Detecting High Temperature Hydrogen Attack (HTHA) Before It’s Too Late

High temperature hydrogen attack is a defect of concern, and it is difficult to detect with conventional ultrasonic testing due its very small size, especially in its early stage.

Figure 1: Examples of high temperature hydrogen attack damage

After the infamous 2010 Tesoro Anacortes Refinery disaster, a fatal accident from a heat exchanger explosion, industry has turned a spotlight on the original culprit: high temperature hydrogen attack. High temperature hydrogen attack, also referred to as hot hydrogen attack or methane reaction, occurs in an environment containing hydrogen and high temperatures, typically in low alloy steels. High temperatures change the atomic form of hydrogen which makes it permeate into the steel, react with carbon, and form methane. Because the methane gets trapped in the metal, it ends up creating a microscopic bubble at the grain boundaries in the steel; this is stage one. As bubbles start to grow, they start to coalesce as stage two, ultimately leading to fissures and cracking at stage three. High temperature hydrogen attack commonly occurs in welds and heat affected zones in materials that never received heat treatment. Many refinery owners have started additional inspections after the component failure that was not anticipated to be susceptible to high temperature hydrogen attack. However, these defects are very small, and it is quite difficult to detect with conventional ultrasonic testing or any other volumetric non-destructive testing method.

An advanced, portable tool capable of performing all ultrasonic techniques required to pinpoint high temperature hydrogen attack: time of flight diffraction/time of flight diffraction ultra-low angle, phased array ultrasonic testing, and total focusing method with an intuitive software for fast configuration.

Figure 2: Gekko® phased array ultrasonic testing instrument

Combining different examination techniques is key for detection, and API’s recommended practice (API RP 941) is evolving towards this. Phased array ultrasonic testing has been recognized as one of the best non-destructive testing techniques for high temperature hydrogen attack inspection, while time of flight diffraction and time of flight diffraction ultra low angle offer rapid screening of large areas with a high probability of detection due to its capacity to sense small back-scattered signals. As for total focusing method, it enables an improved characterization of indications, and ultimately allows making the distinction between high temperature hydrogen attack damage and other types of defects. This type of inspection is usually performed with a single axis encoded scanner and optimized probes for the application. While total focusing method is based on the extraction of the amplitudes of the full matrix capture matrix, another technique called phase coherence imaging uses the phase information from those signals. The phase coherence imaging shows high values when all the elements contribute in phase. This is the case for small diffractors that tend to send energy in all directions such as the small crack encountered with high temperature hydrogen attack.

Eddyfi Technologies offers advanced non-destructive testing flaw detectors like the Gekko® which are regularly used to characterize high temperature hydrogen attack cracking. The proposed solution includes:

• Gekko units to handle high channel count dual probes, high resolution sector scanning, live total focusing method, time of flight diffraction/time of flight diffraction ultra-low angle, intuitive software configuration, and automatic time corrected gain,
• 1-axis scanner for data encoding and defect positioning ,
• Dedicated phased array ultrasonic testing probes with optimized passive focusing for improved sensitivity.