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High Temperature 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 forms voids at the grain boundary.

MC + 4H = M + CH4


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


  1. Supports in the Inspection of Large and Wide Areas

  2. Provides Accessibility with Convenience as Only One Side External Access is Required (Opening of Equipment or Removal of Catalyst is not Required)

  3. Depth Of Attack Can Be Estimated


  1. Deep Expertise Required In Interpretation

  2. Very Initial Micro Level Degradation (Decarburization) Cannot be Estimated



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 from 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 from both sides overcomes the limitations encountered while testing only from outside.

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