Overview of Hydrogen Induced Cracking (HIC)
Hydrogen Induced Cracking (HIC) is a common form of wet H2S cracking caused by the blistering of a metal due to a high concentration of hydrogen. The blistering damage tends to form parallel to the surface and to the direction of hoop stress.
HIC usually occurs due to the effects of aqueous hydrogen charging of steel in wet H2S refinery process environments. It can occur 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. The H2S prevents the hydrogen recombination reaction that would normally occur so, rather than bubbling off from the corroding surface, the hydrogen atoms are forced into the metal structure causing corrosion and weakness.
The damage occurs when the hydrogen collects at inclusions or impurities in the steel. It tends primarily occur in steels that have a hardness of 22 or more on the Rockwell C scale.
As far as damage mechanisms go HIC is usually, but not always, fairly innocuous. It usually isn’t damaging until it becomes extensive and begins to affect the material properties. Once the ductility of the metal has reduced to a significant amount, the metal will form stepwise internal cracks connecting adjacent hydrogen blisters. These can become dangerous should they propagate into a weld.
On the surface, HIC is often horseshoe shaped and no bigger than the cuticle of your small finger. Regular inspection and testing should be performed in order to eliminate the chances of hydrogen corrosion. The conventional method for detecting wet H2S cracking is Wet Fluorescent Magnetic Particle Inspection (WFMPI) which is able to detect sub-surface cracks in the steel caused by HIC. When it comes to cracked piping and other components which cannot be inspected using WFMPI, Phased Array Ultrasonic Testing (PAUT) is the most convenient and reliable non-destructive method available.
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AE monitoring has been done for HIC affected vessels, with limited success. The limitation, which greatly affects the confidence level of results, is that traditional AE testing stresses, i.e. Kaiser affect overpressure and felicity effect types of re-stressing do not generate the types of stress necessary to generate elastic strain waves at the HIC laminar crack tips.
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November/December 2003 Inspectioneering JournalBy 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.
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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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