HTHA and the API 941 Nelson curves
In hot, high-pressure hydrogen service — hydrotreaters, reformers, hydrocrackers — steel can be attacked from the inside out. High-temperature hydrogen attack is insidious because the damage is internal and permanent. The Nelson curves are how the industry stays clear of it.
The mechanism
At elevated temperature, atomic hydrogen dissolves into the steel and reaches the carbides. There it reacts to form methane:
Methane molecules are too large to diffuse out, so they accumulate at grain boundaries and inclusions, building pressure that nucleates fissures and voids. The steel simultaneously loses carbon (decarburization), dropping its strength. The result is irreversible loss of integrity that may not be visible from the surface.
The Nelson curves
API 941 collects decades of plant experience into operating-limit curves: for each steel, a line in the space of temperature versus hydrogen partial pressure separating safe from susceptible operation.
Designing against HTHA
Selection is a matter of staying below the curve with margin for the actual metal temperature and hydrogen partial pressure — including process upsets, not just nameplate conditions. Existing equipment near its limit is monitored with advanced ultrasonic techniques (TOFD, velocity ratio, attenuation) able to detect early subsurface fissuring before through-wall damage develops.
Open the calculatorHydrogen damage lab →Screen a service condition against the Nelson curves, and assess embrittlement, HIC and permeation for the full picture of hydrogen damage.Don’t confuse the mechanisms
HTHA is a high-temperature, permanent, carbide-driven attack. It is distinct from low-temperature hydrogen embrittlement (reversible toughness loss) and from wet-H₂S cracking (HIC/SSC). A hydrogen integrity assessment has to identify which mechanism applies, because the controlling variables and the defences are completely different.
Frequently asked
- What is the difference between HTHA and hydrogen embrittlement?
- High-temperature hydrogen attack (HTHA) occurs above ~200 °C: atomic hydrogen reacts with carbides to form methane internally, causing decarburization and fissuring — permanent metallurgical damage. Low-temperature hydrogen embrittlement is a loss of ductility/toughness from dissolved hydrogen that is largely reversible once the hydrogen leaves. They are different mechanisms with different defences.
- What are the Nelson curves?
- API 941 publishes operating-limit curves of temperature versus hydrogen partial pressure for carbon steel and various Cr-Mo alloys. Operating below a steel’s curve is considered safe from HTHA; above it, that steel is susceptible and a more resistant alloy is required.
- Why do Cr-Mo steels resist HTHA?
- Chromium and molybdenum form stable carbides that do not readily give up their carbon to react with hydrogen, so methane formation is suppressed. Increasing Cr-Mo content raises the safe operating envelope — which is why 1¼Cr, 2¼Cr and higher grades are used as the duty gets more severe.
References
- API RP 941, "Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants" (Nelson curves).
- G.A. Nelson, original operating-limit curves for steels in hydrogen service.
- API RP 571, "Damage Mechanisms Affecting Fixed Equipment in the Refining Industry."
- R.D. Merrick, "Refinery Experiences with HTHA," NACE.
Related guides
- CALPHAD phase diagramsHow Gibbs-energy minimization computes equilibrium phase diagrams.
- CO₂ corrosion ratede Waard–Milliams and NORSOK M-506 for sweet corrosion of carbon steel.
- NACE MR0175 sour serviceThe H₂S threshold, SSC severity regions and hardness limits.
- Pourbaix (E–pH) diagramsImmunity, corrosion and passivation from thermodynamics.