An Inspection Planning Process for Equipment Susceptible to Sulfide Stress Cracking

Introduction

When an inspection test plan specifies a wet hydrogen sulfide (H₂S) damage inspection, the damage mechanism was most likely defined from a corrosion study or damage mechanism review. Wet H₂S damage is a form of material degradation caused when atomic hydrogen diffuses into steel in wet H₂S process environments and collects at any internal material imperfections or discontinuities, leading to corrosion and cracking. Wet H₂S damage can manifest itself in numerous forms, including:

• Hydrogen blistering
• Hydrogen induced cracking (HIC)
• Stress-oriented hydrogen induced cracking (SOHIC)
• Sulfide stress cracking (SSC)

This article will focus on the inspection planning phases for the sulfide stress cracking (SSC) mechanism as a result of the wet hydrogen environments and will not address other mechanisms associated with wet H₂S damage. NACE defines SSC as a “cracking of a metal under the combined action of tensile stress and corrosion in the presence of water and H₂S. SSC is a form of hydrogen stress cracking resulting from absorption of atomic hydrogen that is produced by the sulfide corrosion process on the metal surface. SSC usually occurs more readily in high-strength steels or in hard weld zones of steels” [1].
The inspection planning process discussed herein assumes that the pressure equipment will be available for entry during a maintenance outage. With the damage mechanism defined, this article will help supplement when there is insufficient guidance or plant-recommended practices on what a full inspection scope consists of. It can be standard practice for the Inspection Planner to look at drawings, specify weld locations to be inspected, specify weld preparation requirements, and apply an appropriate nondestructive examination (NDE) method for the SSC inspection. Going beyond this simple planning process, SSC inspection planning is multi-faceted, consisting of personnel qualifications, roles and responsibilities, examination process, documentation, acceptance criteria, and, when applicable, defect removal and repair processes. This detailed planning will help define the inspection process and provide guidance on the necessary actions in anticipation of potential SSC damage, which has proven to help minimize discovery costs during maintenance outages.

Equipment Planning

When planning for an SSC inspection, the equipment should have already been evaluated as part of a corrosion study or damage mechanism review where the corrosion engineer and inspection personnel have identified damage mechanism susceptibilities and likely locations of damage. With the damage mechanism(s) assigned and the inspection and test plan (ITP) driving the due date, the first thing to do is get all the required documentation in order. Some of the items that will be listed may seem simple, but later in the article, the planning process will reveal the relevance of these items.

1. Gather the Manufacturer’s Data Report (e.g., U-1 form), construction drawings, historical write-ups, and previous wet H₂S inspection reports.
2. Mark up the construction or isometric drawing with the applicable welds that are subject to wet H₂S cracking defined by the corrosion engineer or corrosion control document (CCD). The areas subject to the damage mechanism should be specified in the CCDs. The marked-up drawings should also be labeled for the applicable welds that will be tested (e.g., LS-1 or NW-2).
3. After the drawings are marked for inspection, create a health summary for the equipment and, from the historical content and NDE reports, highlight the areas that have a history of repairs and the presence of wet H₂S cracking. Begin to compare the highlighted fields to the summary. Make sure that the current inspection plan covers previous areas that have had damage. If there are any discrepancies, proceed to consult the corrosion engineer on the best path forward and ensure the variance is resolved.
4. Next, record the equipment information such as thickness, material grade and specification, post-weld heat treatment condition, corrosion allowance, or any calculated minimum required thickness that can be used outside of nominal minus corrosion allowance. If the inspector is unsure of what minimum thickness to use, consult a pressure equipment engineer or a reliability engineer on the acceptable minimum thickness for the various components of the equipment.
5. Summarize the cleaning and surface preparation process previously used for the NDE method and decide if the technique is acceptable for the current plan. This step is important because the SSC leaves a sulfide scale on the surface, which may need to be removed depending on the corrosion expert’s recommendation or plant’s preference.