Sonomatic completed a detailed inspection for detection of high temperature hydrogen attack (HTHA) on vessels using phased array ultrasonic testing (PAUT). HTHA was not detected on this campaign, but hydrogen induced cracking (HIC) was identified in the heat affected zone (HAZ) of the head to shell weld of one vessel.
What is HTHA?
High temperature hydrogen attack (HTHA), also called hot hydrogen attack, is a problem which can impact steel operating at elevated temperatures (typically above 400°C) in hydrogen environments, in refinery, petrochemical and other chemical facilities and, possibly, high pressure steam boilers.
The main factors influencing HTHA are the hydrogen partial pressure, the temperature of the steel and the duration of the exposure.
HTHA is the result of hydrogen dissociating and dissolving in the steel, and then reacting with the carbon in solution in the steel to form methane. This can result in either surface decarburisation, when the reaction mostly occurs at the surface and draws carbon from the material, or internal decarburisation when atomic hydrogen penetrates the material and reacts with carbon to form methane, which accumulates at grain boundaries and/or precipitate interfaces and cannot diffuse out of the steel. This causes the fissures and cracking which are typical of HTHA.
Inspecting for HTHA
A HTHA campaign is one of the most challenging non-destructive testing (NDT) inspections relying not only on correct technique selection but operator competency and experience. The most suitable techniques for HTHA inspections depend on the specific area of inspection. It is widely recognised that inspection procedures using a combination of PAUT using advanced acquisition algorithms such total focusing method (TFM) and time-of-flight-diffraction (TOFD) ultrasonic methods are fundamental for achieving a robust inspection regimen for HTHA assessment.
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