Decarburisation of Carbon Steel Pipe Fittings in High-Temperature Hydrogen: Mechanism, Detection, and Prevention
Decarburisation is the loss of carbon from the surface of a steel fitting by chemical reaction with the surrounding atmosphere. In high-temperature hydrogen service, surface decarburisation occurs when hydrogen reacts with carbon in the steel surface to form methane gas: Fe₃C + 4H → 3Fe + CH₄. This is chemically the same reaction as HTHA (high-temperature hydrogen attack), but decarburisation is distinguished from HTHA by its location — decarburisation occurs at the exposed surface and progresses inward, while HTHA occurs internally at grain boundaries and carbide interfaces throughout the section. Both decarburisation and HTHA can occur simultaneously in the same fitting, but they have different consequences and require different inspection approaches.
Surface Decarburisation vs Internal HTHA
Surface decarburisation progresses from the outer or inner bore surface of the fitting inward, creating a carbon-depleted zone (the decarburised layer) that consists almost entirely of ferrite — the soft, low-carbon phase — rather than the normal ferrite-pearlite microstructure. The decarburised layer is softer and weaker than the base metal but is not cracked — it is a zone of carbon depletion rather than fissuring. Surface decarburisation reduces the effective load-bearing wall thickness of the fitting because the decarburised ferrite has a yield strength of approximately 200–250 MPa compared to 250–350 MPa for the normal ferrite-pearlite microstructure — a reduction of approximately 20–30%. Internal HTHA, in contrast, creates internal fissures and voids that can propagate to catastrophic fracture — it is far more dangerous than surface decarburisation. In carbon steel fittings in hydrogen service above approximately 300°C, both mechanisms may be active simultaneously: surface decarburisation progressing inward from the bore, and HTHA initiating at internal carbide interfaces throughout the section.
Detection by Metallography
Surface decarburisation is detected by metallographic examination of a cross-section taken from the fitting. The decarburised layer appears as a zone of bright ferrite (no pearlite — the carbon-containing phase) adjacent to the bore surface, transitioning to normal ferrite-pearlite microstructure at the decarburisation depth. The depth of decarburisation is measured from the surface to the point where the microstructure returns to the normal pearlite content. Decarburisation depth measurement is performed on etched metallographic sections at 100–400× magnification. ASTM E1077 provides a standardised method for measuring decarburisation depth. In hydrogen service, a decarburisation depth exceeding 5–10% of the nominal wall thickness is typically treated as evidence of inadequate material protection or operating outside the Nelson curve limits for the steel grade, and warrants investigation of operating conditions rather than simply acceptance.
Prevention: CrMo Alloying and Alloy Carbides
The mechanism by which CrMo alloying prevents HTHA — the formation of stable alloy carbides (Cr₂₃C₆, Cr₇C₃) that are far less reactive with hydrogen than iron carbide (Fe₃C) — also prevents or significantly slows surface decarburisation. In P11, P22, and P5 CrMo steels, the surface decarburisation rate in hydrogen service is orders of magnitude slower than in carbon steel WPB at the same temperature and hydrogen partial pressure, because the surface carbides are alloy carbides rather than iron carbides. This is the second reason (after HTHA resistance) why CrMo alloy steel pipe fittings are specified for hot-wall hydrogen service — they resist both internal HTHA fissuring and surface decarburisation. For carbon steel WPB fittings used inadvertently in hydrogen service above the Nelson curve limit, the combination of surface decarburisation and internal HTHA can reduce the effective wall thickness and fracture resistance simultaneously, creating conditions for catastrophic failure without any advance warning from conventional external inspection.
Scale and Oxide Barrier Effects
In air or steam at elevated temperature, a protective oxide scale (Fe₂O₃, Fe₃O₄) forms on the carbon steel surface that acts as a partial barrier to hydrogen diffusion and slows surface decarburisation. In pure hydrogen or hydrogen-rich gas (partial pressure of hydrogen predominant), this oxide scale is reduced: H₂ + Fe₂O₃ → Fe + H₂O. The reduced iron surface is then directly exposed to the hydrogen atmosphere, and decarburisation proceeds without the oxide barrier. This is why steam-oxidised carbon steel surfaces show less decarburisation than hydrogen-reduced surfaces at the same temperature — the oxide on steam-treated surfaces provides a partial diffusion barrier. In practice, hydrogen service fittings cycle between hydrogen exposure (during operation) and air exposure (during shutdowns and maintenance), which alternately reduces and re-oxidises the surface. Each cycle can progress decarburisation slightly deeper — the cumulative decarburisation over many cycles may be greater than the decarburisation from continuous hydrogen exposure at the same conditions.