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Hydrogen Embrittlement of
Subsea Structures
David Jones Minton, Treharne & Davies Ltd.
Overview
• Hydrogen Embrittlement is a failure mechanism typically linked to corrosion and corrosion control processes,
• It is often encountered in the marine environment particularly in the offshore energy sector,
• It can affect a variety of metals but particularly high strength steels.
Effects of Hydrogen Embrittlement
• Hydrogen Embrittlement can result in:- • Reduced load bearing capacity of components,
• Cracking of components,
• Failure at unexpectedly low loads often below the Yield Stress of the material,
• Catastrophic brittle failure.
Why is Brittle Failure
Dangerous?
• In brittle failure a material cracks or collapses without plastic or elastic deformation,
• Brittle failures are typically sudden and occur with little or no warning,
• Examples of brittle failures range from shattered glasses to faults formed in the crust of the earth.
How Does Hydrogen Embrittlement Occur?
• The hydrogen atom is the smallest atom in existence,
• At ambient temperatures hydrogen can diffuse into steel,
• It accumulates at defects in the microstructure of the steel,
• It makes the steel brittle.
What Hydrogen Embrittlement Looks Like
What Hydrogen Embrittlement Looks Like
1
Hydrogen
1.00794
Causes of Hydrogen Embrittlement
• The embrittlement is caused by the introduction of hydrogen gas into the component,
• Hydrogen can be introduced by a variety of mechanisms including: -
– As a by-product of a chemical reaction
– From the use of Cathodic Protection
– By the action of Sulphate Reducing Bacteria and microbial communities
Chemical Reactions
• Hydrogen can be released by mechanisms that include: – Any industries handling hydrogen gas – Electroplated components – Environments containing Sour Gas (hydrogen sulphide)
• Metallurgical investigation, in particular hardness testing, can help determine the potential susceptibility of materials to this cracking mechanism
Cathodic Protection
• Cathodic Protection is used to prevent metal structures from corroding by using sacrificial anodes.
• The anode is attached to the metal structure (cathode) so in corrosive environments it preferentially corrodes and protects the structure.
• This reaction creates an electrical potential which causes hydrogen ions (H+) to be ‘pulled’ into the metal structure, leading to hydrogen embrittlement.
Cathodic Protection
• The larger the difference in electric potential, the more hydrogen that can be “pulled” into the cathode.
Sulphate Reducing Bacteria
• Sulphate Reducing Bacteria (SRB) consume organic debris on the sea bed which leads to the release of hydrogen sulphide.
• The hydrogen sulphide can react with steel to release hydrogen which can be absorbed into the steel.
• This can result in cracking, known as ‘Sulphide Stress Cracking’ or SSC.
Sulphide Stress (Corrosion?) Cracking
• SSC is NOT a form of stress corrosion cracking, it is a hydrogen cracking mechanism
• Hydrogen sulphide (H2S) reacts with the iron in steel creating iron sulphide (FeS) and liberating hydrogen (H2)
• In the presence of H2S the hydrogen diffuses into the steel
• If the steel is hard (>248HV) and is under stress then SSC can develop
Microbial Communities
• There are many varieties of organism that form on subsea structures that have a direct effect on hydrogen uptake.
• Microbial films can form directly on subsea surfaces and results in anaerobic areas in which bacteria can produce hydrogen.
• When in contact with steel the hydrogen can be absorbed which leads to hydrogen embrittlement.
What SRB Look Like
0.5 micron
Failure due to Hydrogen Embrittlement
• Once hydrogen has entered a component it can cause failure of that component by Hydrogen Induced Stress Cracking (HISC)
• This results from the application of a stress to an embrittled material
Hydrogen Induced Stress Cracking (HISC)
• In order for Hydrogen Induced Stress Cracking to occur an applied stress and susceptible microstructure and hydrogen need to be present.
• Applied stresses can occur as stress from service conditions, or from residual stresses resulting from during production (e.g. welding)
• The microstructure of the steel will influence it’s susceptibility to HISC.
• Ferritic steel microstructures and heat affected zones caused during welding are typical most susceptible.
Hydrogen Induced Stress Cracks
Hydrogen Induced Stress Cracks
Hydrogen Induced Stress Cracks
Case Study – Brittle Failure in a Process Plant – 1
• Welds in a Sour Gas processing plant failed shortly after plant start up
• Release of process gases posed the risk of fire, explosion and poisoning
Case Study – Brittle Failure in Process Plant – 2
• Dye Penetrant inspection revealed branching and meandering cracks
• Microstructural Inspection revealed fine cracks typical of brittle failure
• Hardness testing discovered hardness levels in the welds and heat affected zones above 300HV
• SSC was determined to be the cause of the cracks
Case Study – Brittle Failure in Process Plant – 3
• Incorrectly designed/performed welding process created changes to the previously sound parent metal adjacent to the weld
• This was a physical change rendering the parts unfit for service, a clear diminution of value
• Does this describe damage as envisioned by the policy?
Summary
• For failure by hydrogen embrittlement to occur an applied stress, susceptible microstructure and hydrogen are required.
• Applied stress can be difficult to avoid due service conditions and residual stresses.
• Different microstructures are more susceptible to hydrogen embrittlement and HISC
• Cathodic corrosion protectors, sulphur reducing bacteria and microbial communities cause uptake of hydrogen which can lead to HISC.