High Temperature Hydrogen Attack (HTHA)

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High Temperature Hydrogen Attack (HTHA) is an insidious condition that can occur in process equipment exposed to hydrogen at elevated temperatures (at least 400F or 204C), under dry conditions, when hydrogen disassociates into nascent (atomic) hydrogen, which is then driven into the steel by the temperature and pressure of the environment. The atomic hydrogen then reacts with unstable carbides in steel to form methane gas, which accumulates in the microstructural grain boundaries, eventually leading to cracking. This is often hazardous as the equipment usually contains hydrocarbons at high pressures and temperatures.

HTHA is a time-temperature-pressure dependent phenomenon. This means the longer that a piece of equipment is exposed to temperatures and hydrogen partial pressures above its resistance limit, the more damage to the steel will accumulate; and the higher the temperature rises above the limit of the steel, the more rapidly the damage will occur.

Susceptible Areas

HTHA affects carbon and low alloy steels, but is most commonly found in carbon steel and carbon-1/2 Mo steel that is operating above its corresponding Nelson Curve limits. Areas that are hotter, often near the outlet nozzle of catalytic equipment or the inlet nozzle of an exchanger that is cooling the process, are areas of concern for HTHA. Welds often suffer from HTHA degradation as well.


Typically HTHA can be avoided by choosing the proper alloy steel or stainless steel cladding to resist the combination of hydrogen partial pressure and temperature, or by adjusting the operating conditions to stay below the Nelson Curve limit for the existing materials of construction. However, there have been several cases where HTHA was found even though operating conditions were below the Nelson Curve.

Inspection Techniques

It can often be difficult to predict the specific areas to inspect for HTHA, since the damage can be very localized. A corrosion or materials specialist, experienced in this particular phenomenon, should be consulted for identifying susceptible equipment, selecting inspection locations, and estimating remaining life of equipment in this service.

Inspection techniques for finding advanced stages of HTHA at the surface include WFMT, MT, and in-situ metallography (e.g., field metallographic replication). Advanced ultrasonic backscatter testing (AUBT) has also been successfully used to find earlier stages of HTHA.



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Articles about High Temperature Hydrogen Attack (HTHA)
January/February 2022 Inspectioneering Journal

This article describes HTHA predictive modeling advances that are designed to demonstrate a direct integration with inspection such that modeling and inspection can be leveraged together in a way never done before.

May/June 2021 Inspectioneering Journal

Hydrogen-induced damage mechanisms have been studied endlessly, leading to changes in equipment design and development of new inspection practices. However, the highly localized nature and difficulty of detection of HTHA add additional complexities.

Authors: Jose Aparicio
July/August 2020 Inspectioneering Journal

This article explores the API RP 581 third edition approach to HTHA damage factor calculations to obtain a probability of failure that subsequently affects risk. Sample calculations are provided to affirm an understanding of the concept.

March/April 2020 Inspectioneering Journal

Nelson curves, which are the foundation of the industry’s HTHA programs, can be tied to mechanistic models from several sources, allowing the recreation of Nelson curves for different operational histories and time durations.

January/February 2020 Inspectioneering Journal

Inspectioneering and PinnacleART co-hosted their 5th Meeting of the Minds roundtable last November in Atlanta. This meeting’s focus was on notable technologies and how they are changing the MI landscape.

Authors: Jeremiah Wooten
Partner Content

Statistical techniques are meant to complement the work of SMEs and can provide deeper insights into the inspection data collected as part of a reliability program. Pinnacle has combined traditional methods, expertise, and Machine Learning to...

July/August 2019 Inspectioneering Journal

Gasification technology has emerged in oil and gas due to the high combustion energy of Syngas and less environmental concerns than flue gas. This article discusses proper inspection and corrosion monitoring techniques for gasification units.

September/October 2018 Inspectioneering Journal

This article discusses the advances in NDE that have given owners increased confidence on how to assess equipment integrity in HTHA service providing risk prioritization of equipment as well as providing the ability to make more reliable...

May/June 2018 Inspectioneering Journal

HTHA of hydrogen-containing equipment can be prevented with appropriate material selection and fabrication, appropriate welding procedures, regular inspection of equipment using proven, effective technologies and equipment operated by qualified...

Authors: Samer E. Ibrahim
November/December 2017 Inspectioneering Journal

Hydrogen is a common culprit of equipment damage in the process industries. As hydrogen-induced damage can occur in multiple forms, it’s critical to identify the specific damage mechanism you’re dealing with before undertaking measures to...

January/February 2016 Inspectioneering Journal

The enormous decline in oil prices over the past 14 months has definitely slowed projects and changed the energy and production landscape. Despite this, refineries, petrochemical plants, and chemical facilities must continue to run safely,...

Authors: Greg Alvarado
Partner Content

Download Pinnacle's Economics of Reliability Report - Refining to see how reliability is affecting profitability.

November/December 2015 Inspectioneering Journal

To ensure the mechanical integrity and fitness-for-service (FFS) of equipment, facility managers, reliability engineers, and inspection technicians must understand the HTHA damage mechanism.

November/December 2014 Inspectioneering Journal

This article provides background on HTHA, discusses some current developments in HTHA inspection and mitigation, and describes how one refiner is instituting an HTHA risk management plan for its refineries along with challenges they have encountered.


I've crunched some numbers and have come up with the three most popular damage mechanisms, based on the activity of our readers, that Inspectioneering publishes information on.

Authors: Nick Schmoyer

In many ways, fertilizer plants are no different than most process industry plants including refineries and petrochemical facilities. Information in documents like API 510, 570, 653 and RPs 580, 581, 571, 577, 579, etc. is essential.

Authors: Greg Alvarado
January/February 2014 Inspectioneering Journal

The intent of these two articles is to share lessons learned from recent experiences setting up a comprehensive HTHA review process across multiple refineries, and to help other operators define and mitigate the HTHA risk to an acceptable level.

Authors: Mike Urzendowski
Partner Content

With the IMS Suite, you can calculate optimal intervention periods and lay out efficient maintenance strategies. Click here to learn more.


Last week we mentioned three important tips to consider when addressing high temperature hydrogen attack (HTHA). This week, we'd like to provide the reader with a few helpful resources that deal with the topic of HTHA.

Authors: Jeremiah Wooten

One of Inspectioneering's editorial themes this month is High Temperature Hydrogen Attack (HTHA). It is a highly contentious issue that the industry is still struggling to fully understand.

Authors: Jeremiah Wooten
November/December 2013 Inspectioneering Journal

A recent High Temperature Hydrogen Attack (HTHA) failure at a refinery in the state of Washington was a “wake up call” to our industry.

Authors: Mike Urzendowski
July/August 2012 Inspectioneering Journal

We have recently learned of one organized joint industry project (JIP) that was announced at the Spring 2012 API Refining meeting. As information has become available we have decided to present it to our readership.

July/August 2011 Inspectioneering Journal

Tesoro Corporation announced the release of the TOP (Triangle of Prevention) Investigation Team Report on the April 2, 2010, incident at the Anacortes Refinery in Washington State.

Partner Content

Learn how you can safeguard integrity, reliability, instrumentation safety, and more.

November/December 2010 Inspectioneering Journal

On April 6, 2010, a tragic accident occurred at the Tesoro Refinery in Anacortes, WA, in the Naphtha Hydrotreater process unit (NHT). During routine operations involving an on-line switching of unit heat exchanger feed trains, seven employees were...

November/December 2010 Inspectioneering Journal

High Temperature Hydrogen Attack (HTHA) is a long known and still occurring degradation issue for fixed equipment construction materials in the hydrocarbon process industry where hydroprocess plants (hydrogen plus hydrocarbons) are in service....

Authors: John Reynolds
November/December 2010 Inspectioneering Journal

Per a report in Downstream Today, by Whitney Pipkin Skagit Valley Herald, Mount Vernon, Knight Ridder/Tribune Business News on August 30, 2010, "Tesoro Anacortes Refinery announced Friday what investigators determined was the likely cause of an...

November/December 2010 Inspectioneering Journal

The following references are from the American Petroleum Institute. They are widely used in the petroleum refining and petrochemical industries for managing equipment in HTHA service and are available in the public domain.

November/December 2010 Inspectioneering Journal

Understanding, predicting, detecting and controlling high-temperature hydrogen attack (HTHA), have been elusive goals of materials engineers and scientists for over 70 years. The destruction of low alloy steel components exposed to hydrogen through...

January/February 2005 Inspectioneering Journal

HTHA falls into multiple categories of corrosion mechanisms, including environmentally assisted cracking, hydrogen assisted cracking, and high temperature degradation. Sometimes HTHA is confused with low temperature hydrogen cracking mechanisms...

Authors: John Reynolds
November/December 2004 Inspectioneering Journal

Accounts with shop validation on carbon steel samples prior to field trials, on an in-service C 1/2 Mo vessel, were reported at a recent industry conference. The studies were successful in the laboratory and appear to make sense in field trials on a...

July/August 2004 Inspectioneering Journal

Decarburization is the antithesis of carburization and rarely results in equipment failure. However, surface decarburization is often a sign that something more serious is going on, ie high temperature hydrogen attack (HTHA), which is well covered...

Authors: John Reynolds
November/December 1995 Inspectioneering Journal

Nelson Curve changes in the late 1980's provided cause for Shell Oil Company to look at more reliable NDE non-destructive evaluation methods for assessment of materials/equipment in high temperature hydrogen service. The primary change motivating...

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