Wet H2S Damage

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Wet Hydrogen Sulfide (H2S) Damage is a common problem in the oil & gas and petrochemical/chemical manufacturing industries. It can occur when carbon-steel equipment becomes exposed to wet H2S service environments, and it can come in several different forms. Wet H2S corrosion can be a particularly dangerous form of corrosion because damage caused by it takes place on the interior of vessels, it can occur without warning, and it can only be detected using complicated inspection methods.

Hydrogen Sulfide (H2S) is a colorless, flammable, and extremely hazardous gas with a strong "rotten egg" odor. Hydrogen sulfide is particularly dangerous to equipment when moisture is present. H2S damage can lead to rapid and extensive damage to metals, including uniform corrosion, pitting, and stepwise cracking.

Causes of Wet H2S Damage

Wet H2S Damage occurs due to the effects of aqueous hydrogen charging of steel in wet H2S process environments. This process can happen at relatively low temperatures, largely as a result of atomic hydrogen from wet H2S corrosion reactions which enter the steel and collect at inclusions or impurities within the steel. This happens because the H2S prevents the hydrogen recombination reaction that would normally occur, forcing the hydrogen atoms into the metal structure, leading to corrosion and weakness.

Wet H2S damage primarily occurs under acidic conditions, which are present in most oil refining environments. Any equipment that runs in conditions that are both above 50 ppm of H2S content and below 180F temperature in aqueous sour waters is likely susceptible to wet H2S cracking.

Hydrogen Blistering

Hydrogen blistering refers to the physical description of a metal with subsurface cavities that were created by accumulated hydrogen gas. Physically, these cavities appear as subsurface “bubbles” or “blisters.”

Blisters form as a result of atomic hydrogen diffusing through metal and accumulating in voids. Being highly reactive, hydrogen atoms combine with each other inside of voids and form hydrogen gas (H2). The buildup of hydrogen gas then increases the pressure beneath the surface of the metal to form blisters.

Types of Wet H2S Cracking

The most common forms of wet H2S cracking are hydrogen induced cracking (HIC), stress-oriented hydrogen induced cracking (SOHIC), and Sulfide Stress Cracking (SSC).

Hydrogen Induced Cracking (HIC) is a form of tiny blistering damage caused by a high concentration of hydrogen in steel. The blistering damage tends to form parallel to the surface and to the direction of hoop stress. Because of this, it usually doesn’t become damaging until it either becomes extensive and affects material properties, or gives rise to cracking that propagates into a weld or begins to go step-wise through the wall. On the surface, HIC is often horseshoe shaped and no bigger than the cuticle of one’s small finger.

Compared to HIC, Stress-Oriented Hydrogen Induced Cracking (SOHIC) is much more insidious. SOHIC is made up of a series of HIC cracks that are stacked perpendicularly in the direction of through wall cracks and driven by high residual or applied stresses. Because this damage can easily lead to integrity failures, facility owners should take measures to prevent or mitigate it when possible.

Sulfide Stress Cracking (SSC) occurs at locations where atomic hydrogen is able to diffuse at sites of high internal stress, such as grain boundaries, inclusions and regions of triaxial stress at notches. When placed in proximity to tensile stresses, embrittlement and the beginnings of brittle fracture may occur.

Detection & Prevention/Mitigation of H2S Damage

The most common NDE method for detecting wet H2S cracking is Wet Fluorescent Magnetic Particle Inspection (WFMP). This method is able to detect sub-surface cracks in the steel that are caused by HIC, SOHIC, and SSC. For cracked piping and other components which cannot be inspected using WFMP, an alternative technique is Phased Array Ultrasonic Testing (PAUT).

Although detection is important, new stainless alloys can be implemented to replace traditional steels in applications where corrosion can be particularly severe. When coupled with chemical inhibitors, these alloys are effective at mitigating corrosion, although they may in some cases still be susceptible to SSC.

Equipment that is specifically susceptible to SOHIC can be made more resilient by incorporating post weld heat treatment (PWHT) and/or by being alloyed up. HIC-resistant steels and polymeric coatings have also been successfully used to prevent damage. In more aggressive environments, another solution might be using stainless steel clad materials, as they are more resistant to this sort of damage.

Note on the Risks of Exposure to H2S

H2S can cause possible life-threatening situations if not properly handled. Workers exposed to H2S can experience serious short term and long term effects, including rapid unconsciousness, coma, and even death. In petroleum refining environments where exposure to H2S is possible, all workers should employ appropriate procedures for identifying, monitoring, and preventing H2S exposure.

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Articles about Wet H2S Damage
January/February 2022 Inspectioneering Journal

Within one unit, TriLat combines the power of two probes containing three angle beam sets to identify and quantify cracking at early stages. The result is inspection speeds up to ten times faster than traditional AUT systems, depending on probe size.

January/February 2022 Inspectioneering Journal

This edition of Damage Control will offer practical steps to mitigate different forms of wet H2S damage and help to minimize long-term inspection and maintenance costs related to wet H2S damage.

November/December 2021 Inspectioneering Journal

This issue of Damage Control offers a perspective on how to assess the different forms of wet H2S damage using modern FFS and computational analysis techniques with the safe operation of damaged pressure vessels, piping, and associated components.

September/October 2021 Inspectioneering Journal

This article summarizes the fundamentals of wet H2S-related damage mechanisms, offers some practical inspection guidance, and reviews a notable industry failure caused by different forms of wet H2S damage.

January/February 2019 Inspectioneering Journal

This article provides a summary of the 2019 API Inspection & MI Summit keynote address, which offers a past, present, and future outlook on fixed equipment mechanical integrity from an industry professional with 50+ years of experience.

Authors: John Reynolds
Partner Content

Tri-Lateral Phased Array is a novel robotic phased array ultrasonic testing technique for the on-stream inspection of fixed equipment in wet hydrogen sulfide (H2S) service. TriLat identifies and quantifies wet H2S damage in the base metal of...

November/December 2003 Inspectioneering Journal

There are a variety of forms of wet H2S cracking. In this short article I will focus on two of the most common forms: hydrogen induced cracking and stress-oriented hydrogen induced cracking (HIC/ SOHIC). HIC is often fairly innocuous (but not...

Authors: John Reynolds
May/June 1995 Inspectioneering Journal

Exposure of carbon steel equipment to wet H2S service environments can lead to various forms of attack, e.g. hydrogen blistering and hydrogen induced cracking (HIC), stress oriented hydrogen induced cracking (SOHIC) and sulfide stress cracking...

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