HTHA – Hydrogen Attack
HTHA – High temperature hydrogen attack is a form of damage commonly observed in steels exposed to high pressure hydrogen at elevated temperatures. The damage occurs as hydrogen atoms diffuse into steels, react with carbon, form methane gas internally in the material, which results in decarburization and fissuring (micro-cracking). This damage leads to loss of tensile strength and toughness. More importantly, it can cause catastrophic failure without prior warning, especially if occurring in the weld H.A.Z.
Extensive research has been carried out to investigate a reliable means to evaluate equipment that fall into the category of high risk potential for HTHA damage.
Advanced NDT Solutions has adopted a multiple ultrasonic technique approach, that includes industry accepted techniques and in addition supplemental Phased Array & TOFD technology to further evaluate and characterize suspect damage.
Technique #1 – Ultrasonic Backscattering
This technique is based upon the principle that ultrasound will be scattered by changes in the grain boundaries resulting from the presence of hydrogen damage. The extent of this damage can be directly correlated to a through-wall depth value of affected material thickness and its location in relation the ID/OD surfaces. Using a combination of 10 Mhz and 5 MHz transducers.
Pattern #1:Continuous decrease of backscattering from OD to ID surface.
Pattern #2:Large discrete reflections but no hydrogen attack.
Pattern #3:Hydrogen attack which has progressed to mid-wall.
Pattern #4:Hydrogen damage at the internal surface.
Technique #2 – Velocity Ratio Calculations: HTHA damage greater than 20% of material thickness will alter the relationship between the ultrasonic velocities of longitudinal and shear-wave transit times. Normal undamaged material should have a ratio within the range of 0.54 – 0.55. Formula Ratio = Velocity L / Velocity S
Technique #3 – Spectral Analysis (FFT) Ultrasonic spectrum analysis is accomplished by converting the first back-wall signal from a time domain to a signal attenuation (dB) with regards to a frequency domain. A clean location will show no increase in ultrasonic attenuation as frequency is increased on the other hand a HTHA damaged region will.
Technique #4 – High Frequency Shearwave of Welds & HAZ Recent research has revealed that utilization of a high frequency shear-wave examination of welds and the HAZ can also help detect HTHA damage. This technique utilizes a pitch catch ultrasonic technique utilizing a pair of 10MHz 45° shear-wave probes.
Technique #5 – Phased Array Ultrasonic Supplemental Phased Array techniques indicate all locations to contain sound material with no indication of HTHA. Images below show Sectorial display -30° to +30° sweep of inspection locations.