Wet H2S Cracking | Materials and Corrosion Control
| Damage Mechanism | Wet H2S Cracking |
| Damage Description | · Hydrogen Blistering: Surface bulges on the ID, the OD or within the wall thickness of a pipe or pressure vessel.
· Sulfide Stress Cracking (SSC): Cracking of metal under the combined action of tensile stress and corrosion in the presence of water and H2S. · Hydrogen Induced Cracking (HIC): neighboring or adjacent blisters that are at slightly different depths · Stress Oriented Hydrogen Induced Cracking (SOHIC): Arrays of cracks stacked on top of each other. The result is a through-thickness crack that is perpendicular to the surface and is driven by high levels of stress |
| Affected Materials | Carbon Steel and low alloy steels. |
| Control Methodology | · Consider effective barriers including alloy cladding and coatings.
· HIC-resistant steels can be used to minimize the susceptibility to blistering and HIC damage. · SSC can generally be prevented by limiting the hardness of welds and heat affected zones to 200 HB maximum through preheat, PWHT, weld procedures and control of carbon equivalents. · PWHT can help to minimize susceptibility to SOHIC. PWHT has limited value in preventing blistering and HIC damage from initiating but is beneficial in reducing residual stresses and strength levels that might otherwise contribute to crack propagation. · Specialized corrosion inhibitors can be used. |
| Monitoring Techniques | · Identify piping & equipment where conditions likely to promote wet H2S damage. · Inspection for wet H2S damage should generally focus on weld seams and nozzles. · Crack detection is best performed with WFMT, EC, RT or ACFM techniques. · UT & external SMUT can be used. · AET can be used for monitoring crack growth. · Visual Inspection: blistering · Monitor installed coupons for degradation |
| Inspection Frequency | · Field sampling of the free water phase should be performed on periodic basis.
· UT, coupons and visual inspection at T&I |
| KPIs | · NDT Inspection: Absence of cracks or blisters |
| Reference Resources (Standards/GIs/BPs) | · API RP 571 (DM #2)
· API RP 941 · NACE RP0296 |