Internal Methane Bubble Formation in Hydrogen-Damaged Steel
Methane-filled cavities formed during High-Temperature Hydrogen Attack (HTHA).
These microscopic voids strongly reflect ultrasonic waves and can eventually
lead to catastrophic material failure.
⚠ The Anacortes Tragedy
On April 2, 2010, a catastrophic failure occurred at the Tesoro
Anacortes Refinery in Washington State. A heat exchanger ruptured
violently, killing seven workers.
Investigators later identified the likely culprit as
High-Temperature Hydrogen Attack (HTHA), a dangerous
form of material degradation that can remain hidden deep within steel
components for years before suddenly causing catastrophic failure.
The most dangerous defects are often the ones that remain invisible
until the moment they become fatal.
⚛ What is High-Temperature Hydrogen Attack?
At elevated temperatures and hydrogen pressures, atomic hydrogen can
diffuse into carbon steel. Once inside the metal, hydrogen reacts with
cementite (iron carbide), an important strengthening constituent of
steel.
$$Fe_3C + 4H \rightleftharpoons 3Fe + CH_4$$
The reaction generates methane gas \((CH_4)\) inside the material.
Unlike atomic hydrogen, methane molecules are too large to diffuse
through the crystal lattice.
Consequently, methane accumulates at grain boundaries, microscopic
interfaces separating individual crystals within the steel.
🔬 Physics Insight
Atomic hydrogen is extraordinarily small and can diffuse through steel
relatively easily. Methane molecules, however, are much larger.
Once methane forms, it becomes trapped. The trapped gas builds pressure
inside the material, creating microscopic cavities and fissures that
gradually weaken the steel structure.
🔍 From Microscopic Voids to Macroscopic Failure
As methane accumulates, internal pressure rises. Tiny voids begin
forming along grain boundaries.
Over time these voids grow, connect, and form networks of microcracks.
What initially appears to be healthy, load-bearing steel gradually
transforms into a brittle, weakened structure.
The danger lies in the fact that the damage develops internally.
External inspection may reveal little or no evidence until failure
becomes imminent.
🚨 Why HTHA is Dangerous
Traditional visual inspections are often incapable of detecting hydrogen
attack because the damage begins deep inside the material.
A component may appear perfectly sound while extensive internal cracking
is already developing beneath the surface.
📡 Ultrasonic Physics to the Rescue
Engineers use ultrasonic testing to detect these hidden defects.
Ultrasonic waves propagate through steel and interact with internal
features such as cracks, voids, and methane-filled cavities.
The effectiveness of ultrasonic testing depends upon differences in
acoustic impedance.
Acoustic impedance is defined as
$$Z = \rho c$$
where:
- \(\rho\) = density of the medium
- \(c\) = speed of sound in the medium
📈 Why Gas Voids Reflect Sound So Strongly
When an ultrasonic wave encounters a boundary between two materials with
different acoustic impedances, part of the wave is reflected.
The intensity reflection coefficient is
$$R_I= \left( \frac{Z_2-Z_1} {Z_2+Z_1} \right)^2$$
In hydrogen attack, one medium is steel and the other is methane gas.
Because the acoustic impedance of methane is extremely small compared to
steel,
$$Z_{gas} \ll Z_{steel}$$
the reflection coefficient approaches unity:
$$R_I \approx 1$$
This means methane-filled cavities behave almost like perfect acoustic
mirrors, reflecting most of the incident ultrasonic energy.
💡 Physical Interpretation
Imagine shouting at a concrete wall and hearing a strong echo. A
methane-filled void inside steel produces a similar effect for
ultrasonic waves.
The enormous impedance mismatch creates powerful reflections that reveal
otherwise invisible internal damage.
🧪 Must Experiment: Interactive Ultrasonic Inspection Lab
Theory becomes much clearer when you can see ultrasonic waves interact
with defects in real time.
Try the Interactive Ultrasonic NDT Laboratory developed for Passion of
Physics. Create artificial defects, switch between Pulse-Echo and Phased
Array modes, and observe how internal discontinuities generate
reflections, A-Scans, and B-Scans.
The same physical principles are used to detect hydrogen attack,
methane-filled cavities, and grain-boundary cracking in industrial
pressure vessels and refinery equipment.
🔬 Launch Ultrasonic NDT Simulation
📊 The Characteristic "Grass" Signal
Experienced inspectors often identify hydrogen attack through the
appearance of ultrasonic backscatter known as grass.
Instead of receiving a clean signal from the back wall of a component,
the instrument displays numerous small echoes generated by
methane-filled voids distributed throughout the material.
The denser the damage, the stronger and more widespread this backscatter
becomes.
✅ Key Takeaways
- Hydrogen diffuses into steel at elevated temperatures.
- Hydrogen reacts with cementite to produce methane gas.
- Methane becomes trapped inside the material.
- Internal pressure creates voids and grain-boundary cracking.
- Gas-filled cavities strongly reflect ultrasonic waves.
- Ultrasonic backscatter provides an early warning of HTHA.
-
Physics allows engineers to detect invisible damage before
catastrophic failure occurs.
🎯 Challenge
Open the Ultrasonic NDT Lab and place several defects of different
sizes. Observe how the A-Scan changes as defect density increases. Can
you identify the point where the signal begins to resemble the "grass"
pattern associated with hydrogen damage?
🎥 Related Video
Watch an excellent explanation of hydrogen damage and ultrasonic
inspection by Steve Mould
▶ Watch on YouTube