High Temperature Hydrogen Attack (HTHA) | Materials And Corrosion Control
| Damage Mechanism | High Temperature Hydrogen Attack (HTHA) |
| Damage Description | · HTHA results from exposure to hydrogen at elevated temperatures and pressures. The hydrogen decomposes into atomic hydrogen at the surface and is able to enter the steel. At the elevated operating temperatures, this hydrogen has affinity to carbon. Under these circumstances, carbides in steel may decompose to release the carbon and form methane (CH4) at grain boundaries when reacting with the hydrogen. Methane molecules cannot diffuse through the steel. The resulting decarburization lowers material strength.
· Methane pressure builds up, forming bubbles or cavities, and microfissures at grain boundaries. Eventually, fissures may combine to form cracks. Blistering at internal locations of pressure vessels and piping may also occur; but this is considered an advanced stage of HTHA · Failure can occur when the cracks reduce the load carrying ability of the pressure containing part. |
| Affected Materials | In order of increasing resistance: carbon steel, 1Cr-0.5Mo, 1.25Cr-0.5Mo, 2.25Cr-1Mo, 2.25Cr-1Mo-V, 3Cr-1Mo, 5Cr-0.5Mo.
C-½Mo and Mn-½Mo steels are considered to have the same resistance to HTHA as plain carbon steel. Equipment: reactors are the focus, but also piping & heat exchangers in the high pressure hydrogen circuits should also be considered. |
| Control Methodology | · Use Cr-Mo steels in accordance to the applicable Nelson Curve described in API RP 941. · Verify that the proper steel has been installed and that the proper weld metal has been used in all the areas where HTHA is likely. PMI the unit for rogue material in HTHA environment · Follow temperature limits of API RP 941 curves for specific alloys, with 50°F (28°C) safety factor · Effect of Cladding: The permeability of hydrogen in steel is equal to the product of its solubility and diffusivity. |
| Monitoring Techniques | · Visual Inspection for blistering · Temperature reading/trend from PI-system · Metallographic replication and hardness checks at internal locations of pressure vessels and piping to check for surface decarburization, i.e. early sign of HTHA · Time Of Flight Diffraction (TOFD) and Advanced Ultrasonic Backscatter Technique (AUBT) for fissuring or internal cracking · Focus inspection on Inter-Critical Heat Affected Zone (ICHAZ) of reactor welds, especially nozzle welds & downstream of mixing tees, as these locations are more susceptible to HTHA |
| Inspection Frequency | · Visual examination at every T&I for gross cracking and blistering at internal locations of pressure vessels and piping
· Monitor temperature trends every shift · Metallographic replication and hardness checks at internal locations of pressure vessels and piping to check for surface decarburization at every T&I or as advised by CSD Metallurgical Specialist. · AUBT at susceptible locations of pressure vessels and piping based on material type and temperature margin with respect to the Nelson curve limit if metallurgical replication shows evidence of carburization, or at every other T&I or as advised by CSD Metallurgical Specialist · A combination of the above after every major process upset or temperature excursion above the Nelson curve limits, or as advised by the CSD Metallurgical Specialist. |
| KPIs | · Number and duration of temperature exceedances above the Nelson Curve limits minus 28°C safety margin; Note: Shell use 15°C safety margin
· Number and duration of temperature exceedances above the Nelson Curve limits (without invoking the 28°C safety margin) · High Susceptibility: > above the existing Nelson Curve limits · Medium Susceptibility = <15°C (25°F) below the exiting Nelson Curve limits · Low Susceptibility: 15 to 30°C (25-50°F) below the exiting Nelson Curve limits · Not Susceptible: >30°C (50°F) below the exiting Nelson Curve limits · Number of Visual Inspections · Number of metallographic replication and hardness checks · Number of AUBT inspections · Surface decarburization depth (as established by taking successive replicas, i.e., excavations, but without compromising Tmin). [This is a leading indicator for HTHA. Its presence signals borderline conditions for potential internal decarburization and fissuring] · Pv Parameter (Refer to Section below about this more detailed susceptibility indicator) |
| Reference Resources (Standards/GIs/BPs) | · API 571 (DM #10)
· API RP 941 · API RP 58i Base Resource Document for Risk-Based Inspection |
Section below is obsolete; little or no value added; also describes other issues such as temper embrittlement. Also, PDMs should not include charts; References/Resources address this requirement.
Pv Parameter:
This single parameter was developed to relate time at temperature and a hydrogen partial pressure:
Pv = log (PH2) + 3.09 x 10-4 . T . (log (t) + 14)
Where PH2 is the hydrogen partial pressure in kgf/cm2 (1kgf/cm2 = 14.2 psia)
and T is the temperature in K (K = C + 273)
and t is time in hours
This parameter can be used to define the susceptibility of a material to damage from HTHA.
This susceptibility is based on 200,000 hours of service data at a given combination of temperature and hydrogen partial pressure.
