8.2 HTHA, High-Temperature H2/H2S Corrosion & Sulfidation

High-Temperature Hydrogen Attack (HTHA) & Sulfidation

High-temperature refining processes—such as hydrotreating, hydrocracking, catalytic reforming, and coking—expose pressure vessels to severe chemical environments containing pressurized hydrogen ($H_2$) and reactive sulfur compounds at elevated temperatures. Two of the most critical thermal degradation mechanisms evaluated under API 510 and API RP 571 are High-Temperature Hydrogen Attack (HTHA) and Sulfidation.

---

1. High-Temperature Hydrogen Attack (HTHA)

High-Temperature Hydrogen Attack (HTHA) is an irreversible metallurgical degradation mechanism that occurs in carbon steel and low-alloy steels exposed to hydrogen at elevated temperatures (typically $> 400^\circ\text{F} / 204^\circ\text{C}$) and high hydrogen partial pressures ($P_{H_2} > 50\text{ psia} / 0.35\text{ MPa}$).

+-----------------------------------------------------------------------------+
|                        THE HTHA METALLURGICAL SEQUENCE                      |
|                                                                             |
|   1. HYDROGEN DISSOCIATION:                                                 |
|      Molecular H2 dissociates into atomic hydrogen at the steel surface:    |
|      H2 <===> 2H (atomic hydrogen permeates the iron crystal lattice)       |
|                                                                             |
|   2. SOLID-STATE METHANE REACTION:                                          |
|      Atomic H reacts with iron carbide (cementite) at grain boundaries:     |
|      Fe3C + 4H  ===>  3Fe + CH4 (Methane Gas)                               |
|                                                                             |
|   3. CAVITY FORMATION & PRESSURE ACCUMULATION:                              |
|      CH4 molecule is too large to diffuse out. Trapped methane gas builds    |
|      internal hydrostatic pressures exceeding tens of thousands of psi.     |
|                                                                             |
|   4. MICROFISSURING & DECARBURIZATION:                                      |
|      Sub-microscopic cavities coalesce into grain-boundary microfissures.   |
|      Decarburization robs steel of carbon, destroying tensile & creep strength.|
|                                                                             |
|   5. CATASTROPHIC BRITTLE RUPTURE:                                          |
|      Microfissures link into macrocracks, causing sudden rupture with       |
|      ZERO prior plastic deformation or wall thinning!                       |
+-----------------------------------------------------------------------------+

Decarburization Modes

HTHA causes two distinct decarburization phenomena:

1. Surface Decarburization: Occurs at high temperatures and lower hydrogen partial pressures. Atomic hydrogen reacts with carbon at the external/internal surface, producing methane that escapes into the process stream. The result is a soft, low-strength ferritic surface layer without internal fissures.
2. Internal Decarburization & Microfissuring: Occurs at higher hydrogen partial pressures. Methane forms internally along grain boundaries and non-metallic inclusions. The trapped gas cannot escape, driving void nucleation, microfissuring, and catastrophic embrittlement.

The Incubation Period

HTHA exhibits a dangerous incubation period—a duration spanning months, years, or decades during which sub-microscopic methane cavities slowly nucleate. During incubation, no macrocracks exist, and standard mechanical properties appear unaffected. Once microfissures link up, mechanical degradation accelerates exponentially, leading to sudden, catastrophic vessel rupture (e.g., the 2010 Tesoro Anacortes refinery catacarb exchanger disaster).

---

2. Nelson Curves & API RP 941 Operating Limits

API Recommended Practice 941 (Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants) provides the globally recognized Nelson Curves. These empirical curves plot Operating Temperature vs. Hydrogen Partial Pressure to define the safe operating threshold for various metallurgy grades.

+-----------------------------------------------------------------------------+
|                   NELSON CURVE ALLOY RESISTANCE HIERARCHY                   |
|                                                                             |
|   [HIGHEST RESISTANCE]                                                      |
|      ^   9.0Cr - 1.0Mo                                                      |
|      |   5.0Cr - 0.5Mo                                                      |
|      |   3.0Cr - 1.0Mo - V / 2.25Cr - 1.0Mo - V                             |
|      |   2.25Cr - 1.0Mo (SA-387 Gr 22)                                      |
|      |   1.25Cr - 0.5Mo (SA-387 Gr 11)                                      |
|      |   1.0Cr - 0.5Mo                                                      |
|      |   Carbon-0.5Mo (Discontinued for new construction)                   |
|      |   Carbon Steel (100% PWHT)                                           |
|      |   Carbon Steel (Non-PWHT) [MOST RESTRICTIVE / LOWEST LIMIT]          |
|   [LOWEST RESISTANCE]                                                       |
+-----------------------------------------------------------------------------+

The Role of Chromium & Molybdenum Alloy Additions

Plain iron carbide (cementite, $Fe_3C$) is thermodynamically unstable in the presence of dissolved atomic hydrogen. Alloying with Chromium (Cr) and Molybdenum (Mo) forms complex alloy carbides (such as $Cr_7C_3$, $Cr_{23}C_6$, $Mo_2C$, and $M_{23}C_6$) that are vastly more stable and resistant to methane conversion:

• Molybdenum: Increases carbide stability and solid-solution creep strength.
• Chromium: Dramatically stabilizes carbides against hydrogen attack and provides oxidation/sulfidation resistance.

Crucial API RP 941 Code Revisions for Carbon Steel

Following multiple catastrophic industry ruptures of carbon steel equipment operating below the historical Nelson Curve:

1. Elimination of the Single Carbon Steel Curve: API RP 941 abolished the legacy unified carbon steel curve.
2. Two Distinct Carbon Steel Curves:
   • Curve 1 (Non-PWHT Carbon Steel): Shifted downward and to the left to significantly lower temperatures and pressures. Non-PWHT carbon steel is now recognized as highly vulnerable to HTHA due to high residual welding stresses.
   • Curve 2 (PWHT Carbon Steel): Applies only to carbon steel that has received full Post-Weld Heat Treatment ($100%$ PWHT per code rules).
3. C-0.5Mo Degradation Warning: Carbon-0.5Mo steel is no longer recommended for new hydrogen equipment because numerous failures occurred significantly below its historical Nelson Curve.

---

3. Advanced NDE Methodologies for HTHA Detection

Conventional NDE methods—such as visual inspection, standard pulse-echo thickness gauging, and Radiographic Testing (RT)—are completely incapable of detecting early-stage HTHA microfissuring ($< 0.1\text{ mm}$ size).

+-----------------------------------------------------------------------------+
|                        ADVANCED HTHA NDE TOOLKIT (API RP 941)               |
|                                                                             |
|   [STAGE 1: METHANE BUBBLES]  -----> Ultrasonic Velocity Ratio (Vl / Vs)   |
|   [STAGE 2: MICROFISSURING]   -----> Advanced Backscatter (AUBT) & TOFD    |
|   [STAGE 3: MACROCRACKING]    -----> Phased Array UT (PAUT) & Angle Beam UT|
|   [SURFACE DECARBURIZATION]   -----> In-situ Metallographic Replication     |
+-----------------------------------------------------------------------------+

Advanced HTHA Inspection Techniques

---

4. Sulfidation and High-Temperature H2/H2S Corrosion

Sulfidation (also known as high-temperature sulfidic corrosion) is the reaction of elemental sulfur and sulfur compounds (hydrogen sulfide $H_2S$, mercaptans, thiophenes, and aliphatic sulfides) with iron and steel at temperatures above $500^\circ\text{F} (260^\circ\text{C})$.

$\text{Primary Reaction: } Fe + S \longrightarrow FeS \quad \text{or} \quad Fe + H_2S \longrightarrow FeS + H_2$

+-----------------------------------------------------------------------------+
|                     SULFIDATION: H2-FREE VS. H2/H2S ENVIRONMENTS             |
|                                                                             |
|   [H2-FREE SULFIDATION]                       [H2 / H2S SULFIDATION]        |
|   - Crude units, vacuum towers, cokers        - Hydrotreaters, hydrocrackers|
|   - Governed by MODIFIED McCONOMY CURVES      - Governed by COUPER-GORMAN   |
|   - Forms protective FeS / coke scale         - H2 strips protective scale  |
|   - Rate peaks at 750°F-800°F (399°C-427°C)   - Rates continue accelerating |
|   - Severe Si-content dependency (< 0.10% Si) - Extremely aggressive attack |
+-----------------------------------------------------------------------------+

The Critical Role of Silicon Content in Carbon Steel

One of the most critical and heavily tested topics under API 571 and API 510 is the Silicon Content Effect in carbon steel exposed to high-temperature sulfidation ($> 500^\circ\text{F}$):

+-----------------------------------------------------------------------------+
|               SILICON EFFECT ON CARBON STEEL SULFIDATION RATES              |
|                                                                             |
|   [LOW SILICON CARBON STEEL: Si < 0.10%] (e.g., A53 Pipe, Rimmed/Semi-Killed)|
|   ---> Sulfidation rate is 2x to 5x HIGHER than standard carbon steel!      |
|   ---> Forms porous, non-adherent, non-protective iron sulfide scale.       |
|                                                                             |
|   [SILICON-KILLED CARBON STEEL: Si >= 0.10%] (e.g., SA-516 Gr 70, SA-106 B) |
|   ---> Normal baseline sulfidation rate. Forms dense, protective FeS scale. |
+-----------------------------------------------------------------------------+

[!CRITICAL] Silicon-Killed Steel Mandate: Carbon steel components (vessel shells, nozzles, pipe fittings) manufactured from semi-killed or rimmed steels containing less than $0.10\text{ wt}%$ Silicon corrode at catastrophic rates ($2\times \text{ to } 5\times$ normal). API 510 inspectors must identify low-silicon components in hot sulfidic services using Positive Material Identification (PMI) with portable Optical Emission Spectrometry (OES) or Laser-Induced Breakdown Spectroscopy (LIBS).

McConomy vs. Couper-Gorman Curves

1. Modified McConomy Curves: Predict corrosion rates in $H_2$-free sulfur streams (e.g., crude distillation units). Corrosion rates increase from $500^\circ\text{F}$ up to $750^\circ\text{F}-800^\circ\text{F}$ ($399^\circ\text{C}-427^\circ\text{C}$), above which dense coke and iron sulfide scales form a diffusion barrier that retards further corrosion.
2. Couper-Gorman Curves: Predict corrosion rates in $H_2/H_2S$ systems (e.g., hydroprocessing units). Because excess hydrogen prevents the formation of protective coke and destabilizes the iron sulfide scale, corrosion rates do not plateau at $750^\circ\text{F}$ but continue rising steeply.

Materials Selection & Mitigation for Sulfidation

Corrosion resistance increases exponentially with increasing Chromium content:

$\text{Carbon Steel } (< 0.10\%\text{ Si}) \ll \text{CS } (\ge 0.10\%\text{ Si}) < 5\text{Cr}-0.5\text{Mo} < 9\text{Cr}-1\text{Mo} < 12\text{Cr (Type 410)} \ll 300\text{-Series SS}$

Austenitic stainless steels (18Cr-8Ni, e.g., Type 304L/316L) and nickel alloys with $> 20%\text{ Cr}$ provide virtually complete immunity to high-temperature sulfidation in most refining services.

---

5. Summary Comparison: HTHA vs. Sulfidation