Inspectioneering

Wet Hydrogen Sulfide (H2S) Cracking 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.

Wet H2S cracking 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 cracking 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.

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.

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.

 

Related Topics

Amine Cracking Ammonia Stress Corrosion Cracking Blistering Brittle Fracture Carburization Caustic Cracking Cavitation Chloride Stress Corrosion Cracking Cooling Water Corrosion Corrosion Fatigue Corrosion Under Insulation (CUI) Cracking Decarburization Embrittlement Erosion Corrosion Fatigue Graphitization High Temperature Hydrogen Attack (HTHA) Hydrochloric Acid Corrosion Hydrofluoric Acid (HFA) Corrosion Hydrogen Assisted Cracking Hydrogen Embrittlement Hydrogen Induced Cracking (HIC) Hydrogen Stress Cracking Hydrogen Sulfide (H2S) Corrosion Microbiologically Induced Corrosion (MIC) Naphthenic Acid Corrosion (NAC) Phosphoric Acid Corrosion Polythionic Acid Stress Corrosion Cracking (PASCC) Spheroidization Stress Assisted Corrosion Stress Corrosion Cracking (SCC) Sulfidation Corrosion Sulfuric Acid Corrosion Temper Embrittlement Thermal Fatigue

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Articles
  • November/December 2003 Inspectioneering Journal
    By John Reynolds at Intertek

    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 always), while SOHIC is a type of cracking that can easily lead to failure and needs to be mitigated. HIC is a form of tiny blistering damage that is mostly parallel to the surface and to the direction of hoop stress, hence is usually not damaging until it is extensive and affects material properties or gives rise to step-wise cracking that propagates into a weld or begins to go step-wise through the wall.

  • May/June 1995 Inspectioneering Journal
    By Dr. Russell D. Kane at CLI International Inc., and Dr. Michael S. Cayard at Flint Hills Resources

    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 (SSC). Documented cases of leaks and failures of pressure containing equipment have been attributed to these forms of corrosive damage.

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