High Temperature Hydrogen Attack (HTHA)

High Temperature Hydrogen Attack (HTHA)

High Temperature Hydrogen Attack (HTHA)

Independent inspection services for reliable detection and monitoring

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.

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 fl ight diffraction (TOFD)

With time of fl ight 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


Operating conditions

Inspection strategy


At or above Nelson Curves

High inspection coverage: backscatter mapping, velocity ratio measurement, ToFD, 45° shear wave inspection


Up to 25 °F below Nelson Curve

Medium inspection coverage: backscatter mapping, manual, velocity ratio measurement, ToFD, 45° manual. (no requirement to open vessel)


Between 50 °F below Nelson Curve

Low inspection coverage: manual backscatter, ToFD


More than 50 °F below Nelson Curve



Other topics

Minimum Intervention Strategy for Inspection (MISI)

Minimum Intervention Strategy for Inspection