December 2020, Vol. 247, No. 12

Features

Avoiding Hydrogen Embrittlement, Corrosion Cracking Offshore

By Ben Burgess, Director, William Hackett Lifting Products  

Product standards and guidelines for the offshore oil and gas sector naturally have been based on international or harmonized standards, such as EN 818, ASME and DNV2.7-1.  

These standards and guidelines define the minimum acceptable product specifications and are based on how products typically will be used.   

For any products intended for offshore use, not only do they need to comply with these standards, but they also need to have material properties suitable for the specific risks of the offshore environment.  

Risk of Hydrogen   

Based on our experiences, combined with the manufacturing expertise of McKinnon Chain and the outcome of detailed technical analysis by  industry, we have identified that as material hardness exceeds 38HRC (hardness Rockwell C), the risk of hydrogen embrittlement and stress-induced cracking increases as the hardness values rise.  

Through our discussions and meetings with customers and third-party verification organizations, we are aware that the subject of material properties of lifting appliances is a growing issue, with recent incidents:  

1)  Norway: Two oil company’s fleets of G10-welded chain slings for their container fleet had to be withdrawn.  

2)  United States: A global oil company had to withdraw a number of lifting appliances and introduce an inspection regime prior to every lift.  

Fit for Purpose  

While chain and link products may be fully compliant with the relevant international standards, the reality is that they may be unsuitable for use in the offshore environment.  

 If a high-grade alloy steel lifting product does not have the correct material attributes for offshore lifting applications, the risks of its failure due to hydrogen embrittlement and stress-induced corrosion cracking (SICC) increases  because the material becomes inherently susceptible to these degradation mechanisms.  

Typically, when a product fails due to hydrogen embrittlement, it is instantaneous; therefore, the risks are severe.  

Meeting specific international standards, however, should not be seen as a guarantee that specific equipment is fit for purpose in an offshore environment.   

Specific environmental and performance considerations for equipment used in the offshore industry need to be an important consideration as part of the specification and selection process. (Figure 1) 

What is hydrogen embrittlement, and how does it happen?  

 Hydrogen embrittlement (HE) is a phenomenon that causes a loss of strength and toughness in a material through the introduction of hydrogen into the material, making it brittle.   

While there are several factors that can influence embrittlement,  such as temperature, in this briefing we are focusing on the impact of hydrogen embrittlement and its causes, namely, the ingress of hydrogen atoms into the metal, reducing its toughness and load bearing capacity at ambient temperatures hydrogen atoms can diffuse into steel.  

There are three main contributing factors of environmental conditions: sources of hydrogen, mechanical stresses and material susceptibility.   

1)  Environmental hydrogen sources: (trigger, cannot be avoided): Hydrogen can enter steel through the manufacturing process, including steel making, heat treatment and finishing processes.  

 It can also enter through the environment by way of corrosion in water and  cathodic processes linked to the pH content of moisture in the air. In addition, hydrogen sources simply cannot be avoided in an offshore environment, including seawater and resulting corrosion.   

As corrosion takes place, atomic hydrogen is produced and diffuses into the steel structure, where it is drawn toward the higher stress areas such as the crown of links and chain.   

As hydrogen diffuses into the steel, the tendency for the hydrogen is to accumulate at hydrogen traps such as inclusions, grain boundary areas, microstructural imperfections and areas of high stresses. Hydrogen atoms then interact with the dislocations in steels, resulting in localized deformation and fracture.  

In other cases, hydrogen atoms can interact with atomic bonds between iron atoms, resulting in decohesion and fracture.  

As a result, the internal strains created by the hydrogen can cause failure of the product when the yield strength of the steel is  locally exceeded.  

As the internal strains caused by the hydrogen increases, the load bearing ability of the steel decreases, the applied stress being a combination of two factors. In this scenario, the product may not be able to withstand the stresses of the product’s intended working load limit (WLL).  

2)  Mechanical stresses (trigger, cannot be avoided): When high-strength steels are subjected to sustained tensile loads under normal ambient temperatures, dissolved or absorbed hydrogen is attracted to the regions of high-tensile stress.   

As it diffuses to high-stress regions, it is absorbed on planes of weaknesses, such as grain boundaries, where it reduces the attractive forces between the iron atoms. When the force required for decohesion of these planes is reduced to less than that required to cause plastic flow, slow cracking occurs.   

A threshold stress exists above which hydrogen fracture can take place. As the strength of certain steel increases, the threshold stress is reduced.  

3)  Material condition (can be controlled): The susceptibility of steel to hydrogen embrittlement is related to its composition, microstructure, strength and hardness, otherwise known as the material property.   

It is important to note that the material property of the steel is pivotal in the process of hydrogen embrittlement while the hydrogen source and the mechanical stresses are triggers in the process. For certain combinations of properties, it is found that the steel is not susceptible to hydrogen embrittlement.  

Reducing Risk  

Selecting the correct equipment for lifting applications in the offshore environment in which it will be used is crucial for project delivery and safety.  

Choosing the correct equipment based on the operational environment (onshore/offshore) is the essential and primary consideration for operators and those with technical and corporate obligations for employee safety and successful project delivery.  

Onshore, products designed and manufactured for lifting in a stable and typically noncorrosive environment may be well-suited to higher strength steels, which offer increased WLL at a reduced unit mass.  

However, the issue is that unlike onshore applications, the products being used offshore will be constantly subjected to a corrosive environment and dynamic load amplification stresses as the norm.   

For offshore lifting applications, high-strength steel grades can be inherently susceptible to hydrogen embrittlement, if the material properties exceed certain thresholds.   

The fundamental way to mitigate the risk of hydrogen embrittlement is therefore to ensure that the material’s condition of a lifting product is suitable for the environment in which it will be operating.   

Since offshore environmental conditions of salt spray combined with the mechanical stresses cannot be avoided, attention must be given to material susceptibility and the insidious effect of hydrogen within the lifting appliance  itself.  

It has been well documented that higher tensile and higher hardness steels  are more susceptible to hydrogen embrittlement due to the hardness of the steel, which is related to the steel chemistry and, in particular, the carbon content plus the processing parameters of the product.  

Grade 10, 12 or even higher master links are now being produced, and all have a higher tensile strength and correlating hardness compared to lower grades. Using a higher-grade product could reduce the product mass and cost, if it were to replace a larger diameter product of a lower grade.   

While the use of these products is totally appropriate for use in controlled, industrial or construction environments onshore, these are not designed for the extreme conditions for which steel is exposed to in the offshore environment. Offshore, the products would typically face an increased susceptibility to embrittlement.  

This has led to significant product recalls, safety alerts and contractors having to remove assets from service while investigations are completed with organizations posting public safety alerts.  

To put this into context, a Grade 8 master link, when correctly heat-treated, will provide product ductility, toughness, tensile strength and resistance to shock absorption at hardness levels that enable the steel within the product to withstand the extreme conditions of the offshore environment, including hydrogen embrittlement.  

If the rate of corrosion on the surface of the steel can be reduced, then it follows that the amount of hydrogen entering the steel will also be reduced. This, in turn, will reduce any given material’s susceptibility to hydrogen embrittlement.  

However, it is important to ensure that through the manufacturing process, any potential influences of hydrogen on the material should also be avoided in the application of the corrosion protection process.  

Focus must be placed on the material (steel) and the end-to-end manufacturing process involved in the production of the offshore lifting equipment. Best practice includes, among others, following a clean steel manufacturing practice followed by optimum product processing parameters that will ensure a product with required material property to confirm suitability for offshore application.   

Processes related to hydrogen formation, such as pickling and electroplating, as well as heat treatment in hydrogen-rich atmospheres, should be avoided because they introduce an additional risk. 

Production Grups  

For anyone involved in the specification and selection process of offshore lifting appliances, there are some key considerations that should be taken to ensure selection of equipment for offshore environments is fit for purpose.  

As a general guide, the lower the hardness of the steel is, the less susceptible it is to hydrogen embrittlement. We recommend that hardness is always considered in product selection offshore.  

This is a result of the different manufacturing processes. For example, on bolts, the coatings used and load distribution on the head of the product and across the thread can act as a stress raiser. On the hoist load chain, a prescribed tensile strength must be achieved from a set diameter to fit within load sprockets. 

Lifting Recommendations  

1)  Master links: Maximum hardness of ≤38HRC while complying to DNV and API, with 100% proof testing, magnetic particle inspection (MPI) and stress-relieving heat treatment of the final product with certification enables real-time traceability to enable supply chain integration.  

The William Hackett HA range of Gr8 master links and quad assemblies have DNVGL-type approval (TAS000013Z Rev 3) for use in lifting sets for the offshore environment. The HA range has WLL from 4.1 tonnes to 250 tonnes, offering the most comprehensive range of links available on the market today.  

These HA links comply with international standards EN1677-4, DNVGL-ST271-271, DNVGLST273-273, EN12079-2 and API 2CCU. The HA range of links are individually proof load tested to 2.5 times WLL, individually MPI tested, Charpy impact tested  >42J @ -40°C and are suitable for use in the temperature range from -40°C to 200°C without reduction in WLL.   

2)  Welded, chain slings and marine lifting chain: The product attributes as the individual components of the sling are manufactured from the same base material to produce a consistent set or attributes.   

We recognize that there are different stresses applied to the individual component of sling and, therefore, recommend the following and that all G8 components fit within a range of 34 to 38HRC:  

Master links and intermediary links: ≤38HRC  

Chain: We recommend a marine G8 chain with a hardness of ≤38HRC.  

3) Marine Grade 8 chain: William Hackett and McKinnon Chain have developed a chain compliant with EN818-2, with material properties that give a maximum hardness of 38HRC.   

The chain is supplied with a zinc-tough corrosion protection finish and is suited to general lifting and rigging applications in the offshore environment.   

4)  Hoisting load chain: The same considerations apply to all chain-based lifting products and the material requirements for offshore lifting. We recommend G8 load chain that fully complies with EN 818-7 with a hardness range from ≤39 to 40HRC. Given that Grade 10/V has greater tensile attributes and less ductility than Grade 8, it is more susceptible to HE and SICC and not recommended.  

5)  Load attachment bolts: Grade 12.9 have proved to be susceptible to hydrogen embrittlement and associated safety alerts issued.   

Grade 10.9 bolts are acceptable and are referenced by a number of organizations (Lloyds Register and DNV) with associated hardness ranges.  

William Hackett Lifting Products has designed and developed a range of load attachment products with L7 (AISI 4140/4142/ASTM A320/A320M) bolts specifically for offshore and low-temperature lifting applications with ≤34HRC.  

6) Proof testing: Unless the harmful residual stresses from the manufacturing process are removed prior to the product going into service, plus the stresses caused by inspection processes that involve proof testing, an unsupported product in excess of its WLL will increase the residual stress in the product and thereby increase the risks relating to the product’s integrity.  

Conclusion  

The well-being and safety of both personnel and the environment cannot be overstated for company directors and heads of function.   

Hydrogen embrittlement and the stress-induced cracking that arises from the internal processes within steel are known and well-documented, and therefore difficult to claim during an investigation as an unknown consideration.  

William Hackett believes it is essential that the responsible organizations ensure that personnel and environment are not put at risk through a lack of detailed instruction regarding the requirement for a steel offshore lifting product to be  manufactured from a material with suitable properties for the environment in which it is used.  

Caution should be applied, and the personal well-being and corporate risks considered, when a manufacturer or third-party verification organization approves a product for offshore lifting that is not in line with the known science and industry best practice.   

Commercial pressures are never a valid decision-making criterion during an incident investigation.  

Specifically, with regard to the design and manufacturing processes associated with the industry leading range of HA DNV-type approved master links, we have proven that risks of hydrogen embrittlement have been minimized based on:  

• Material properties and a maximum hardness of ≤38HRC   
• Track record in the supply of HA links for major offshore operators through the supply of over 550,000 master links since 2004 without one suspected case of hydrogen embrittlement