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Carbon Steel Degradation in High Temperature Hydrogen Service

The purpose of this alert is to inform you that there have been several reports of cracking -related issues with carbon steel piping and equipment in high temperature, high pressure hydroprocessing service at operating conditions where carbon steel was previously thought to be resistant to high temperature hydrogen attack (HTHA). One published report of such incidents can be found in the paper PVP2010-25455, Proceedings of the 2010 ASME Pressure Vessels and Piping Conference, July 18-22, 2010, Bellevue, WA. API RP 941, Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants, 7th Edition, 2008, Figure 1, shows the operating limits for steels in hydrogen service to avoid decarburization and fissuring from HTHA. One curve on that graph is for carbon steel. At temperatures and hydrogen partial pressures below the curve, HTHA is not expected to occur in carbon steel.

Prior to these recent reports, the only reported failures of carbon steel below the API RP 941, Figure 1 curve were in cases of exceptionally high stress, as discussed in Sections 5.2 and 5.3 of API RP 941. All of the new reports of HTHA involve carbon steel equipment that was not postweld heat treated. Past research summarized in API TR 941, The Technical Basis Document for API RP 941, states that non-postweld heat treated welds not only retain high residual welding stresses but also have lower carbide stability in the weld heat affected zone that further increases HTHA susceptibility. The API RP 941 Task Group of the API Subcommittee on Corrosion and Materials is now in the early stages of collecting and verifying data and information to determine if the recommended practice might need to be altered as a result of this new information.

API is notifying all refining operating companies of this new issue should owner-operators decide to alter their inspection plans or risk assessments for carbon steel piping and equipment, especially if not postweld heat treated and/or highly stressed, and particularly in hydroprocessing services. Section 6 of API RP 941 provides recommended practices for inspection of equipment that may be susceptible to HTHA.

If any of your operating sites have experienced unexpected cracking issues associated with carbon steel equipment that may be due to HTHA, please bring those to the attention of API by participating in the Corrosion and Materials Subcommittee and the RP 941 Task Group. The form found in Annex F of API RP 941, Datasheet for Reporting High Temperature Hydrogen Attack (HTHA) of Carbon and Low-alloy Steels, provides a recommended format for internal company data collection.

For information on API's Refining Standards and the API RP 941 Task Group, please contact Steve Crimaudo, Manager, Downstream Standards, at

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