A complex but valuable inspection exercise
Hydrogen sulphide (H2S) damage remains one of the more challenging degradation mechanisms encountered in industrial assets, particularly in pressure equipment, piping systems, and welded components operating in sour service environments. The detection and characterization of this type of damage requires not only advanced ultrasonic techniques, but also a high level of interpretation skill and understanding of the underlying damage morphology. In one recent inspection case, H2S-related damage was identified using Ultrasonic Testing Phased Array (UT PA), with Time of Flight Diffraction (TOFD) considered as a complementary method to support the evaluation.
H2S damage can manifest itself in several forms, including hydrogen-induced cracking (HIC), sulphide stress cracking (SSC), High Temperature Hydrogen Attack (HTHA), blistering, stepwise cracking, and crack-like indications in the heat affected zone or base material. These mechanisms often develop internally and may not be visible from the surface, making volumetric non-destructive testing essential. However, detecting such damage is far from straightforward. Unlike simple planar defects, H2S-related indications are often irregular, discontinuous, and scattered through the material in complex orientations.
In this case, Phased Array Ultrasonic Testing proved to be highly effective because of its ability to generate multiple angles and focal laws in a single scan. This allowed detailed sectorial views of the material and enabled the inspection team to detect suspicious reflectors at various depths and positions. The advantage of PAUT lies not only in detection sensitivity, but also in imaging capability. The generated S-scan helped to localize the indications and assess whether the reflectors were consistent with embedded cracking associated with sour service degradation.
Where appropriate, TOFD can provide valuable additional information. Although TOFD is traditionally very strong for sizing well-defined crack tips, it can also be useful in confirming the presence of crack-like defects when H2S damage develops into more significant or connected cracking. However, the application is complex. Diffuse hydrogen damage does not always produce classic, sharp diffraction signals, which means the data must be interpreted carefully and in correlation with PAUT results, material history, operating conditions, and damage mechanism assessment.
What makes this type of inspection especially demanding is that success does not depend solely on equipment performance. It requires experienced technicians, correct scan planning, proper calibration strategy, optimized probe selection, and a clear understanding of the limitations of each method. Surface condition, geometry, grain structure, weld profile, and access restrictions can all influence the reliability of the results.
The identification of H2S damage through UT PA, supported where relevant by TOFD, demonstrates the value of advanced NDT techniques in proactive asset integrity management. At the same time, it underlines that this is not a routine inspection task. It is a complex technical exercise that requires expertise, careful interpretation, and close alignment between inspection execution and engineering assessment. When performed correctly, it can provide critical information for safety, fitness-for-service evaluation, and risk-based maintenance planning.
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