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