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.

Microbial Community

Microbial Community

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.