High Temperature Hydrogen Attack: Safe use of carbon steel

Health and Safety Executive - Safety alert

Department Name:
Chemicals, Explosives and Microbiological Hazards Division

Bulletin No:
CEMHD 2 - 2018

Issue Date:
December 2018

Target Audience:
Chemical processing and production, Manufacturing (general)

Key Issues:
Carbon steel and susceptibility to High Temperature Hydrogen Attack – the use of such steel should be limited to applications of lower temperatures and pressures.


Where carbon steel is used as a material of construction in applications involving hydrogen (gaseous or liquid, including where it is a constituent part of a fluid), the service conditions should be restricted to limit the effect of High Temperature Hydrogen Attack (HTHA). This degradation mechanism attacks the material structure over time and is accelerated where the process conditions are more arduous. The effect is most notable in welds and other stressed areas and is particularly acute where welds are not stress relieved (post weld heat treated, PWHT).


The effect of hydrogen on carbon steel and its alloys has been known for some years, whereby at elevated temperatures and pressures, atomic hydrogen enters the microstructure of the steel, and reacts with the carbon present to form larger methane molecules. That process continues, with the hydrogen penetrating further through the steel structure, so that over time, tiny pockets of methane coalesce, leading to fissuring and ultimately crack development.

The issue led to the development of curves for various carbon steel alloys, plotted against axes of temperature and hydrogen partial pressure. These 'Nelson' curves are detailed in a publication from the American Petroleum Institute (API), Recommended Practice 941. The curves attempt to describe limit conditions for different grades of carbon and alloy steel, where exposure to adverse conditions above the curve will lead to HTHA effects on the micro-structure.

The U.S. Chemical Safety and Hazard Investigation Board (CSB) determined HTHA was the damage mechanism responsible for an incident in the USA in 2010, at the Tesoro Anacortes refinery, where the shell of a heat exchanger ruptured catastrophically, killing seven workers. CSB were of the opinion that the heat exchanger was operating significantly below the relevant Nelson curve applicable at the time. Challenged by the CSB findings, API added a curve for non-PWHT carbon steel, the 8th edition of API RP941 published in February 2016. CSB were of the opinion that this did not sufficiently address the issue, and in their Safety Alert of August 2016, as well as stating that operators should identify susceptible equipment and verify operating conditions, stated that operators should:

  • 'Replace carbon steel process equipment that operates above 400 °F and greater than 50 psig hydrogen partial pressure', and
  • 'Use inherently safer materials, such as steels with higher chromium and molybdenum content.'

The temperature and pressure conditions stipulated by CSB differ considerably from those detailed in the 'Nelson' curves. As a result of these differences, and in order to determine the relative validity or otherwise of the CSB and API positions, the Health and Safety Executive (HSE) commissioned independent research work with TWI Ltd. Two separate work-streams were initiated; firstly, reviewing HTHA mechanism and its progression, including the background information used to derive the 'Nelson' curves, and secondly consideration of the effectiveness of current non-destructive testing (NDT) approaches to detect HTHA. Both workstreams involved a review of academic papers as well as consultation with third parties and end users, so are regarded as sufficiently authoritative. The associated reports are available free of charge from the HSE website via the links below;

Considering the independent reports, HSE supports the CSB approach that inherently safer materials should be employed where HTHA is possible – this is consistent with the hierarchal approach to risk management and will apply where new or replacement equipment is being installed. For existing plant, API 941 8th Edition can be regarded as 'relevant good practice', and the Nelson curves can be used to help define a safe operating envelope for steels on hydrogen service. However, the curves should be used with some caution, and their use is dependent on a number of factors which should be taken in to consideration when assessing risk and developing a management strategy:

  1. Data used in plotting process conditions:
    1. needs to be accurate, taking in to account hot spots etc.;
    2. captures the entire history of the plant, and
    3. is representative of all operating conditions (including start up, shutdown, faults/trips, stripping/cleaning and other transient operation).
  2. The curves are not to be regarded as a 'no attack' line, below which the threat of HTHA is eliminated – operating close to but beneath the curves still comes at some risk;
  3. Time of exposure is an additional variable not currently taken in to account in the curves. It is anticipated that the time variable will be incorporated in to future editions of RP 941. In addition, operators should be aware that as understanding of the mechanism increases, further evolution of the curves is likely, and more restrictions may come about.
  4. The standard and curves should be applied with a degree of conservatism, especially where consequences of failure will lead to a major accident. Closer proximity to the applicable curve should involve higher levels of scrutiny, perhaps involving more frequent examination and testing.
  5. Equipment at threat of HTHA should be suitably installed and maintained, minimising applied stress. For example, pipework should be supported in accordance with an appropriate design code, such as ASME B31.3.
  6. NDT is continually developing, and currently requires complimentary, validated techniques to detect HTHA. The damage mechanism is very difficult to diagnose in the early stages, and unless newer, more advanced techniques are used at an appropriate time, may already threaten integrity by the time it is detected.

Action required:

Where operators have equipment susceptible to HTHA, they should take the following precautions:

  • Operators should adopt a hierarchal approach of elimination, prevention, control and mitigation. For new installations or where equipment is replaced, this would entail installing equipment of improved metallurgy (eg, alloyed with chromium and molybdenum) so that plant is more resistant to attack. Alternatively, operating at a reduced temperature and/or hydrogen partial pressure would also reduce the risk.
  • Operators should adhere to the limits prescribed by the 'Nelson' curves in the most up to date edition of API RP941. Continued operation above the relevant curve for the material, or significant excursion above it, is not acceptable and may result in enforcement. This would also apply where curves were adjusted or removed because of industry experience. In such instances, operators should plan for replacement with a higher grade of material and provide a short-term assurance of fitness for service.
  • Operators should apply a risk-based approach, so that proximity to the relevant curve, the age of the installation, and uncertainties such as:
    • missing historical process data;
    • the extent and quality of PWHT;
    • the limitations of NDT techniques;
    • etc.
    all lead to more conservative assumptions. The risk from failure due to HTHA should remain ALARP.
  • Operators should have sufficient instrumentation so that they can accurately state the worst process conditions that apply. Where excursions outside of a prescribed operating envelope take place, ie, excess temperature and/or hydrogen partial pressure, a competent person should be consulted at the earliest opportunity and the potential damage from HTHA assessed.
  • Where liquid service is encountered, whether continual or mixed with vapour, one of the assessment routes in API RP 941 Annex G should be adopted.
  • Where examination is part of the management strategy, operators should:
    • use validated techniques that are proven to detect the level of HTHA damage anticipated, recognising the limitations of all techniques to detect the early stages of HTHA;
    • Examine plant at sufficient intervals so that the progression of HTHA is monitored, predicted and does not threaten integrity;
    • Use competent personnel who have direct experience of HTHA detection, and have been trained using HTHA damaged samples;
    • Not rely on one single NDT technique to diagnose HTHA and employ at least two complimentary methods.
    • Examine all susceptible areas, such as structural welds, support attachments, hot spots etc., and not rely on a sampling approach.

Relevant legal documents:


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