHydrogen Induced Cracking
• Refers to mechanical damage of metal
• Caused by the presence/Interaction with Hydrogen
• Can be classified in to following types 1) Hydrogen Blistering 2) Hydrogen Embrittlement 3) Hydrogen Attack 4) Decarburization
Hydrogen Damage
Hydrogen Blistering
H+
H H2 H
H+
H
H
H
H
H
ee
H H2 H
VoidAir
Acid Electrolyte
• Occurs due to penetration of H2 in to metal
• Results in to local deformation and in extreme cases, complete destruction of the vessel wall
H H2 H
Hydrogen Embrittlement• Caused by the penetration of H2 in to a metal
• loss of ductility - Embrittlement
• Tendency increases with Hydrogen Concentration
• Catastrophic failure occurs if hydrogen concentration > 6 ppm
• Steels with >= 75 ksi are prone to Hydrogen Embrittlement
• Steels with >=110 ksi are very much susceptible to Hydrogen Embrittlement
Hydrogen Embrittlement• Absorption of hydrogen due to presence of H2 gas
– Hydrogen Stress Cracking
• Absorption of hydrogen due to corrosion reaction – Stress Corrosion Cracking
• Corrosion due to the presence of H2S -- Sulfide Stress Corrosion Cracking
Hydrogen Attack & Decarburization• Elevated temperature phenomenon
• Dissolved Hydrogen diffuses in to steel and reacts with carbon or carbides to from Methane Bubble (CH4) C [Fe] + 4 H [Fe] = CH4
• Formation at surface is known as Surface decarburization
• Formation within the metal is known as Internal decarburization
• Initially CH4 forms in micro voids grows to cracks
• Rate of reaction depends on(1) Amount of H2 (2) % C in the alloys(3) Amount of CH4
Hydrogen Attack & Decarburization
Hydrogen Induced Cracking (HIC)
• Step Wise Cracking (Blistering)
• Hydrogen Stress Cracking
• Hydrogen Embrittlement
• Hydrogen Induced Step Wise Cracking
All are known as “Hydrogen Induced Cracking (HIC)”
Mechanism of Cracking
• In the Presence of H2S, the process of cracking (HIC) can be divided in to 5 stages
1) Corrosion : Hydrogen Production 2) Hydrogen Diffusion 3) Hydrogen trapping in defaults 4) Increasing of pressure 5) Cracking
Hydrogen Induced Cracking (HIC)
Main external Factors: -Acidity of the environment-H2S content
• Metallurgical Factors : -Susceptible Microstructure -Inclusions in steel ( S, P, As etc)
Hydrogen Induced Cracking (HIC) Factors leading to Cracking
• Inclusions in the steel (Cracking often begins near inclusions)
-- Sulphide inclusions ( S= 0.001% Max)-- Oxide Inclusions ( O= 0.002% Max)
Hydrogen Induced Cracking (HIC)
• Susceptible Microstructure -- Hardened micro structure-- No PWHT
Factors leading to Cracking
• Most serious type of stress corrosion cracking - due to hydrogen (occurs in sour service)
• Hydrogen usually picked up from sulfide solutions
• Occurs in cathodically protected structures if the level of protection is too high
• H2S poisons the oxide surface - allows Hydrogen to diffuse in to steel
• Hardness exceeding 22HRC (appx. 248HV) prone to cracking
Hydrogen Induced Cracking (HIC)Summary
• Link-up of cracks at different planes to form steps across the thickness
• Cracks reduce the effective wall thickness
• The term “HIC” has been widely used and adopted by NACE international
• Several service failures
Hydrogen Induced Cracking (HIC)Summary
HIC TESTING
Standard Test ---- HICGeneral
• NACE TM 0284 – 96
• Std Test method for evaluating HIC resistance of pipeline and pressure vessel steels
• Provides results at simulated environment
• No acceptance or rejection criteria
Standard Test ---- HICTest Specimens – Pipeline Steels
Standard Test ---- HICTest Specimens – Pipeline Steels
Standard Test ---- HICTest Specimens – Pressure Vessel Plate
Standard Test ---- HICReagents
Solution “A” -- Nitrogen gas for purging, H2S gas, 5.0% Sodium Chloride (NaCl)
0.50% Acetic Acid (CH3COOH)
Solution “B” -- Nitrogen gas for purging, H2S gas, Synthetic Sea Water
Standard Test ---- HICTest Procedure
• Test Specimen Exposure
Wide faces vertical Separated from
vessel
Standard Test ---- HICTesting Apparatus
H2S Cylinder
Flow meter
Test Specimens
Trap Trap
10 % NaOH Solution
Solution
Standard Test ---- HICEvaluation of Test Specimens
• Sectioning of test specimen • Surface examination• Section to be polished metallographically and
etched• Include all identifiable cracks visible at 100X in
the calculations• Measurement of cracks (Crack length and
thickness)• Cracks separated by 0.5 mm - considered as
single crack
Standard Test ---- HICEvaluation of Test Specimens
• Crack Sensitivity Ratio (CSR) = [Σ(a x b)/ (W x T)] x 100%
• Crack Length Ratio (CLR) = (Σa / W) x 100%
• Crack Thickness Ratio = (Σb / T)x 100%
T
w
ba
ab
Standard Test ---- HICReporting Test Results
• Method of testing • Test solution used• pH of the test solution before introduction of H2S• pH of the test solution after completion of the test• Individual CSR, CLR and CTR
Preventive Measures
Hydrogen Induced Cracking (HIC)
• Using Clean Steel -- Killed Steel
• Using Coating (Metallic, Inorganic and Organic)
• Using Inhibitors
• Removing poisons
• Substituting alloys (Ni, Cr etc.)
• Reduce corrosion rate
• Baking
•Practicing proper welding
Preventive Measures
Hydrogen Induced Cracking (HIC)
• Using Clean Steel -- Killed Steel
• Using Coating (Metallic, Inorganic and Organic)
• Using Inhibitors
• Removing poisons
• Substituting alloys (Ni, Cr etc.)
• Reduce corrosion rate
• Baking
•Practicing proper welding