10.1 Hydrogen Blistering & Hydrogen-Induced Cracking (HIC)

Wet H2S Damage Fundamentals & Cathodic Hydrogen Charging

1. Classification of Wet H2S Damage Mechanisms (API RP 571 Section 3.67)

In petroleum refining and petrochemical processing, the term Wet H2S Damage encompasses four distinct degradation mechanisms caused by aqueous electrochemical corrosion in hydrogen sulfide ($H_2S$) containing streams:

1. Hydrogen Blistering
2. Hydrogen-Induced Cracking (HIC)
3. Stress-Oriented Hydrogen-Induced Cracking (SOHIC)
4. Sulfide Stress Cracking (SSC)

While all four mechanisms share a common root cause—the absorption and diffusion of nascent atomic hydrogen into carbon or low-alloy steel—they differ fundamentally in their metallurgical drivers, stress requirements, crack morphologies, and mitigation protocols.

2. Aqueous Sour Corrosion and Hydrogen Charging Chemistry

When carbon steel is exposed to an aqueous electrolyte containing dissolved hydrogen sulfide, electrochemical corrosion proceeds through coupled anodic and cathodic half-cell reactions:

• Anodic Dissolution of Iron: $Fe \rightarrow Fe^{2+} + 2e^-$
• Cathodic Hydrogen Ion Reduction: $2H^+ + 2e^- \rightarrow 2H^\bullet \quad (\text{nascent atomic hydrogen})$
• Iron Sulfide Scale Precipitation: $Fe^{2+} + S^{2-} \rightarrow FeS \quad (\text{mackinawite / pyrrhotite scale})$

In benign aqueous acidic environments (such as pure hydrochloric or sulfuric acid without sulfides), the atomic hydrogen generated on the steel surface rapidly undergoes recombination to form harmless molecular hydrogen gas: $H^\bullet + H^\bullet \rightarrow H_2 \uparrow$ The resulting $H_2$ gas bubbles off the metal surface into the process fluid, resulting in minimal hydrogen diffusion into the steel.

3. The Catalytic Poisoning Role of Sulfides and Cyanides

In sour environments, this benign recombination pathway is blocked:

• Sulfide Recombination Poisons: Dissolved sulfide species (hydrosulfide ions $HS^-$ and sulfide ions $S^{2-}$) adsorb directly onto the metallic surface, chemically poisoning the catalytic recombination of atomic hydrogen into molecular hydrogen.
• Cyanide Acceleration ($CN^-$): In downstream cracking units (such as Fluid Catalytic Cracking [FCC] and delayed coking gas recovery plants), hydrogen cyanide ($HCN$) dissolves in alkaline wash waters. Cyanide ions act as aggressive depassivators, reacting with protective iron sulfide surface films to form soluble ferrocyanide complexes: $FeS + 6CN^- \rightarrow [Fe(CN)_6]^{4-} + S^{2-}$ By continuously stripping the protective $FeS$ scale and exposing fresh bare steel, cyanides dramatically accelerate both the corrosion rate and the hydrogen charging flux into the metal.
• Lattice Absorption: Because the recombination reaction ($2H^\bullet \rightarrow H_2$) is poisoned, high surface concentrations of nascent atomic hydrogen build up. Driven by a steep chemical potential gradient, atomic hydrogen—having an atomic radius of only ~0.053 nm—readily absorbs into the body-centered cubic (BCC) ferrite crystal lattice and diffuses rapidly through interstitial spaces.

Hydrogen Blistering Mechanics

1. Internal Hydrogen Trapping and Cavity Pressurization

As atomic hydrogen diffuses interstitially through the bulk steel plate, it encounters internal metallurgical discontinuities, microstructural interfaces, and laminations:

• Non-Metallic Inclusions: In standard commercial plate steels, manganese sulfide ($MnS$) inclusions and silicates are rolled into elongated, planar ribbon-like stringers.
• Laminar Interfaces & Banding: Segregation of carbon, manganese, and phosphorus during ingot casting creates alternating microstructural bands of ferrite and pearlite parallel to the plate rolling direction.

When diffusing atomic hydrogen reaches these internal micro-voids and inclusion interfaces, the chemical environment permits the recombination reaction to proceed: $H^\bullet + H^\bullet \rightarrow H_2 \quad (\text{molecular hydrogen gas})$

2. The Molecular Diffusion Barrier and Immense Pressure Generation

Crucially, while atomic hydrogen ($H^\bullet$) diffuses rapidly through the BCC iron lattice, the diatomic molecular hydrogen molecule ($H_2$) has a kinetic diameter of approximately 0.29 nm and cannot diffuse through the metal lattice at ambient and moderate operating temperatures.

• The trapped molecular hydrogen gas is physically locked inside the internal laminar void.
• As atomic hydrogen continues to enter the void and recombine, internal gas pressure escalates rapidly.
• Thermodynamic equilibrium pressures can build to thousands of pounds per square inch (psi), frequently exceeding 10,000 to >50,000 psi (70 to >350 MPa).
• When this localized internal gas pressure exceeds the yield strength of the overlying steel, the metal undergoes permanent plastic deformation, bulging outward to form visible subsurface blisters.

3. Blister Manifestation and Failure Modes

• Subsurface & Surface Blisters: Blisters can range in diameter from a few millimeters (micro-blisters) to over a foot across (macro-blisters). Blisters located close to the inner or outer surface produce distinct, dome-shaped surface bulges.
• Blister Rim Cracking: Extreme hoop stresses around the periphery (rim) of a blister generate through-thickness shear cracking. These cracks can propagate from the blister rim to either the internal process-wetted surface or the external atmospheric surface.
• Blister Rupture: If an internal blister links to the process surface, sour process fluid enters the cavity, accelerating localized pitting and crevice corrosion. If a blister ruptures through to the external surface, loss of containment occurs.

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Hydrogen-Induced Cracking (HIC)

1. Stepwise Cracking Mechanism

Hydrogen-Induced Cracking (HIC), historically referred to as stepwise cracking or blister cracking, is an advanced, severe form of wet $H_2S$ degradation.

• Purely Pressure-Driven: HIC is driven entirely by internal hydrogen gas pressure accumulated within adjacent laminar micro-voids. No externally applied mechanical stress or welding residual stress is required for HIC to initiate and propagate.
• Stepwise Crack Propagation:
  1. Atomic hydrogen collects at multiple elongated manganese sulfide ($MnS$) inclusions located on adjacent, parallel rolling planes across the plate thickness.
  2. Molecular $H_2$ gas pressurization initiates planar cracks that propagate parallel to the plate rolling surface along these inclusion stringers.
  3. As adjacent parallel planar cracks grow, the intense triaxial stress fields at their overlapping crack tips interact.
  4. High shear stresses between the crack planes force the steel to rupture along through-thickness shear planes (typically oriented at approximately 45° to the rolling direction).
  5. This alternating sequence of planar laminar cracking connected by through-thickness shear links creates a distinctive stepwise (staircase) fracture morphology through the vessel wall.

2. Rolled Plate Steels vs Forgings and Seamless Pipe

HIC is overwhelmingly prevalent in rolled plate steels (such as ASTM A516 Grade 70 or ASTM A285 Grade C) used in pressure vessel shells, heads, and large-diameter welded pipe.

• Hot-rolling elongates ductile Type II manganese sulfide inclusions into broad, paper-thin planar sheets that act as enormous hydrogen collectors.
• In contrast, seamless pipe (e.g., ASTM A106 Grade B) and forgings (e.g., ASTM A105) exhibit significantly lower HIC susceptibility because their three-dimensional metal flow and lower inclusion aspect ratios do not produce continuous planar trap planes.

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Critical Factors Governing Hydrogen Blistering and HIC

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Prevention, Mitigation & Materials Specification

1. HIC-Resistant Steel Specification (NACE TM0284)

For new construction in wet sour service, pressure vessels and piping should be fabricated using HIC-resistant steel tested and certified in accordance with NACE TM0284 (Evaluation of Pipeline and Pressure Vessel Steels for Resistance to Hydrogen-Induced Cracking).

• Ultra-Low Sulfur Metallurgy: Modern steelmaking utilizes ladle refining, desulfurization, and vacuum degassing to depress bulk sulfur content to $S \le 0.0010\text{ wt}\%$ (10 ppm), eliminating the raw material needed to form manganese sulfides.

• Inclusion Shape Control (Calcium Treatment): Liquid steel is treated with calcium (typically wire-fed calcium-silicon, $CaSi$). Calcium reacts preferentially with sulfur and oxygen to form complex calcium aluminates and calcium sulfides ($CaS$). Unlike manganese sulfide, which is soft and plastically deforms into flat stringers during hot rolling, calcium sulfides remain hard, indeformable, and spherical throughout rolling operations. Spherical inclusions present minimal planar surface area and do not act as crack initiators.

• Controlled Chemistry & Segregation Control: Continuous casting with electromagnetic stirring and soft reduction prevents centerline macro-segregation of carbon, manganese ($Mn < 1.20\%$), and phosphorus ($P \le 0.010\%$), suppressing hard microstructural bandings.

• Acceptance Criteria (purchaser-defined): NACE TM0284 defines the test solutions (Solution A, pH about 2.7; Solution B, pH about 4.8) and how to measure the crack ratios; the purchaser sets the limits. Commonly specified maximums are:

  • Crack Length Ratio (CLR): Typically $\le 15\%$
  • Crack Thickness Ratio (CTR): Typically $\le 5\%$
  • Crack Sensitivity Ratio (CSR): Typically $\le 1.5\%$

• What PWHT Does Not Do: RP 571 notes that PWHT helps minimize SSC and SOHIC but is not effective against blistering and HIC, which are driven by internal hydrogen pressure rather than stress.

2. Corrosion Barrier Coatings and Metallic Cladding

• Alloy Cladding / Weld Overlay: Isolating the carbon steel substrate from the sour aqueous electrolyte using roll-bonded cladding or weld overlay of corrosion-resistant alloys (CRAs):
  • Nickel-Base Alloy 625 (UNS N06625)
  • 300-Series Austenitic Stainless Steels (Type 304L, 316L)
• Austenitic alloys exhibit negligible corrosion rates in wet $H_2S$ (generating zero cathodic hydrogen) and possess extremely low hydrogen diffusion coefficients, acting as a complete barrier to hydrogen permeation.

3. Chemical Process Inhibition and Water Washing

• Ammonium Polysulfide (APS) Injection: In FCC gas recovery units, overhead sour waters contain elevated cyanides ($HCN / CN^-$). Injecting ammonium polysulfide ($[NH_4]_2S_x$) chemically converts aggressive cyanide ions into benign, non-corrosive thiocyanate ions ($SCN^-$): $CN^- + S_x^{2-} \rightarrow SCN^- + S_{x-1}^{2-}$ Neutralizing the cyanide allows a stable, protective iron sulfide ($FeS$) passivation film to establish, reducing the hydrogen charging flux.
• Continuous Water Washing: Introducing clean, low-oxygen wash water upstream of condensers and sour overhead systems dilutes cyanides ($CN^- < 10\text{ ppmw}$), reduces ammonium bisulfide concentrations ($NH_4HS < 2\text{ to }5\text{ wt}\%$), and controls pH within safe operating boundaries.

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Non-Destructive Examination (NDE) Methodologies