Hydrogen induced cracking (HIC) and Stress-oriented hydrogen induced cracking (SOHIC) are both forms of wet hydrogen sulfide (H2S) cracking.
When left untreated, both forms of corrosion can be very dangerous. Untreated HIC or SOHIC can lead to the steel losing its mechanical properties, such as its strength or ductility. This is why it’s important to perform HIC testing on vulnerable equipment regularly.
Hydrogen induced cracking (HIC) can occur in metal equipment due to high concentrations of hydrogen. This type of cracking tends to appear on the surface as small blisters connected by cracks and it can occur when atomic hydrogen enters into the steel.
This atomic hydrogen can form during wet H2S corrosion reactions. The hydrogen atoms are small enough that they can diffuse into the steel. From there they are able to collect at impurities in the steel and are prevented from recombining by the continued presence of the H2S. This allows them to be driven further into the steel and prevents them from diffusing back out into the environment.
The cracks caused by HIC are usually fairly small; smaller than the width of a pinky finger. In small amounts, HIC isn’t too dangerous. It’s only once the cracking begins to become extensive and affect the material properties that it tends to become problematic.
Stress-oriented hydrogen induced cracking (SOHIC) is an even more dangerous form of HIC. It is, in effect, several HIC cracks occurring in a single piece of equipment. The cracks tend to occur perpendicularly, and are driven by stresses, either residual or applied. It also tends to occur most often around the heat affected zone of a weld, which is often the weakest point of a piece of equipment.
Like basic HIC cracking, SOHIC cracking occurs when atomic hydrogen diffuses into equipment and collects at impurities within the steel. It can become a problem in nearly any acidic, hydrogen rich environments, particularly those where the parts per million (ppm) of H2S is over 50, and the temperature is under 180 °F.
HIC and SOHIC can cause a number of problems should they be allowed to propagate uncontrolled. For this reason, regular testing should be done on equipment that service in a hydrogen rich environment, or otherwise vulnerable to any form hydrogen corrosion.
One effective NDT method to test for either form of hydrogen cracking is magnetic particle testing. It can easily detect most forms of hydrogen cracking, including both HIC and SOHIC. It can even detect subsurface cracks caused by HIC or SOHIC. For equipment that cannot be inspected with magnetic particle testing, the next best form of HIC testing is phased array ultrasonic testing (PAUT), which is also convenient and reliable.
Hydrogen corrosion can be treated with a hydrogen bake-out. This involves heating up the equipment to high temperatures. This process is able to drive atomic hydrogen out of the equipment, thus helping to prevent hydrogen corrosion.