Hydrogen Attack Repair
It is not acceptable to repair cracks attributable to the terminal stage of hydrogen attack (see Section 738) by local grinding and weld build-up. Cracks are likely to develop rapidly adjacent to the repair, in the material that had sustained a lesser degree of hydrogen attack. Furthermore, fitness-for-service analysis (see Section 750) is not applicable to cracks attributable to hydrogen attack. Therefore, the component of the vessel exhibiting the cracks (or the entire vessel) must be replaced before the vessel returns to service.
It may be possible to retain in service for a limited duration a vessel that has exhibited the initial stages of hydrogen attack detected by specialized ultrasonic examination (UT) or metallurgical study. However, this must be discussed with a pressure vessel or materials engineer before making a decision. If the decision is to return the vessel to service, it is necessary to schedule re-examinations at recommended intervals to monitor the progression of the hydrogen attack. It must be recognized that the vessel cannot be retained in service indefinitely, because the attack will continue to progress. Therefore, it is advisable to schedule replacement at an early opportunity that will not unnecessarily interrupt production.
All vessels, or components of vessels, that are replaced due to hydrogen attack should be redesigned with a material that is more resistant to hydrogen attack.
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