Full Description

There are several forms of wet H₂S cracking of carbon steel that are mechanistically very similar which are: hydrogen blistering, hydrogen induced cracking (HIC), and stress oriented HIC (SOHIC). Blistering is the physical bulging of the steel typically in a single plane, HIC has a stepwise (through-thickness crack linkage) nature that is created from laminar (in-plane) cracking, and SOHIC is the arrangement of small HIC cracking perpendicular to the stress in a stacked array and is typically located in the residual stress fields of welds. These three types of damage result from atomic hydrogen, which is generated by a surface corrosion reaction, permeating into the steel. The hydrogen then combines at inclusions, discontinuities or other imperfections forming hydrogen molecules that are too large to diffuse out of the steel. Lower strength carbon steel plate is most susceptible to hydrogen blistering, HIC, and SOHIC damage, especially when exposed to an aqueous hydrogen environment, such as sulfide, cyanides, and hydrofluoric acid. These mechanisms typically occur below 300F and are common in the oil, gas, and petroleum refining industries, particularly if older high sulfur or phosphorous steels are present. Blistering and HIC damage have typically not lead to breaches of containment in pressurized equipment; however, SOHIC has on rare occasions lead to breaches in containment.

This Bulletin focuses on the background of blister damage, HIC, and SOHIC. A review of WRC Bulletin 396, "Research Report on Characterization and Monitoring of Cracking in Wet H₂S Service," along with detailed analyses of laboratory tests undertaken on steel removed from severely HIC damaged vessels are included. In addition, the results of round robin NDE testing of HIC damaged plates in the MPC FFS Joint Industry Program are discussed. The results of testing and WRC Bulletin 396 indicated that HIC damaged steel has considerable remaining strength. However, with HIC comes the added uncertainty of potential for SOHIC. Therefore new rules were developed for API 579-1/ASME FFS-1 that include both the recognition that HIC damaged material has remaining strength, but also the need to perform cracklike flaw calculations. A commentary on the new rules in Part 7 of API 579-1/ASME FFS-1 issued in 2007 is also included in this paper.

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Published: 2007 Number of Pages: 65 File Size: 1 file , 16 MB