High Temperature Hydrogen Production: Selecting the Right Components

Hydrogen production doesn’t usually involve the high temperatures and corrosive fluids that can threaten the reliability of equipment in oil and gas refineries. However, high-temperature hydrogen attack, stress corrosion cracking, and hydrogen embrittlement are still concerns, and material selection is critically important. Materials that may be adequate for other operations may be unsuitable for hydrogen operations.

The quality of your components can be the difference that keeps your hydrogen production equipment running safely. In this post, we’ll go over some of the material concerns in hydrogen production, high-temperature hydrogen processes and the challenges they create, and how to choose the right materials and components to keep your hydrogen plant up and running.

Challenges of Hydrogen Production: High-Temperature Hydrogen Attack

While corrosive media and high-temperature processes are normally associated with refineries, not hydrogen plants, there are still potential hazards to watch out for in hydrogen production. High-temperature hydrogen attack (HTHA) and stress corrosion cracking (SCC) are two potential hazards that are especially dangerous because of how difficult they can be to detect.

HTHA occurs in steel exposed to hydrogen under high pressure and temperatures (above 200°C)—like those found within steam-methane reformers. At this temperature, hydrogen dissolves into the steel, where it reacts with the carbon in the alloy. This results in either surface decarburisation, where the reaction draws carbon to the surface of the material, or internal decarburisation, where hydrogen penetrates the steel and reacts with carbon to form methane. The methane cannot diffuse out of the steel, so it accumulates and creates voids in the material, eventually causing cracking.

Surface decarburisation decreases the hardness of the steel near the surface, which is usually not a major concern. However, internal decarburisation can cause serious deterioration of mechanical properties and even catastrophic failure if not mitigated quickly.

Because HTHA is caused by the reaction of hydrogen with carbon, a steel’s susceptibility to HTHA is heavily influenced by its composition. In particular, elements like chromium (Cr), molybdenum (Mo), and vanadium (V) that tie up carbon in stable precipitates help avoid HTHA. Increasing the content of these elements increases the steel’s resistance to HTHA. Likewise, the crystalline structure of austenitic stainless steels makes them resistant to HTHA.

Challenges of Hydrogen Production: Stress Corrosion Cracking

While HTHA is a danger in any high-temperature hydrogen process, stress corrosion cracking (SCC) is not normally considered a major issue in hydrogen production plants. The hydrogen in these processes is much cleaner than that used in refinery processes, but SCC is still possible.

While there may not be large amounts of caustics in the hydrogen production process, even tiny amounts can lead to SCC. Steel piping used in high-temperature steam methane reformers can be susceptible to SCC if there is any place for material to accumulate in the piping system. This is why it’s a good idea to have piping and tubing systems evaluated by fluid handling experts who can identify these points and design them out of the system. Like HTHA, SCC can also be prevented by using corrosion-resistant stainless steels high in elements like chromium and molybdenum.

How to Prevent Equipment Failures with High-Quality Materials and Components

For decades, hydrogen producers have used the Nelson curves in API RP 941 to guide the selection of materials resistant to HTHA. However, HTHA is complex, and failures have occasionally occurred even where the API RP 941 curves did not predict them. For example, the latest revision of API RP 941 (2016) reports 13 new failures below the carbon steel Nelson curve.

Current thought in the industry is that because the Nelson curves can be unreliable, the best approach to preventing HTHA is using inherently safer materials. Furthermore, hydrogen plants should not rely on inspections to detect HTHA, as the cracks are initially microscopic and highly localized.

Avoiding HTHA and SCC failures requires a proactive approach. Once cracks begin to occur, it’s probably too late. Prevent HTHA and SCC failures by following these guidelines:

• Identify all carbon steel components in hydrogen service that present potential hazards in the case of a failure.
• Validate the actual operating conditions (hydrogen partial pressure and temperature) for the identified components.
• Replace all carbon steel components that operate above 200°C and greater than 50 psia hydrogen partial pressure.
• Use inherently safer materials, like steels with higher chromium and molybdenum content.

Inherently Safer Materials for High-Temperature Hydrogen Production

Stainless steel is an alloy of iron, chromium, and other elements. When chromium is added to steel, it forms a surface passivation layer that protects the steel from corrosion. Stainless steel contains between 10.5% and 30% chromium, depending on the type. In general, the more chromium, the better the resistance to HTHA and SCC.

Even within a specific grade of stainless steel, though, there can be considerable differences in properties from one product to another. Most stainless steel is made with the lowest allowed percentages of expensive elements like chromium and molybdenum. Swagelok's stainless steel specifications, however, are at the high ends of the ranges specified by ASTM standards. This means Swagelok components provide better resistance to HTHA and SCC compared to products from other sources—even when they specify the same grade of steel.

Unlike most 304 and 316 stainless steels, Swagelok 304 and 316 are not cross-listable. Many manufacturers sell a stainless steel composition that meets specifications for both 304 and 316. However, because Swagelok’s stainless steels are made to tighter specifications, Swagelok 316 tubing and other components are superior to typical cross-listed 304/316.

Look to Local Experts for the Best Metallurgy for Oil and Gas Corrosion Prevention

If you have questions about which components and materials are best for your hydrogen operations, Edmonton Valve & Fitting can help. Our expert Field Advisors can perform an onsite assessment to understand your process and make sure you get the right components for your process. We can also make specific recommendations regarding piping and tubing system design, instrumentation, and custom components to improve reliability and protect against HTHA and SCC.

In hydrogen production, high-temperature component quality is key to safety and reliability. Our goal is to ensure you have the best available systems and components so you don’t have to worry about the challenges of high-temperature hydrogen production. All our products are backed by Swagelok’s industry-leading Limited Lifetime Warranty, so you can rest assured that you’re covered if anything does go wrong.