Overview of Stress-Oriented Hydrogen Cracking (SOHIC)

In a previous blog, we discussed Hydrogen Induced Cracking (HIC) and how to detect it.

Stress-Oriented Hydrogen Induced Cracking, or SOHIC, is a type of hydrogen damage. It occurs in a fairly wide range of process conditions, particularly in pipelines and process vessels. Due to its unpredictable nature, understanding which assets are susceptible and how it occurs is key. Applying proper prevention and detection methods is critical to avoid catastrophic incidence, downtime, significant capital expenditures and injuring personnel.

But what exactly is SOHIC, and how does it differ from other types of hydrogen damage?


There are precursor stages that ultimately lead to SOHIC, which is a type of Wet H2S (hydrogen sulfide) Damage.

SOHIC starts when carbon or low-alloy steel is exposed to wet H2S conditions (acidic conditions such as those at oil refineries). Any equipment that runs in conditions that are both above 50 ppm of H2S content and below 180 degrees Fahrenheit in aqueous sour waters is likely susceptible to wet H2S cracking, according to leading industry journal, Inspectioneering.

This atmosphere results in the formation of atomic hydrogen atoms, which, under pressure, are subsequently diffused—or forced—into the material surface. The fact that hydrogen atoms are the smallest of all atoms makes them even more conducive to diffusion. Unfortunately, as these atoms combine to form molecules, they become too large to escape, resulting in the buildup of internal hydrogen. These hydrogen molecules collect at defects, impurities or stresses, hence stress-oriented, in the material welds, which can, but not always, cause surface bulges or blisters. These can be identified with the naked eye, and often accompanied by hydrogen-induced cracking (HIC). This type of cracking is of a smaller nature, typically much less damaging than SOHIC, quietly lurking beneath the surface. Though SOHIC develops more slowly, HIC can transpire very rapidly, thus coming as an extremely unpleasant surprise. By the time HIC is identified, SOHIC may be well under way.

SOHIC diagram

SOHIC becomes most dangerous when cracks begin to form and stack perpendicularly. This intricate network of cracking may extend through the wall of the asset. These cracking arrays are often concentrated at the weakest surface locations, particularly the low-hardness, heat-affected zone (HAZ) weld regions.

Eventually, these cracks can lead to leaks or ruptures of the pipeline or asset, in-service failure and unscheduled downtime, injuries or worse.

According to industry research, at least nine pipeline failures over the past couple decades are believed to have been caused by SOHIC. (King, “Trends in Oil and Gas Corrosion Research and Technologies, 2017).

It should be noted that HIC and SOHIC are different from High-Temperature Hydrogen Attack, which we covered in a previous post, and it’s also distinct from sulfide stress cracking, a form of brittle breakdown.

But moving on, what can be done in terms of SOHIC incident prevention?'

Mitigation & Detection

When it comes to HIC and SOHIC prevention, industry recommendations include the use of HIC-resistant steel. However, HIC-resistant steel stirs controversy, as some studies have suggested that these HIC-resistant steels are even more susceptible to SOHIC than conventional steels—but we won’t dive into that today. Instead, let's touch on approaches to find and mitigate SOHIC.

For equipment that is considered to be susceptible to SOHIC, it is recommended that it be post-weld heat treated. When regularly performed, post-weld heat treatment diffuses hydrogen from the area, which can alleviate pressure in the process structure. The temperature and length of time heated depends on a variety of factors, including material composition and thickness.

Additional guidelines suggest application of polymeric coating and stainless steel cladding.

A useful resource for operators, API Recommended Practice 571, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry, provides guidance to pressure equipment integrity personnel. This document is designed to help in overall management of pressure equipment integrity from identification of damage to conducting examinations. While it also provides nondestructive examination and testing method suggestions, the most direct route in early, effective detection is engagement with experienced inspection professionals who will assist your facility in selecting and deploying the best method for your individual facility or asset.

stepwise cracking detected by PAUT

Regular, proactive inspections utilizing advanced nondestructive testing techniques, such as Phased Array Ultrasonic Testing, can identify SOHIC and other difficult-to-detect conditions. When performed by an expert inspection technician, PAUT quickly provides detailed, high-resolution data that can be used for effective SOHIC detection—and simultaneously—many other process indications.

Although SOHIC is a relatively common problem, it can have critical implications for infrastructure.

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