Inspectioneering Journal

Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants

This article appears in the November/December 2010 issue of Inspectioneering Journal.

Editor’s Note: 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. To learn more visit the API website at

This recommended practice (RP) summarizes the results of experimental tests and actual data acquired from operating plants to establish practical operating limits for carbon and low alloy steels in hydrogen service at elevated temperatures and pressures. The effects on the resistance of steels to hydrogen at elevated temperature and pressure that result from high stress, heat treating, chemical composition, and cladding are discussed. This RP does not address the resistance of steels to hydrogen at lower temperatures [below about 400°F (204°C)], where atomic hydrogen enters the steel as a result of an electrochemical mechanism.

This RP applies to equipment in refineries, petrochemical facilities, and chemical facilities in which hydrogen or hydrogen-containing fluids are processed at elevated temperature and pressure. The guidelines in this RP can also be applied to hydrogenation plants such as those that manufacture ammonia, methanol, edible oils, and higher alcohols.

Hydrogenation processes usually require standards and materials that may not be warranted in other operations of the petroleum industry. At certain combinations of elevated temperature and hydrogen partial pressure, both chemical and metallurgical changes occur in carbon steel, which in advanced stages can render it unsuitable for safe operation. Alloy steels containing chromium and molybdenum can be used under such conditions.

The steels discussed in this RP resist high temperature hydrogen attack (HTHA) when operated within the guidelines given. However, they may not be resistant to other corrosives present in a process stream or to other metallurgical damage mechanisms operating in the HTHA range. This RP also does not address the issues surrounding possible damage from rapid cooling of the metal after it has been in high temperature, high pressure hydrogen service (e.g. possible need for outgassing hydroprocessing reactors). This RP will discuss in detail only the resistance of steels to HTHA.

Presented in this document are curves that indicate the operating limits of temperature and hydrogen partial pressure for satisfactory performance of carbon steel and Cr-Mo steels in elevated temperature, hydrogen service. In addition, it includes a summary of inspection methods to evaluate equipment for the existence of HTHA.

API TR 941, Revision / Edition: 8 - 9/00/08 Technical Basis Document for RP 941

Reports covering a half-century of comprehensive research on hydrogen attack have been reviewed. The major investigators were found to agree about what information would be needed to model the curves presented in API RP 941. However, they concluded that quantification of key, very complex material property and performance inputs is not possible. Prediction of attack limits from first principles therefore remains elusive. With the benefit of hindsight, the curves in API RP 941 are explained herein. A series of reasonable assumptions appear to justify Nelson’s placement of the lines for carbon and low alloy steels.

The approach proposed here is applied to these common steels and agrees with trends in attack thresholds established by experience. It is based on the obvious and long-held notions that if the methane pressure in voids is low compared to the material’s strength or methane forming reaction rates are low, attack does not occur. The approach is flexible and can be applied to all carbon and low alloy steels. It can also be used as a starting point to estimate the effect of applied stress on time-dependent behavior.

Application of these models to refinery equipment, especially clad components, has been attempted and the results are credible. Ferritic and austenitic stainless steel overlay and cladding are clearly effective. However, practical implementation of the principles is impeded by uncertainties regarding diffusivity, solubility, absorption rates, and fluxes of hydrogen and the effects of stress and materials strength.

Among the stumbling blocks to successful modeling of hydrogen attack is the lack of knowledge of relevant concentrations and activities of carbon and alloy elements remaining in solution after heat treatment. Also, there is scant knowledge of details about void nucleation and the rates of the methane forming reactions in voids. Local compositions at grain boundaries and the compositions of carbides are probably important, but are not known with certainty. The manner and rate of the evolution of hydrogen attack damage have not been studied quantitatively.

Prediction of attack boundaries is difficult since materials of a grade differ in critical respects and those that have been attacked in service have never been fully characterized (as discussed in Appendix A). Systematic laboratory studies of the effects of heat treatment and stress could build confidence in the conclusions offered here and provide valuable information for life assessment and risk evaluation.

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