High Temperature Hydrogen Attack (HTHA)

High temperature hydrogen attack (HTHA) is a phenomenon which occurs within some industrial processes when the presence of hydrogen, subjected to high temperature and pressure, makes plants and systems susceptible to damage. This reaction can cause an industrial plant to suffer serious failure, which may lead to both expensive repairs and loss of production.

HTHA can be detected in a number of ways, but the reliability of some of the techniques used in its detection is questionable. We have developed a series of procedures featuring the latest advances in ultrasonic imaging and data processing techniques in order to achieve a greater level of confi dence in both HTHA inspection results and inspection repeatability.

The following key components can lead to HTHA:

  • Presence of Hydrogen
  • Presence of high pressure
  • Operating at temperatures above ~200°C
  • Presence of stress
  • Material Chemistry
  • Duration of exposure

The following damages mechanisms are also related to a hydrogen service environment and are taken into account in the Sonotech inspection approach:

  • Hydrogen Embrittlement
  • Blisters

Hydrogen Embrittlement

Occurs at lower temperatures and often related to start stop cycles, where hydrogen does not diffuse completely out of the material at the stop cycles and building up its concentration, which can lead to cracking. The diffusion of hydrogen can lead in cladded pressure vessels to disbonding of the cladding.


Blisters can for example occure if there is already a crack in the weld or parent material, for example from the manufacturing phase, where two H atoms will form together an H2 molecule, which create a bubble.

Available techniques in accordance with American Petroleum Institute (API 941)


The backscatter technique is used to detect suspicious areas affected by hydrogen attack. The front of the attack in the material can be measured and imaged. The remaining wall thickness (un-attacked) is assigned a color.

A typical display of the backscatter technique

Time of flight diffraction (TOFD)

With time of flight diffraction (TOFD) testing it is possible to inspect welds for defects resulting from HTHA. It can be used for the detection of both macro and micro cracks. Macro cracks can easily be identifi ed with TOFD. Micro cracks, however, can be challenging to discriminate from other types of weld defects. Also here a verifi cation is found by cross-referencing the results to a Pulse-Echo or Phased Array UT examination.

Velocity ratio measurement

Velocity ratio measurement is based on the principle that the longitudinal and transverse sound velocities are affected by HTHA, by means of which the percentage of attack can be measured. By cross-referencing the results with the backscatter measurement, a confi rmation of the amount of attack is established. In addition, the methodology used allows for a differentiation between small inclusions and HTHA.

Ts = Unattacked wall thickness

Ta = Attacked wall thickness

Cl = Longitudinal sound velocity in unattacked material

Ct = Transverse sound velocity in unattacked material

Cla = Longitudinal sound velocity in attacked material

Cta = Transverse sound velocity in attacked material

Areas for application

Backscatter and velocity ratio measurement are used for the

inspection of the parent material. Time of flight diffraction

is used for the inspection of the weld material.

Benefits at a glance

– Digital techniques

– High reliability and reproducibility

– Clear presentation of results

– Numerous different geometries can be examined

– Damage progress can be accurately monitored.

Inspection strategy

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