Detection of Hydrogen Induced Cracking (HIC)
Hydrogen-induced cracking (HIC) is a phenomenon associated with the dimerization of nascent atomic hydrogen at metallurgical defects, such as nonmetallic inclusions, large precipitates, or other discontinuities in the steel (Figure 1).
The molecular hydrogen formed in situ produces blisters, and adjacent blisters subsequently are connected on different planes in the metal by a transgranular plastic shear mechanism (Figure 2). In the Petroleum Industry, HIC usually is associated with wet hydrogen sulfide (H2S) environments (sulfide stress-cracking).1
Hydrogen-induced cracks in an ASTM 515(60) steel plate as they appear under scanning electron microscopy (SEM) observation.
A stepwise crack between two hydrogen-induced cracks in an ASTM 515(60) steel plate as it appears under SEM observation.
In the past, HIC has been easily confused with laminations or “dirty steel” (volumetric inclusions or contaminates and segregates) causing many vessels to be unnecessarily condemned and replaced. The most common inspection approach for the detection of HIC has been inspecting the base material with a compression wave looking for hydrogen blisters. This inspection has been performed both manually and automated and has yielded varied results.
Inspection using a compression wave (0°) showing blisters and step wise cracking.
The addition of angle beams to the compression wave for the HIC inspections allows the examiner to look for flaws emanating from the ends of the blisters to help differentiate the blisters from laminations or dirty steel.
Combined inspection using compression wave (0°) and shear waves (45°) showing blisters and step wise cracking.
Phased array inspections for the detection of hydrogen blisters have taken the benefits from performing the compression and shear wave approach to a new level. Through beam steering and beam focusing, this inspection technique can now easily differentiate a lamination or “dirty steel” from a blister as the shape of the defect can be clearly seen on the sector scan presentation. Using conventional methods as a screening tool to identify areas with possible blisters and then following up with phased array to validate and size the affected areas, is the ideal approach and the best use of time and technology.
Phased array inspection showing blisters and step wise cracking detected using compression wave (-30° to +30°)
1. J.L. Gonzales, R. Ramirez, J.M. Hallen, R.A. Guzman, “Hydrogen-Induced Crack Growth Rate in Steel Plates Exposed to Sour Environments,” Corrosion 53, 12 (1997): p. 935.