Methane Bubbles
Methane bubbles are gas-filled cavities that form within steel during high-temperature hydrogen attack (HTHA). This degradation mechanism occurs when hydrogen gas penetrates steel at elevated temperatures and chemically reacts with carbon in the material to produce methane gas. Because methane diffuses through steel much more slowly than atomic hydrogen, it accumulates at internal sites—particularly at grain boundaries and around carbide particles—creating localized pressure that generates void-like cavities.
Formation and Distribution
The formation of methane bubbles is a time-dependent process that becomes significant at temperatures typically above 200°C, depending on steel composition and hydrogen pressure. Atomic hydrogen dissolves into the steel and migrates toward areas rich in carbon, where it combines to form methane molecules. These methane molecules cannot readily escape the material, leading to bubble nucleation and growth at metallurgical discontinuities. The bubbles can coalesce over time, potentially creating larger cavities and networks that degrade mechanical properties.
Detection
Methane bubbles can be detected using ultrasonic nondestructive testing (UT), which identifies the acoustic impedance changes associated with gas-filled voids. Other detection methods include radiography and microscopy of sectioned samples. Early detection is valuable because methane bubble formation can lead to loss of ductility, cracking, and ultimately component failure if left unchecked. Regular inspection of equipment exposed to high-temperature hydrogen environments helps prevent catastrophic degradation.