Material Selection for Special Services: Wet H2S (Liquid Hydrogen Sulfide)

Wet H2S damage is a phenomenon affecting materials in a variety of industries, especially those related to oil and gas, refining, and chemical processing. This gas, in its liquid form combined with water, known as “Wet H2S”, represents a significant challenge for material selection, due to its ability to accelerate corrosion and embrittlement of metallic components. In environments with Wet H2S, the integrity of industrial facilities is crucial, as failure in material selection can result in catastrophic failures, operational risks, and high maintenance costs.

In this article, we will explore this phenomenon, how it affects materials, and what strategies exist to prevent or mitigate this problem.

Damage Mechanism of Wet H2S

Hydrogen sulfide (H2S) is a colorless, toxic, flammable gas commonly found in industrial environments. In the presence of water, whether in the form of steam, condensate or liquid, it dissolves and generates hydrogen sulfide, a weak acid that promotes corrosion in metallic materials, especially in steels.

When H2S dissolves, it releases protons (H+), which easily react with compounds susceptible to oxidation, such as the iron present in steels, and this process accelerates the deterioration of the materials.

It is important to note that the real damage is not caused by the H2S itself, but by the released protons, which penetrate the interstices of the steel, weakening its structure and favoring corrosion mechanisms.

Factors that influence damage by wet H2S

Damage by wet H2S depends on several interrelated factors, including:

• Material composition: carbon steels, low-alloy steels and certain stainless steels are particularly susceptible. The presence of inclusions or impurities in the material increases vulnerability.
• H2S concentration: at higher concentrations, the gas becomes more aggressive.
• Temperature and pressure: High temperatures can accelerate chemical reactions, while high pressure facilitates the solubility of H2S in water.
• pH of the medium: An acidic environment (low pH) intensifies corrosion.
• Presence of water: As mentioned above, without water, H2S cannot generate the corrosive medium necessary to cause moisture damage.
• Applied stresses: external or internal stresses increase the risk of sulfide-induced cracking depending on the damage mechanism present.

Types of damage caused by wet H2S

Wet H2S damage presents in different ways depending on the environmental conditions and materials involved. Common types of damage include:

• Blisters: These form when hydrogen, generated by the reaction of H2S with the metal, is trapped beneath the surface of the material, creating internal cavities or blisters that can weaken the structure. This occurs mostly in welded pipes and is not as common in seamless pipes.

This type of damage is mainly affected by inclusions/laminations, but is not related to the hardness of the material nor is it necessary for the material to be under tension. To mitigate it, preheating and cladding can be used (post-weld heat treatment is not applicable in this case).

• HIC (Hydrogen-Induced Cracking) and SWC (Step-Wise Cracking): Hydrogen-induced cracking (HIC) occurs when hydrogen accumulates in internal planes, parallel to the surface of the metal. This can lead to a step-wise fracture known as SWC. This type of damage is also particularly affected by inclusions and laminations and is not related to the hardness of the material. Mitigation will not be by post-weld heat treatment, but rather through preheating and cladding.
• SSC (Sulfide Stress Cracking): this type of cracking occurs when sulfide ions interact with metallic materials under stress, causing brittle fractures. It is common in carbon and low-alloy steels exposed to H2S. In this case, it occurs when the material is subjected to stress and hardness is the main problem. It is mitigated by preheating and post-weld heat treatment.
• SOHIC (Stress-Oriented Hydrogen-Induced Cracking): this phenomenon is similar to HIC, but oriented by stresses applied to the material, resulting in cracking patterns that can significantly compromise mechanical strength. It typically occurs due to the presence of residual stress and is not related to the hardness of the material. Post-weld heat treatment can minimize its effects, however, it is mainly mitigated by cladding and preheating.

Material Selection Criteria

When selecting a suitable material for wet H2S environments, factors such as corrosion resistance, mechanical properties, and resistance to hydrogen embrittlement must be considered.

There are several industry standards, such as NACE MR0175/ISO 15156, that define requirements for materials used in wet H2S environments. Ensuring compliance with these standards is essential to ensure durability and operational safety.

Some examples of materials commonly used in the construction of equipment and structures that operate in wet H2S environments include:

• Stainless steel (316L, 2205 duplex): Excellent corrosion resistance. 316L is ideal for moderate H2S concentrations and controlled temperature and pressure environments. Duplex 2205, which combines austenitic and ferritic stainless steel, offers better corrosion resistance in more extreme conditions, such as in saline waters or in the presence of high H2S concentrations.
• Nickel alloys (Inconel, Hastelloy): Very resistant to H2S corrosion and hydrogen embrittlement. Inconel is especially useful in high temperature environments, while Hastelloy is corrosion resistant under a wide range of conditions, including the most severe. These alloys tend to be more expensive than stainless steels.
• Carbon and alloy steel: Although these materials are less resistant to corrosion in the presence of H2S, they can be used in some cases if treatments such as coatings are used or if the environmental conditions are not as aggressive. Heat treatment and application of specialized coatings can improve their resistance.
• Coatings and surface treatments: In some cases, protective coatings such as epoxy paints, polymer coatings or nickel layers are used to increase the durability of materials exposed to H2S. Heat treatments can also be used to improve corrosion resistance.

Prevention and mitigation

Preventing wet H2S damage requires a combination of good design practices, material selection, and controlling operating conditions. Some key strategies include proper material selection, use of coatings and linings, environmental control, structural design, corrosion inhibitors, and monitoring and maintenance tasks.

Conclusion

Wet H2S damage represents a considerable challenge in industrial environments, but with a thorough understanding of the mechanisms involved and the implementation of appropriate strategies, its effects can be minimized. The selection of resilient materials, combined with good operating practices and preventative maintenance, is key to ensuring the safety and longevity of equipment exposed to this aggressive environment. Adopting these measures not only protects industrial investments, but also safeguards people and the environment from the risks associated with wet H2S.

For more information:
ASME | Material Selection for Piping Systems and PVs