HTHA
CORE SERVICE OFFERINGS

High-Temperature Hydrogen
Attack (HTHA) Inspection

High-temperature hydrogen attack (HTHA) is observed in steel that is exposed to a temperature of 200 °C or more. At such a high temperature, atomic hydrogen diffuses in steel. This hydrogen reacts with carbon present in the steel and forms CH4. The methane that is formed bubbles and creates voids at the grain boundary.

MC + 4H = M + CH4

These bubbles exert pressure and also coalesce into fissures. The growth of voids and fissures weakens the metal, which leads to a major crack. This reaction decarburizes the steel, produces micro cracks/fissures and lowers toughness of steel but may not cause a loss in thickness.

Advantages

  1. Supports the Inspection of large and wide areas
  2. Provides accessibility and convenience as external access is required only on one side (Opening of equipment or removal of the catalyst is not required)
  3. The depth of attack can be estimated

Limitations

  1. Deep expertise required in interpretation
  2. Ignitial micro level degradation (Decarburization) cannot be estimated

TECHNICAL CAPABILITIES

HTHA relies on detecting the scattering of ultrasound
energy

The technique detects the presence of fissures on the internal side of the low-alloy steel metal surface exposed to hydrogen at high temperature by scanning the outside surface.

The procedure for testing is based on API 941 using different approaches like:

  1. Attenuation Measurement
  2. Velocity Measurement
  3. Spectral Analysis
  4. Analyzing Scattered Signals
  5. Testing Weld Joints and HAZ Using High-Frequency Shear Wave Ultrasound
  6. Advanced Ultrasonic Testing Like Phased Array and TOFD

The extent of damage by HTHA can be assessed using the above techniques as well as other internal techniques such as WFMPI (Wet fluorescent magnetic particle inspection), in-situ metallography and hardness testing. Testing both sides overcomes the limitations encountered while testing only from outside.