Hangbo Alloy Group
Shanghai · Est. 2012
HTHA fissures carbon and low-alloy steels in refinery H2/H2S service. Learn why nickel alloys like C-276, 625 and 800H resist high-temperature hydrogen attack.
High-temperature hydrogen attack (HTHA) is one of the most destructive and least visible failure modes in petroleum refineries, syngas plants, and hydrogen production facilities. In service above roughly 200°C (390°F) and at hydrogen partial pressures beyond about 1.4 MPa (200 psi), atomic hydrogen diffuses into carbon and low-alloy steels, reacts with carbides at the grain boundaries, and forms methane. The methane cannot escape, so it builds pressure until it blisters and fissures the metal from the inside. Nickel-based alloys avoid this mechanism almost entirely, which is why they anchor the clad linings and solid components in hydrocrackers, reactors, and reheaters. This guide explains the mechanism, why nickel works, which grades to specify, and how to detail them for safe, long-lived H2 service.
HTHA proceeds in two linked steps. First, molecular hydrogen at the process surface dissociates into atomic hydrogen, which is small enough to diffuse through the steel lattice. Second, that hydrogen combines with carbon — either from the steel's own cementite (Fe3C) or from carbon in the process stream — to form methane (CH4). Because methane is a large, stable molecule, it accumulates at grain boundaries and inclusions.
The result is a characteristic microstructure: decarburization (loss of hardness and strength) immediately beneath the surface, followed by the nucleation of methane-filled voids and fissures. Cracking is often internal and undetectable by external visual inspection until a component is sectioned — which is why HTHA is called a 'silent' failure.
Engineers control the risk with the API 941 (Nelson) curves, which plot the safe operating envelope of temperature versus hydrogen partial pressure for different steel grades. Carbon steels are limited to the lowest pressure-temperature corners; even 2.25Cr-1Mo with proper post-weld heat treatment survives only to moderate conditions. Beyond those limits, the only reliable answer is a material that does not contain the reactive iron-carbide network that HTHA feeds on — i.e., a high-nickel alloy.
Nickel itself does not form iron carbides, and high-nickel alloys contain little or no free iron at the surface to react with hydrogen and carbon. More importantly, the carbon present in these alloys is tied up as stable, finely dispersed carbides of chromium, molybdenum, and niobium — not as brittle cementite that methane can pry apart. Atomic hydrogen still diffuses through nickel, but there is no carbon-rich grain-boundary phase to convert into methane, so fissuring does not initiate.
A second advantage is stability at the temperatures where HTHA operates. Grades such as Incoloy 800H/800HT and Inconel 601 retain useful creep strength well above 500°C, so a single alloy can satisfy both the structural and the hydrogen-resistance requirement. Chloride and sulfur species that accompany many H2 streams are also far better tolerated by nickel alloys than by stainless steels.
Hangbo Alloy Group stocks and produces these grades as plate, tube, forged flanges, and weld-overlay wire, with chemistry verified against ASTM/ASME and EN 10204 3.1 mill certificates on every heat.
Most refineries do not build reactors from solid nickel alloy — it is far too expensive. Instead they specify a thin clad or weld overlay of C-276, C-22, or 625 over a carbon-steel pressure boundary. The overlay must be free of dilution-induced porosity, so qualified procedures and proper purge are essential; our welding guide covers heat input and shielding-gas basics that also apply to overlay.
Where solid nickel alloy is used (tubes, fittings, instrument hardware), keep carbon low and avoid unnecessary post-weld heat treatment that could sensitize adjoining stainless. Always check the component against the latest API 941 envelope for its actual hydrogen partial pressure, not the nameplate design value, because streams swing during startups and upsets.
For anything touching sour, high-H2S environments, confirm sulfide-stress-cracking resistance too — our NACE MR0175 sour-service guide and sour-gas selection guide explain the complementary limits.
HTHA is generally a concern above about 200°C (390°F) with a hydrogen partial pressure above roughly 1.4 MPa (200 psi). The exact safe limit for each steel grade is read from the API 941 (Nelson) curves, which are updated periodically. Nickel alloys sit outside the curve entirely because they do not form the iron-carbide network HTHA needs — for the broader sulfide limits that often accompany H2 streams, see our NACE MR0175 sour-service guide.
Yes. C-276 contains essentially no free iron-carbide phase for hydrogen and carbon to convert into methane, so it is immune to HTHA and also resists the chlorides and wet H2S that often appear in the same equipment. Its nominal chemistry and corrosion profile are detailed in our Hastelloy C-276 composition guide.
They resist HTHA, but hydrogen embrittlement is a different, lower-temperature mechanism driven by dissolved atomic hydrogen at ambient-to-moderate temperature, not by methane formation at grain boundaries. Nickel alloys are far more tolerant than high-strength steels, yet designers should still control hydrogen in high-strength fasteners and cold-worked sections — our hydrogen embrittlement guide covers the distinction.
Almost always clad or weld overlay over carbon steel for large pressure vessels, because solid nickel alloy is costly and harder to qualify for thick sections. Solid forms (tube, fittings, fasteners) make sense where the whole part sees H2 and high temperature. For the underlying material choices, compare our Incoloy 825 high-temperature notes and Alloy 28 chemical-processing guide.
As a dedicated nickel alloy manufacturer serving oil & gas, power generation sectors, Hangbo Alloy Group supplies the grades discussed here across all standard product forms with full ASTM/ASME documentation and EN 10204 3.1 certification. Our material is specified for applications where corrosion resistance, high-temperature strength, and traceable quality are non-negotiable. See our production & testing facilities for more on how every heat is verified.