HYDROGEN INDUCED CRACKING (HIC)
Hydrogen-induced cracking (HIC) refers to the internal cracks brought about by material trapped in budding hydrogen atoms. It involves atomic hydrogen, which is the smallest atom, that diffuses into a metallic structure. In the case of a crystal lattice becoming saturated or coming into contact with atomic hydrogen, many alloys and metals may lose their mechanical properties.
Hydrogen induced cracking (HIC) damage is a degradation mechanism active in the oil and gas industry. The hydrogen atom is the smallest of all atoms and hydrogen attack is similarly insidious and hidden from simple inspection techniques. Hydrogen damage takes place at two temperature regions, elevated temperatures and temperatures at ambient and slightly above. The following details NON-DESTRUCTIVE inspection techniques aimed at identifying, quantifying and tracking this form of damage. The operation consists of utilizing advanced ultrasonic techniques to detect HIC & SOHIC, metal loss and other laminar defects in the shell material of vessels. UT techniques include manual UT, TOFD, Phased Array Ultrasonics and Automated Combined UT (Triplex). Best results are obtained when a number of these techniques are employed together.
Advanced Ultrasonic Techniques for the inspection of Hydrogen Induced Cracking (HIC)
There are a number of factors that must be taken into consideration when planning inspection surveys on different components. Sometimes it is necessary to utilize a combination of ultrasonic techniques and even possibly other NDT test methods to obtain full coverage in region of concern. Advanced Ultrasonic Testing (AUT) covers several ultrasonic techniques and maybe either partially automated or fully automated. The following are a list of factors that should be considered prior to specifying scope of work:
Time of Flight Diffraction (ToFD): This technique is very sensitive for detecting changes in material structure in a single line scan pattern and is ideal for fast screening of welds for the presence of environmental service-related cracking such as HIC/SOHIC. This would then be followed up with alternative techniques for further evaluation. It is important that correct setups and procedures are used.
Phased Array Technology (PAUT): PAUT is a very useful tool for evaluation of HIC damage, either using a 0° Linear Scan Technique or the more familiar Sectorial Scan (-30° to + 30°). Images for both of these techniques are shown below. Once again correct setup, probe frequency and focus are paramount to accuracy of examination.
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