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A Primer on High Temperature Hydrogen Attack (HTHA)

Asset Intelligence Report

We hope this report helps in your pursuit of a higher level of Asset Integrity Intelligence.

Version 2014/July

OverviewHigh temperature hydrogen attack (HTHA) is an intergranular damage mechanism that can occur in process equipment that is exposed to hydrogen at elevated temperatures (at least 400F or 204C), under dry conditions, when hydrogen disassociates into nascent (atomic) hydrogen, which is then driven into the steel by the temperature and pressure of the environment. The atomic hydrogen reacts with unstable carbides in steel to form methane gas, resulting in the formation of gas pockets that lead to material degradation. The damage that results from HTHA ranges all the way from superficial surface decarburiza-tion of the inside diameter (ID) surface to severe material em-brittlement, loss of mechanical properties, and cracking.

HTHA is a time-temperature-pressure function, meaning that the longer that a piece of equipment is exposed to tempera-tures above its resistance limit in a certain hydro-process en-vironment, the more the damage to the steel will accumulate; and the higher the temperature rises above the limit of the steel, the more rap-idly the damage will occur.

Subject to varying conditions, HTHA in-volves the breakdown of molecular hydro-gen on the surface of steel, the subsequent diffusion of atomic hydrogen into the steel, and subsequent chemical reaction between the nascent hydrogen in the steel and dissolved carbon atoms in the steel microstructure. That reaction between hy-drogen and carbon produces methane gas, which collects in microstructure grain boundaries, inclusions, micro-voids, and laminations. Atomic hydrogen has no trouble diffusing through solid steel because of its tiny size. However, the methane gas molecule is much larger and cannot diffuse through the steel, so it collects and builds up to high internal pressures in those tiny cavities. That pressure build-up is enough to cause internal fissuring and eventually crack propagation in the steel as the methane con-tinues to collect and the pressure increases, ultimately causing deterioration in mechanical properties (strength, ductility and toughness).

Susceptible Areas HTHA damage has not only been found in fixed equipment materials in the hydrocarbon process industry, but also in high pressure boiler tubes, hydrogen producing units, synthetic gas units, ammonia plants and other equipment where hydrocar-

bons may not be present but high temperatures are involved.

HTHA affects carbon and low alloy steels, but is most com-monly found in carbon steel and carbon-1/2 Mo steel that is operating above its corresponding Nelson Curve limits (refer to the following section). Areas that are hotter, often near the outlet nozzle of catalytic equipment or the inlet nozzle of an exchanger that is cooling the process, are areas of concern for HTHA. Welds often suffer from HTHA degradation as well.

Nelson CurveThe Nelson Curve, shown below, is a vital component in un-derstanding this process. The American Petroleum Institute published the API RP 941 Nelson Curves in order help define whether or not a given material is vulnerable to HTHA.

The Nelson Curve stipulates that, for a given material, a low-er bound operating envelope (temperature and hydrogen partial pressure) criteria should be established (referred to as

hot hydrogen). Any equipment item that operates below this envelope can be ex-cluded from a HTHA assessment. An exam-ple of this would be to set 400F (204C) and 50 psia hydrogen partial pressure as a lower bound envelope for a screening eval-uation. This example is highlighted by the red lines in the figure on the following page, which uses the API RP 941 Nelson Curve to define what might be included in a typ-ical HTHA program

scope. In this case, all items to the left of and below the red lines would not be included in an HTHA assessment. For this step, design temperatures and partial pressures, or a worst case estimate with process engineering input, can be used, as this is just meant to be a conservative screening.

Note that in API RP 581, similar screening criteria is referenced, but uses 400F and a hydrogen partial pressure of 80 psia. Giv-en some recent HTHA failures being evaluated by the API RP 941 committee, 80 psia may not be low enough, particularly for non-heat treated carbon steels. Until the committee com-pletes its re-evaluation of the carbon steel Nelson curve, the owner-user may want to set criteria that includes a more con-servative buffer zone in the operating envelope. For example, although the curve for a material may be at 400F, the envelope selected may be set at 350F.

Figure 1. API RP 941 Nelson Curves

Prevention/MitigationHTHA can typically be avoided by choosing the proper steel to resist the combination of hydrogen partial pressure and tempera-ture, or by adjusting the operating conditions to stay below the Nelson Curve limit for the existing materials of construction. However, there have been several cases where HTHA was found even though operating conditions were below the Nelson Curve.

Effective Inspection TechniquesEffective inspection techniques for detecting early stages of HTHA involve a combination of various ultrasonic examination tech-niques including a frequency dependent backscatter method in combination with the velocity ratio and spectral analysis tech-niques. For weldments, high frequency shear wave and angle-beam spectrum analysis techniques are needed. This collection of advanced UT techniques is often referred to as Automated Ultrasonic Backscatter Techniques (AUBT), but clearly involves more than just backscatter techniques to be most effective in finding and identifying earlier stages of HTHA damage. Magnetic particle examination, conventional shear wave UT, and time of flight diffraction (TOFD) can be used to detect the later stages of HTHA in weldments where significant cracking is already present. Field metallography and replication can be useful for verification of potential HTHA damage discovered by other nondestructive evaluation (NDE) techniques.

Codes, Standards, and Best Practices API RP 571 Damage Mechanisms Affecting Fixed Equipment in the Refining Industry: This recommended practice discusses the iden-

tification of operative damage mechanisms, assessment of future damage progression rates, and the selection of appropriate NDE techniques for detecting and characterizing equipment damage.

API RP 941 Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants: This practice establishes practical operating limits for carbon and low alloy steels in hydrogen service at elevated temperatures and pressures. Also discussed are the results of high stress, heat treating, chemical composition, and cladding on the resistance of steels to hydrogen at elevated temperature and pressure.

Further Reading Avoiding HTHA Failures: An Owner-User Perspective of the HTHA Risk Mitigation Process - Part 1, November/December 2013

Inspectioneering Journal Avoiding HTHA Failures: An Owner-User Perspective of the HTHA Risk Mitigation Process - Part 2, January/February 2014

Inspectioneering Journal Avoiding HTHA Failures in Existing Equipment, November/December 2010 Inspectioneering Journal 99 Diseases of Pressure Equipment: High Temperature Hydrogen Attack (HTHA), January/February 2005 Inspectioneering Journal Methods for Detection, Characterization, and Quantification of High Temperature Hydrogen Attack (HTHA), November/December 1995

Inspectioneering Journal

All information found in this report is without any implied warranty of fitness for any purpose or use whatsoever. None of the contributors, sponsors, administrators or anyone else connected with this Asset Intelligence Report, in any way whatsoever, can be held responsible for the inclusion of inaccurate information or for your use of the information contained herein. DO NOT RELY UPON ANY INFORMATION FOUND IN THIS REPORT WITHOUT INDEPENDENT VERIFICATION.

Figure 2. Example of API RP 941 Nelson Curve with HTHA lower bound envelope defined.

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