High-Temperature Hydrogen Attack (HTHA) is one of the most serious and least forgiving damage mechanisms in the refining and petrochemical industries. In steel exposed to hydrogen at elevated temperature and pressure — conditions that exist in hydroprocessing reactors, catalytic reformers, and hydrogen-service heat exchangers — atomic hydrogen reacts with carbon in the steel matrix to form methane.
The Detection Challenge in HTHA
Microstructural Degradation Before Fissuring
HTHA begins as decarburisation and methane
formation at a microscopic level — before any fissures large enough for
conventional UT detection have formed. This sub-critical stage is important
because it indicates that the steel has been exposed to hydrogen conditions
that may eventually produce detectable damage — but it is only accessible to
microstructural examination on removed samples, not to in-service NDT.
Velocity and Attenuation Change Detection
As HTHA progresses from microscopic
decarburisation to grain boundary fissuring, changes in the steel's acoustic
properties begin to manifest — small decreases in ultrasonic velocity and
increases in backscatter attenuation that can be detected by specialist UT
measurements before macroscopic damage is visible on standard A-scan
presentation. Advanced UT techniques including velocity measurement,
backscatter analysis, and acoustic microscopy on removed samples can provide
early-stage HTHA indicators.
Fissure and Void Detection — Advanced PAUT
When HTHA progresses to the fissuring stage, grain boundary fissures and
methane voids are detectable by high-frequency PAUT with appropriate focal laws
for the wall thickness under inspection. The fissure orientation —
predominantly at grain boundaries and inclusions throughout the cross-section —
requires a combination of beam angles and frequencies to achieve adequate
sensitivity across the full inspection volume.
API RP 941 — Nelson Curves and
Inspection Requirements
API RP 941 (Steels for Hydrogen Service at Elevated Temperatures and
Pressures in Petroleum Refineries) defines the material selection criteria —
Nelson curves — for hydrogen service equipment. Assets operating above the
Nelson curve for their material specification are at risk of HTHA. Where
operating conditions approach or exceed the Nelson curve, enhanced inspection
is required. Our HTHA inspection programme is structured around API RP 941 risk
assessment, with technique selection and inspection scope matched to the
specific material, operating temperature, and hydrogen partial pressure of each
asset.
Reports include assessment of the
inspection asset against API RP 941 Nelson curve criteria, UT scan data with
acoustic velocity measurements and backscatter analysis where applicable,
indication tables for any detected fissuring, and recommended inspection
interval and monitoring programme based on the identified risk level and
inspection findings.