--- title: "High Temperature Hydrogen Attack" slug: "high-temperature-hydrogen-attack" updated: 2026-10-27T23:12:43Z published: 2026-10-27T23:12:43Z canonical: "ims-handbook.cenosco.com/high-temperature-hydrogen-attack" --- > ## Documentation Index > Fetch the complete documentation index at: https://ims-handbook.cenosco.com/llms.txt > Use this file to discover all available pages before exploring further. # High Temperature Hydrogen Attack ## Summary High-Temperature Hydrogen Attack (HTHA) is defined as an irreversible degradation of the mechanical properties of steel by high temperature reaction of absorbed hydrogen with carbon in the steel, resulting in decarburisation and internal fissuring. ### Detailed Guidance For detailed Guidance, see the attached PDF. A summary of the information, as it pertains to use in IMS is shown below. [PDF document](https://cdn.document360.io/e492a0e0-a4ba-4aca-8fc6-76b95f91fdf3/Images/Documentation/CEN.16.10653 - High Temperature Hydrogen Attack (HTHA).pdf?sv=2026-02-06&spr=https&st=2026-10-28T12%3A43%3A17Z&se=2026-10-28T13%3A43%3A17Z&sr=c&sp=r&sig=b6s6eteScjTesgHsVwQ2%2Bl0%2BrIMKA2so4UCcSS%2BkpsA%3D) ### StF – Step 7 See [Step 7](/v414/docs/step-7-determine-susceptibility-to-failure-stf-1). There are three different StF questionnaires: - StF Non PWHT Carbon Steels - StF 0.5Mo STEELS - Stf ALL STEELS − Excluding 0.5Mo Steels and non-PWHT Carbon Steel An appropriate StF questionnaire will be selected based on the user-provided input, depending on the entered BASE MATL and PWHT **Specs** on the **Component** level. ![](https://cdn.document360.io/e492a0e0-a4ba-4aca-8fc6-76b95f91fdf3/Images/Documentation/image-Z24LF88Q.png?sv=2026-02-06&spr=https&st=2026-10-28T12%3A43%3A17Z&se=2026-10-28T13%3A43%3A17Z&sr=c&sp=r&sig=b6s6eteScjTesgHsVwQ2%2Bl0%2BrIMKA2so4UCcSS%2BkpsA%3D) For a more detailed overview of the requirements for each StF questionnaire, as well as the details for each questionnaire, consult the spreadsheet attached below. [](https://cdn.document360.io/e492a0e0-a4ba-4aca-8fc6-76b95f91fdf3/Images/Documentation/HTHA-material-selection.xlsx?sv=2026-02-06&spr=https&st=2026-10-28T12%3A43%3A17Z&se=2026-10-28T13%3A43%3A17Z&sr=c&sp=r&sig=b6s6eteScjTesgHsVwQ2%2Bl0%2BrIMKA2so4UCcSS%2BkpsA%3D)HTHA-material-selection106.98 KB[](https://cdn.document360.io/e492a0e0-a4ba-4aca-8fc6-76b95f91fdf3/Images/Documentation/HTHA-material-selection.xlsx?sv=2026-02-06&spr=https&st=2026-10-28T12%3A43%3A17Z&se=2026-10-28T13%3A43%3A17Z&sr=c&sp=r&sig=b6s6eteScjTesgHsVwQ2%2Bl0%2BrIMKA2so4UCcSS%2BkpsA%3D) > [!NOTE] > Take Note > > In principle the API RP 941 should be leading. The S-RBI assessment has been adapted to include existing situations where 0.5-Mo Steels and as welded Carbon Steels (non-PWHT) are still in use for historical reasons. ### Confidence – Step 10 See [Step 10](/v414/docs/step-10-assess-the-confidence-1). For HTHA, the Confidence assessment has been extended from 3 to 4 questions and the scoring has been tightened to make it more difficult to extend inspection intervals unless process stability, process monitoring, and inspection are clearly understood. All questions need to be answered positive to gain Very High confidence. Consult Table 64 for guidance. Confidence Assessment for HTHA guidance: | Key Questions | Main Input | Guidance | | --- | --- | --- | | Degradation mechanism can be properly controlled. | Corrosion Engineer | Process Stability: Over the last run, has the unit has been operating - At the operating conditions for the next lower susceptibility level: "Yes" (+0.05) - Within the IOW: "Intermediate" (0.0) - Above the IOW or with exceedances above the IOW: "No" (-0.05) | | Relevant process parameters are reliably monitored. | Operator / Process Engineer and the Corrosion Engineer | Have the operating conditions been verified to be within the IOW? - On a more frequent Schedule than shown below: "Yes" (+0.05) - Per the process monitoring Schedule shown below: "Intermediate" (0.0) - Less often than shown below: "No" (-0.05) **Process Monitoring Validation Schedule for HTHA:** \| **StF** \| **Operating Conditions Validation Intervals for All Steels and 0.5Mo materials** \| **Operating Conditions Validation Intervals for Non-PWHT Carbon Steel** \| \| --- \| --- \| --- \| \| VH \| Risk management \| Risk management \| \| H \| 1 – 2 months \| 3 – 6 months \| \| M \| 12 months \| 12 – 24 months \| \| L \| 5 years \| 5 years \| \| N \| 5 years \| 5 years \| | | **StF** | **Operating Conditions Validation Intervals for All Steels and 0.5Mo materials** | **Operating Conditions Validation Intervals for Non-PWHT Carbon Steel** | | VH | Risk management | Risk management | | H | 1 – 2 months | 3 – 6 months | | M | 12 months | 12 – 24 months | | L | 5 years | 5 years | | N | 5 years | 5 years | | Reliable inspections have been carried out. | Inspection Engineer | - Inspection of >25% of the susceptible areas: "Yes" (+0.05) - Inspection of 25% of the susceptible areas: "Intermediate" (0.0) - No history of inspection or less than 25% of the susceptible areas inspected to date: "No" (-0.05) | | Total percentage of susceptible areas inspected at the last inspection. | Inspection Engineer | - >50%: "Yes" (+0.05) - Between 25% and 50%: "Intermediate" (0.0) - <25%: "No" (-0.05) This question allows the inspection interval to be increased to the maximum, based on the ratio of the amount of the susceptible areas inspected. A 50% inspection sample size is used as it allows the interval to be increased by a factor of two, but this can only be achieved if all the Confidence questions are answered positively. | IMS sums the total points from the four questions. Half scores, i.e., a total of 0.15 are not rounded up to 0.20 but is left at 0.10. ### MII – Step 11 See [Step 11](/v414/docs/step-11-review-the-results-nid-mii-is-inspection-effectiveness). IMS implements the following table to determine the MII. HTHA MII Lookup table: | Inspection Intervals (yrs) | **Confidence Rating** | | --- | --- | | VL | L | M | H | VH | | **Criticality** | E | 0 | 1 | 3 | 4.5 | 6 | | H | 0 | 3 | 6 | 9 | 12 | | MH | 0 | 6 | 12 | 16 | 24 | | M | 6 | 12 | 24 | 30 | 30 | | L | No inspection | No inspection | No inspection | No inspection | No inspection | | N | No inspection | No inspection | No inspection | No inspection | No inspection | The original tables are contained in the full document shown above. Please refer to it for additional guidance and explanations.