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Exploring Assessment Methodologies for HTHA of C-Steels

The Problem

High temperature hydrogen attack (HTHA) leads to the weakening and fissuring of steel, making it an important degradation mechanism to consider for plants operating in hydrogen rich environments above 150°C. Susceptibility is typically assessed by the criteria in API RP 941 and the Nelson curves, which provide temperature-pressure limits to abide by.

Background to Current Concerns

A catastrophic failure of a C-steel heat exchanger vessel at Tesoro in 2010 due to HTHA threw the long-standing Nelson curves into the limelight. Since then, regulating authorities have been scrutinising the use of the curves more closely, and plant operators have been reviewing the adequacy of their procedures for the management of HTHA.

The Nelson curves in API 941 are based around data provided largely from industry experience and essentially represent a pressure-temperature limit for operation. Although useful, the black and white picture painted by the curves does not account for the numerous different situations that can develop in practice.  Beyond the Nelson curves, there is no widely accepted guideline for assessing the impact of HTHA damage on the structural integrity and remaining life of equipment.

HTHA damage can come in various forms and can be difficult to detect. Furthermore, once the degree of severity has been characterised, there remains a question over how to manage the affected plant. Welds are focal points, where damage is more likely to occur due to the susceptible microstructures, residual stresses and higher likelihood of flaws associated with them. Indeed, the latest API 941 document features a new appendix, with a lower operation limit for non-PWHTed welds.

The treatment of cracks and flaws in components operating in hot hydrogen environments is of particular concern. Pre-existing flaws are considered to be present at welds, of a size corresponding to the detection limit for the NDT procedure employed. Significantly, crack progression due to HTHA can occur before any detectable damage in the bulk material, indicating that the Nelson curves are not a reliable method for assessing safety where crack-like flaws are considered to be present.

Figure 1a. Schematic illustration of diffusion and reaction processes taking place in HTHA
Figure 1a. Schematic illustration of diffusion and reaction processes taking place in HTHA
Figure 1b. Micrograph of degraded C-Mn steel due to HTHA
Figure 1b. Micrograph of degraded C-Mn steel due to HTHA
Figure 2. Schematic showing crack development due to HTHA ahead of general attack at the surface
Figure 2. Schematic showing crack development due to HTHA ahead of general attack at the surface

UK Regulatory Action

Following the Tesoro incident report that identified HTHA as the failure mechanism and condemned the use of carbon steel altogether (CSB 2014), the UK HSE commissioned TWI to produce a review of the various aspects of HTHA. This included careful consideration of the data behind the Nelson curves, a critique of the CSB report, and a thorough review of the challenges facing NDT for the detection of HTHA. The full reports are freely available on the HSE website:

RR1133 – Maintaining the integrity of process plant susceptible to high temperature hydrogen attack. Part 1: Analysis of non-destructive testing techniques

RR1134 – Maintaining the integrity of process plant susceptible to high temperature hydrogen attack. Part 2: Factors affecting carbon steels

Engineering Assessment of HTHA in Pressurised Vessels

TWI is currently developing techniques for the effective detection and assessment of HTHA damage in welded components. The studies are focussed on measurement and assessment of crack growth, as the proper treatment of crack-like flaws in components is very important to avoid catastrophic failures in pressurised plant. A large-scale validation experiment has been designed that involves a heated vessel, pressurised with hydrogen gas, containing flaws engineered to grow under the operating conditions. Sophisticated NDT techniques will be used to monitor crack initiation and growth during operation and the results compared to predictions made using data from small-scale experimental data. Information on the Joint Industry Project can be found here.

The state-of-the-art facility developed at TWI’s laboratories in Cambridge is unique in that it allows continuous monitoring and assessment of damage as it develops under realistic operating conditions. The facility gives TWI Members a venue where they can explore, develop and refine their monitoring and assessment approach for HTHA.

Avatar John Rothwell Principal Project Leader - Materials

John Rothwell is a Principal Project Leader in the Materials department of TWI. He obtained an MEng in materials science and engineering at The University of Sheffield and is a qualified welding engineer with over 10 years of experience tackling issues affecting engineering steels and weldments in the oil and gas and thermal power industries.

For more information please email:


contactus@twi.co.uk