Saturday, January 31, 2009
Wet H2S Cracking - Basics
Corrosion Mechanisms:
This is another type of typical corrosion mechanisms in oil & gas industry that occurs on steel. Hydrogen Sulfide (H2S) naturally co-exists with oil/gas in the well reservoir. When gas/oil is extracted during production, the H2S and other gaseous (e.g. CO2) and water are extracted together. H2S dissolved in water produces hydrogen H- ions. H- atom is relatively small and is able to diffuse through the grain boundaries or any defect opening in the steel materials. Two H- atoms combined forms H2 molecule (gas). H2 molecules accumulated at a localized area and trapped. This causes highly localized pressure build-up and thus initiate a crack.
Wet H2S can occur in several forms or morphologies:
(1) Hydrogen blistering - causes localized blisters on the steel surface.
(2) Hydrogen Induced Cracking (HIC) - also known as stepwise crack which is normally parallel to the steel surface.
(3) Stress-Oriented HIC (SOHIC) - with presence of stress, the crack can propagate across the thickness of the steel and causing typically transgranular cracks.
(4) Sulfide Stress Corrosion Cracking - is a combined effect of sulfidic corrosion and cracking due to hydrogen diffusion.
Apprearance:
(1) Hydrogen Blistering
Image source: http://webwormcpt.blogspot.com
(2) Hydrogen Induced Cracking (HIC)
Image source: http://www.clihouston.com/knowledge-base/wet-h2s-cracking.html
(3) Stress-Oriented HIC (SOHIC)
Image source: http://www.clihouston.com/knowledge-base/wet-h2s-cracking.html
(4) Sulfide Stress Corrosion Cracking
Image source: http://octane.nmt.edu/waterquality/corrosion/H2S.htm
Main factors affecting susceptibility:
(1) pH / H2S concentration – higher concentration, higher susceptibility
(2) Temperature – higher temperature, higher susceptibility
(3) Materials properties - higher hardness, higher susceptibility(4) Stress i.e. applied / residual stress due to welding
Preventions / Mitigations:
(1) Use HIC-resistant steels – “high purity steels”
(2) Preheat, PWHT and control of carbon equivalent – limit hardness value to max. 200 HRB.
(3) Specialized corrosion inhibitors or coating.
ARE YOU EXPERT IN WET H2S CRACKING? ANY COMMENT ON THIS ARTICLE? (click Comments below)
Corrosion Mechanisms:
This is another type of typical corrosion mechanisms in oil & gas industry that occurs on steel. Hydrogen Sulfide (H2S) naturally co-exists with oil/gas in the well reservoir. When gas/oil is extracted during production, the H2S and other gaseous (e.g. CO2) and water are extracted together. H2S dissolved in water produces hydrogen H- ions. H- atom is relatively small and is able to diffuse through the grain boundaries or any defect opening in the steel materials. Two H- atoms combined forms H2 molecule (gas). H2 molecules accumulated at a localized area and trapped. This causes highly localized pressure build-up and thus initiate a crack.
Wet H2S can occur in several forms or morphologies:
(1) Hydrogen blistering - causes localized blisters on the steel surface.
(2) Hydrogen Induced Cracking (HIC) - also known as stepwise crack which is normally parallel to the steel surface.
(3) Stress-Oriented HIC (SOHIC) - with presence of stress, the crack can propagate across the thickness of the steel and causing typically transgranular cracks.
(4) Sulfide Stress Corrosion Cracking - is a combined effect of sulfidic corrosion and cracking due to hydrogen diffusion.
Apprearance:
(1) Hydrogen Blistering
Image source: http://webwormcpt.blogspot.com
(2) Hydrogen Induced Cracking (HIC)
Image source: http://www.clihouston.com/knowledge-base/wet-h2s-cracking.html
(3) Stress-Oriented HIC (SOHIC)
Image source: http://www.clihouston.com/knowledge-base/wet-h2s-cracking.html
(4) Sulfide Stress Corrosion Cracking
Image source: http://octane.nmt.edu/waterquality/corrosion/H2S.htm
Main factors affecting susceptibility:
(1) pH / H2S concentration – higher concentration, higher susceptibility
(2) Temperature – higher temperature, higher susceptibility
(3) Materials properties - higher hardness, higher susceptibility(4) Stress i.e. applied / residual stress due to welding
Preventions / Mitigations:
(1) Use HIC-resistant steels – “high purity steels”
(2) Preheat, PWHT and control of carbon equivalent – limit hardness value to max. 200 HRB.
(3) Specialized corrosion inhibitors or coating.
ARE YOU EXPERT IN WET H2S CRACKING? ANY COMMENT ON THIS ARTICLE? (click Comments below)
Labels: Corrosion, Hydrogen Sulfide, Wet H2S Cracking
¶ 5:14 PM
