Cracking under pressure

High temperature hydrogen attack (HTHA) is a damage mechanism responsible for some of the worst petrochemical disasters in modern history. Heat exchangers and pipework systems have catastrophically failed, resulting in fireballs that engulfed surrounding process plants, and in some cases, resulted in several fatalities or at the very least life changing injuries. This damage mechanism can quietly spread through the body undetected until late stages manifest, resulting in extremely compromised service life expectancy, with massively marginalised fitness for purpose predictions. Detection of this damage at the earliest stage is of paramount importance.

How does this damage occur? Pressurised systems, vessels, exchangers, and pipework manufactured from carbon manganese and low alloy steels when operated at elevated temperatures exceeding 205°C and are subject to prolonged exposure to hydrogen develop HTHA. This may be extremely localised and relatively minor damage, or it could be damage that is quite advanced and is progressively increasing towards through-thickness failure. HTHA occurs when hydrogen joins with the carbon in steel to form methane (CH₄). As operating temperatures increase, carbon precipitates more easily toward grain boundaries attracted by hydrogen to form CH₄. This chemical change results in larger atoms which accumulate pressure along the grain boundaries.

When methane pressure exceeds a material’s yield point it forms microscopic gas bubbles along grain boundaries in solid metal. The surrounding grain structure is depleted of carbon, which along with the methane bubble/voids, marginalises the material mechanical properties. This is known as decarburisation. As time progresses, methane void density increases to such a point that micro fissures form, joining voids together. Progressively these accumulate, eventually coalescing to form macro cracks and subsequently larger cracks. The crack mechanism is ductile, very similar to the progression of creep damage. Cracks that propagate extend through the colony of dense methane voids arresting at the edge of the void colony where mechanical strength remains uncompromised. In turn, higher stress concentrations form at the new crack tips, each time stresses being more concentrated, resulting in accelerated methane formation, each time progressively less methane pressure is needed to exceed the material’s yield point. The cycle becomes exponentially accelerated until such a point that the remaining ligament of sound material terminally fails through ductile fracture.

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