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What is high temperature hydrogen attack (HTHA)/hot hydrogen attack?


Frequently Asked Questions

High temperature hydrogen attack (HTHA), also called hot hydrogen attack, is a problem which concerns steels operating at elevated temperatures (typically above 400°F or 204°C) in hydrogen environments, in refinery, petrochemical and other chemical facilities and, possibly, high pressure steam boilers. It is not to be confused with hydrogen embrittlement or other forms of low temperature hydrogen damage.

HTHA is the result of hydrogen dissociating and dissolving in the steel, and then reacting with the carbon in solution in the steel to form methane. This can result in either surface decarburisation, when the reaction mostly occurs at the surface and draws carbon from the material, or internal decarburisation when atomic hydrogen penetrates the material and reacts with carbon to form methane, which accumulates at grain boundaries and/or precipitate interfaces, and cannot diffuse out of the steel. This causes the fissures and cracking which are typical of HTHA.

Surface decarburisation results in a decrease in hardness and increase in ductility of the material near the surface. This is usually only a minor concern for these types of application. However, internal decarburisation, and in particular the formation of methane and consequent development of voids, can lead to substantial deterioration of mechanical properties due to loss of carbides and formation of voids, and catastrophic failure.

The main factors influencing HTHA are the hydrogen partial pressure, the temperature of the steel and the duration of the exposure. Damage usually occurs after an incubation period, which can vary from a few hours to many years depending on the severity of the environment. High temperatures and low hydrogen partial pressures favour surface decarburisation while the opposite conditions (lower temperature, high hydrogen partial pressure) favour fissuring. In addition, the composition of the steel influences the resistance to HTHA; in particular elements that tie-up carbon in stable precipitates such as Cr, Mo and V are very important. Increasing content of such elements increases the resistance to HTHA, and Cr-Mo steels with more than 5% Cr, and austenitic stainless steels, are not susceptible to HTHA.

In 1949, Nelson gathered and rationalised a number of experimental observations on different steels. In the Nelson diagram, boundaries are placed in a temperature/hydrogen partial pressure graph, which delineates the region of safe use for carbon steels, 1.25Cr-0.5Mo steels, etc. This diagram has been updated a number of times by the American Petroleum Institute (API) and published in the API recommended practice 941. More recently, analytical models have been used to predict the kinetics of HTHA with some success (Shih, 1982 and Parthasarathy, 1985).

There is increasing concern that the Nelson curves may not be relevant for the newer steels being used in high temperature hydrogen service, or may be overly conservative, and there are increasing trends towards risk-based inspection of items in hot hydrogen service. For information on how this approach could be applied for your situation, please contact us.

Further reading

  • Job Knowledge 143: High Temperature Hydrogen Attack (HTHA)
  • American Institute of Petroleum, Recommended Practice 941, seventh edition, August 2008
  • B. M Saba, Evaluation of mechanical fitness for service of high temperature hydrogen attack steels, Master's Thesis, Louisiana State University
  • P. F. Timmins, Solutions to Hydrogen Attack, ASM Intl, Metals Park, OH
  • H. M. Shih, A Model for calculation of the Nelson curves for hydrogen attack, Acta Metall Vol 30, P537
  • T. A. Parthasarathy, Mechanisms of hydrogen attack of carbon and 2-1/4Cr-1Mo steels, Acta Metall, Vol 33, P1673, 1985
  • Canale G; De Marco M; Pinca S, The role of welded joint vulnerability and various damage mechanisms active in process and petrochemical plants in reliability analysis with risk based inspection (RBI) approach, Eurojoin 7.Proceedings, 7th European Congress on Joining Technology; also incorporating GNS 5, 5th Edition of Italian Welding Days, Technical Session 2: Reliability of Welded Components and Structures. 40pp. 21 22 May 2009.
  • McLaughlin J E, A qualitative risk-based assessment procedure for high temperature hydrogen attack of C-1/2Mo steel, Proceedings, ASME Pressure Vessels and Piping Conference (PVP2007), and the Eighth International Conference, Creepand Fatigue at Elevated Temperatures (CREEP8), PVP2007-26129, 22-26 July 2007.
  • Krynicki J W; Bagnoli K E; McLaughlin J E, Probabilistic risk based approach for performing on onstream high temperature hydrogen attack inspection. Paper 06580 presented at Corrosion 2006, NACE International 12-16 Mar.2006.

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