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published on

December 2023

Strategies for Preventing High Temperature Hydrogen Attack (HTHA) Failures: Safeguarding Industrial Environments

Oil refining, fertilizer, and hydrogen production facilities often use carbon and low alloy steel for a variety of piping and equipment that were previously within industry standard requirements. However, specific components in high temperature/hydrogen environments may be susceptible to high temperature hydrogen attacks, leading to equipment and piping failures and serious incidents.

It’s vital to know how to prevent high temperature hydrogen attacks for the safety of worksite personnel and the preservation of important equipment. This guide will explore what a high temperature hydrogen attack is and a few means of preventing and mitigating the risk of HTHAs in your organization’s facilities.

What is a High Temperature Hydrogen Attack?

high temperature hydrogen attack (HTHA) – sometimes called a “hot hydrogen attack” or “methane reaction” – is a chemical process that occurs when steel is exposed to hydrogen-rich atmospheres at higher temperatures (usually 204°C/400°F or above). During an HTHA, hydrogen molecules diffuse into a steel alloy, causing two types of damage:

  • Surface decarburization, which occurs when dissolved carbon atoms react with surface hydrogen and escape into the atmosphere as methane gas.
  • Internal decarburization, which occurs when carbides deeper in the steel dissolve due to lost carbon atoms. If left unchecked, the internal decarburization can lead to a significant loss of strength, as well as form methane pockets at steel defects or grain boundaries. In the long term, methane pockets can cause further cracks to form on steel surfaces.

The risk factors of HTHAs include steel temperature, hydrogen content in the atmosphere, stress profile in specific areas, and length of exposure. Steel composition can also impact inherent resistance (or lack thereof) to HTHA failure.
Typical progression of HTHA from almost undetectable to microscopic to macro crack fissuring.

Operating Limits for Steels – API (RP) 941

Organizations use the American Petroleum Institute (API) Recommend Practice (RP) 941 – Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants – to determine the vulnerability of steel equipment and components to high temperature hydrogen attacks.

Specifically, API (RP) 941 uses Nelson curves to determine the process conditions within which steel equipment can safely operate while minimizing the risk of HTHA failure. Nelson curves are based on industry process data that is voluntarily reported to the API.

Generally, organizations and facilities can use API 941 to determine whether a piece of steel equipment, such as steel piping, is safe to use for service, even when exposed to hot hydrogen conditions. However, there is some concern that Nelson curves might not be relevant or accurate for new steels being used in certain services or environments, which we will explore in more detail below.

Effective Means of Preventing HTHA Failures

All organizations that rely on steel components exposed to high temperatures and hydrogen-rich environments must know the best means to prevent HTHA failures. In general, the best way to prevent HTHA failures is to perform regular, non-destructive examinations of relevant equipment and to use the right equipment for hazardous processes in the first place.

Let’s explore both of these individually in more detail.

Perform High Temperature Hydrogen Attack Assessments and Inspections

Engineering assessments are necessary to know what equipment and piping may have been exposed to elevated temperatures and hydrogen.

It is imperative that qualified personnel inspect steel equipment to notice the early signs of and prevent HTHA damage prior to significant damage or equipment malfunctions. Your organization should carry out high temperature hydrogen attack inspections regularly.

HTHA failures can still occur even if your equipment meets the new API Nelson curves. The 2016 eighth edition of the API (RP) 941 standards includes two carbon steel Nelson curves. But there are some limitations. For example, the curve for non-post-weld heat treated carbon steel doesn’t take into account all possible estimated process conditions where catastrophic HTHA failures can occur.

In other words, while the API (RP) 941 standard is valuable and a good benchmark quality to require for steel components, it’s not foolproof. Regular inspections are always important.

Some historical Non-Destructive Examination (NDE) methods have been shown to be unreliable but are still in practice. Newer NDE methods have demonstrated a higher level of accuracy and reliability, so they should be utilized whenever possible.

Depending on your facility and the expertise of available corrosion or material specialists, you may carry out different examination techniques and inspections, such as:

  • Phased Array Ultrasonic Testing (PAUT)
  • Time of Flight Diffraction (ToFD) testing
  • Full Matrix Capture/Total Focusing Method (FMC/TFM)

BakerRisk Materials Engineer uses laboratory equipment to examine specimens and determine if HTHA occurred.

Where to Look for HTHA Failure

Carbon and low alloyed steels – particularly carbon steel and carbon-1/2 Mo steel – are more susceptible to HTHA failures below current Nelson curves. Therefore, specialists should inspect these materials and components often.

Furthermore, steel components that operate in hotter areas, such as steel components or tubing around the outlet nozzles of catalytic equipment, are at an inherently higher risk of high temperature hydrogen attacks. The same is true for any equipment that operates above 400°F and in conditions with more than 50 psi hydrogen partial pressure.

Use the Right Materials for Tubing & Other Process Equipment

Your worksite should and components possible (where it is practical to replace existing steel elements). For example, steel with higher chromium or molybdenum content is less susceptible to HTHA failure.


Performing engineering assessments and regular inspections, using durable steel materials with inherent resistance to HTHAs, and adjusting operating conditions where possible are the best ways to reduce HTHA failures in industrial environments. BakerRisk’s experienced personnel and state-of-the-art tools can help you identify areas of risk in your facility and inspect steel components for HTHA vulnerabilities. Contact one of our representatives today to learn more.