99 Diseases of Pressure Equipment: Wet H2S Cracking (HIC/SOHIC)

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. The damage occurs when atomic hydrogen from wet H2S corrosion reactions enters the steel and collects at inclusions or impurities in the steel. On the surface HIC is often horse shoeshaped and no bigger than the cuticle of your small finger. SOHIC on the other hand is more insidious and is a bunch of short HIC cracks that are stacked perpendicularly in the direction of through wall cracks and driven by high residual or applied stresses. These are the cracks that we really need to find and mitigate in our wet H2S cracking inspection programs. HIC/SOHIC cracking in the refining industry has received more attention in the last 15 years than any form of cracking because of the spectacular failure at the Lamont, Illinois refinery in 1984 that cost many lives. It's fairly well known that above 50 ppm of H2S content, below 180F temperature in aqueous sour waters, that our pressure equipment is susceptible to wet H2S cracking. It's also fairly well known that cyanides in overhead systems of cat crackers and cokers can significantly increase susceptibility.

Thankfully the intense focus on this phenomena over the last decade or so has almost eliminated significant failures, but we must not let our guard down. All susceptible systems should be inspected and maintained according to the latest edition of NACE RP0296. Equipment that is susceptible to SOHIC needs to be post weld heat treated (PWHT) or alloyed up. HIC resistant steels and polymeric coatings have been successfully applied, as well as stainless steel clad materials in more aggressive environments.

Do your operators know the importance of carefully operating and maintaining the water wash or polysulfide treating systems? Do you have operating windows in place that specify process variables limits, like ammonium hydrosulfide, and are these variables adequately measured and monitored?