High temperature hydrogen attack (HTHA) Inspection
High temperature hydrogen attack (HTHA) is observed in steels exposed to temperature above 200 °C. At high temperature atomic hydrogen diffuses in steel. This hydrogen reacts with carbon of steel and forms CH4. The methane so formed bubbles at grain boundary and forms voids at grain boundary.
MC + 4H = M + CH4
These bubbles exert pressure and also coalesce resulting in to fissures. The growth of voids and fissures weakens the metal and the fissures develop in to major crack.
This reaction decarburizes the steel, produces micro cracks/fissures and lowers toughness of steel but not necessarily cause loss in thickness.
Technique / Procedure
Technique used for HTHA relies on detecting the scattering of ultrasound energy due to presence of fissures on the internal side of the low-alloy steel metal surface exposed to hydrogen at high temperature by scanning from outside surface.
The procedure for testing is based on API 941 using different approaches like:
- Attenuation measurement
- Velocity measurement
- Spectral analysis
- Analyzing back scattered signals
- Testing weld joints and HAZ using high frequency shear wave ultrasound
- Advanced ultrasonic testing like Phased array and TOFD
Extent of damage by HTHA can be assessed using above techniques and also testing from inside using other techniques like WFMPI (Wet fluorescent magnetic particle inspection), in-situ metallography and hardness testing. Testing from both the sides overcomes the limitations as encountered during testing from outside only.