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 the steel matrix at elevated temperatures and chemically reacts with carbon present in the material to produce methane gas. Because methane diffuses through steel significantly more slowly than atomic hydrogen, it accumulates at internal microstructural sites, particularly at grain boundaries and around carbide particles.
The accumulation of methane creates localized high pressure within these confined spaces, leading to the formation of microvoids and cracks. This process results in a loss of mechanical integrity, including reduced ductility and tensile strength, which can ultimately lead to catastrophic failure of pressure vessels and piping systems in petrochemical and refining industries.
Detection of methane bubbles is critical for assessing the remaining life of affected equipment. Ultrasonic nondestructive testing is commonly employed to identify these internal defects. The presence of methane bubbles alters the acoustic properties of the steel, allowing inspectors to locate and size the cavities without damaging the component, thereby facilitating maintenance decisions and safety assessments.