12.3 HF Cracking (HSC & HF SCC of Ni Alloys) & Ethanol SCC

Hydrogen Stress Cracking in Hydrofluoric Acid (HSC) — API RP 571 Section 3.41

1. Fundamental Definition and Phenomenological Contrast

Hydrogen Stress Cracking in HF (HSC) is a form of hydrogen embrittlement (HE) that affects high-hardness carbon steels, low-alloy steels, and high-strength fasteners in hydrofluoric (HF) acid alkylation units.

Candidates must distinguish between the two distinct degradation mechanisms governed by hydrofluoric acid:

• Hydrofluoric (HF) Acid Corrosion (API RP 571 Section 3.38): An electrochemical corrosion mechanism causing general wall thinning and localized gouging through the dissolution of carbon steel into iron fluoride ($FeF_2$).
• Hydrogen Stress Cracking in HF (API RP 571 Section 3.41): An environmental cracking mechanism causing sudden, catastrophic brittle fracture in the presence of tensile stress and susceptible, high-hardness microstructures, without requiring significant metal loss.

                    ELECTROCHEMICAL GENERATION & HYDROGEN CHARGING IN HF

       Process Acid Stream: Hydrofluoric Acid (HF) + Hydrocarbons
                                   │
                                   ▼ Electrochemical Corrosion Reaction at ID Surface
               Fe  +  2 HF  ───►  FeF2 (Surface Scale)  +  2 H• (Nascent Atomic Hydrogen)
                                                            │
                         ┌──────────────────────────────────┴──────────────────────────────────┐
                         ▼                                                                     ▼
             Recombination into Molecular H2                                       Lattice Interstitial Diffusion
             H• + H• → H2 (Gas in Process)                                         Atomic Hydrogen Diffuses into Steel
                                                                                               │
                                                                                               ▼
                                              High Tensile Stress + High-Hardness Microstructure (>200 HBW)
                                              (Untempered Martensite/Bainite in Weld HAZ or Gr B7 Bolting)
                                                                                               │
                                                                                               ▼
                                                             HYDROGEN STRESS CRACKING IN HF (BRITTLE FRACTURE)

2. Corrosion Chemistry and Hydrogen Charging

When aqueous hydrofluoric acid contacts carbon steel, an electrochemical reaction generates an iron fluoride ($FeF_2$) surface scale and nascent atomic hydrogen ($H^\bullet$): $Fe + 2HF \rightarrow FeF_2 + 2H^\bullet$ While much of the atomic hydrogen recombines at the surface to form molecular hydrogen gas ($H_2$), a significant fraction dissolves into the body-centered cubic (BCC) iron crystal lattice. The presence of trace contaminants in HF service (such as sulfur, arsenic, or phosphorus species) acts as hydrogen recombination poisons, drastically increasing the rate of hydrogen permeation into the metal.

Once inside the metal, atomic hydrogen diffuses interstitially toward regions of high triaxial tensile stress—specifically the root of notches, weld toe geometric discontinuities, and crack tips. If the localized microstructure exhibits high hardness, the trapped hydrogen lowers the cohesive bonding energy of the crystal lattice, triggering rapid brittle crack propagation.

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Critical Factors and Hardness Thresholds in HF Units

1. The 200 HBW Hardness Limit

The single most critical factor determining susceptibility to HSC in HF service is microstructural hardness:

• Base Metal and Weldments: Steels with hardness levels below 200 HBW (approximately 93 HRB) exhibit excellent resistance to HSC. Susceptibility escalates dramatically when hardness exceeds about 200 HBW, and it rises sharply above 22 HRC (about 237 HBW).
• High hardness in weldments results from fast cooling rates following welding, which transforms the weld metal and heat-affected zone (HAZ) into brittle, untempered martensite or lower bainite.

2. Bolting Vulnerability and API RP 751 Standards

Standard high-strength carbon and alloy steel bolting, such as ASTM A193 Grade B7 (hardness typically 28 to 35 HRC / 270 to 330 HBW), is exceptionally vulnerable to HSC:

• If Grade B7 bolting is exposed to trace HF vapor, flange leaks, or atmospheric wash water, rapid catastrophic brittle failure can occur within hours to days.
• API RP 751 (Safe Operation of Hydrofluoric Acid Alkylation Units) addresses hardness control for fasteners exposed to HF service. Operators specify ASTM A193 Grade B7M (maximum hardness strictly limited to 235 HBW / 22 HRC) or upgrade to solid nickel-base fasteners (e.g., Alloy B-2 [UNS N10665] or Alloy C-276 [UNS N10276]).

3. Steel Chemistry and Carbon Equivalent (CE)

To avoid hard, brittle HAZ formation during fabrication and field repairs, HF-service specifications (see API RP 751) commonly control steel chemistry, including carbon equivalent (CE), along with residual elements. Carbon equivalent is typically calculated with the IIW formula: $CE = C + \frac{Mn}{6} + \frac{Cr + Mo + V}{5} + \frac{Ni + Cu}{15}$

4. Post-Weld Heat Treatment (PWHT)

PWHT of carbon steel weldments in HF service is common practice (RP 571 notes that PWHT reduces SOHIC and preferential corrosion of weld heat-affected zones). PWHT is typically conducted at 1150 °F to 1200 °F (621 °C to 649 °C) with a 1-hour minimum hold time to temper hard microstructures back below 200 HBW and reduce residual welding tensile stresses.

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HF Stress Corrosion Cracking of Nickel Alloys — API RP 571 Section 3.39

                  HYDROFLUORIC ACID SCC OF NICKEL ALLOYS (ALLOY 400)

        Deaerated HF Environment                     Aerated / Oxidizing HF Environment
  (No Dissolved Oxygen or Oxidizers)            (Dissolved O2 > 1-5 ppm, Fe3+, Cu2+ Contamination)
  ┌──────────────────────────────────┐          ┌──────────────────────────────────┐
  │ - Stable CuF2/NiF2 passive film  │          │ - Film destabilization & active  │
  │ - Low general corrosion rate     │          │   transpassive dissolution       │
  │ - IMMUNE TO STRESS CORROSION     │          │ - Cu+ oxidized to Cu2+ oxidizer  │
  │   CRACKING                       │          │ - RAPID INTERGRANULAR /          │
  │ - Standard industry metallurgy   │          │   TRANSGRANULAR HF SCC           │
  └──────────────────────────────────┘          └──────────────────────────────────┘

1. Susceptible Metallurgy

Nickel-Copper Alloy 400 (UNS N04400, Monel 400) and its precipitation-hardenable counterpart Alloy K-500 (UNS N05500) are classic construction materials in HF alkylation units, selected for critical components including acid regenerator overhead condensers, acid relief valves, pump impellers, and flange gaskets.

2. The Critical Role of Oxygen and Oxidizers

In pure, deaerated hydrofluoric acid, Alloy 400 is virtually immune to stress corrosion cracking, exhibiting an exceptionally low general corrosion rate due to the formation of a tenacious cuprous/nickel fluoride protective film.

However, Alloy 400 becomes highly susceptible to severe stress corrosion cracking when the HF acid is contaminated with dissolved oxygen ($O_2$) or other oxidizing chemical species (such as ferric ions $Fe^{3+}$, cupric ions $Cu^{2+}$, or aeration from nitrogen purge contamination):

• Oxygen acts as a strong cathodic depolarizer, raising the electrochemical corrosion potential of the alloy into the transpassive cracking regime.
• Dissolved oxygen converts cuprous ions ($Cu^+$) in the scale into aggressive cupric ions ($Cu^{2+}$), which accelerate localized anodic dissolution at grain boundaries.

3. Vulnerable Components & Alloy K-500 Restrictions

• Vulnerable Locations: HF acid regenerator overhead condensers, acid rerun column top sections, vapor relief lines, and components exposed to atmospheric air ingress during shutdowns or improper nitrogen purges.
• Alloy K-500 Vulnerability: Alloy K-500 is precipitation-hardened to achieve high tensile and yield strengths (often 35 to 40+ HRC). Because of its high internal hardness and precipitation morphology, Alloy K-500 is exceptionally prone to both HF SCC and hydrogen embrittlement. Many owners therefore avoid Alloy K-500 in wetted HF alkylation service, using Alloy 400 or nickel-chromium-molybdenum alloys instead.

4. Prevention and Mitigation of HF SCC in Nickel Alloys

1. Thermal Stress Relief: Cold-worked or welded Alloy 400 components must undergo thermal stress-relief annealing at 1000 °F to 1100 °F (538 °C to 593 °C) for 1 hour followed by slow cooling. This relieves residual fabrication stresses below the threshold required for cracking.
2. Oxygen Exclusion: Strict exclusion of air, oxygen, and oxidizing agents during operations, acid unloading, and turnaround purging.
3. Metallurgical Upgrading: For severe services where oxygen contamination is unavoidable (such as acid regenerator overhead condensers), equipment is upgraded to Alloy C-276 (UNS N10276), which is highly resistant to HF SCC in both aerated and deaerated environments.

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Ethanol Stress Corrosion Cracking — API RP 571 Section 3.28

1. Emergence in Renewable Fuel Distribution

Ethanol Stress Corrosion Cracking (Ethanol SCC) is an environmental cracking mechanism that affects carbon and low-alloy steels handling Fuel-Grade Ethanol (FGE) conforming to ASTM D4806. First identified in the early 2000s, this mechanism caused numerous unexpected ruptures and leaks in ethanol storage tanks, blending headers, railcars, tanker trucks, and transport pipelines.

                      KEY PROCESS DRIVERS OF ETHANOL STRESS CORROSION CRACKING

       Dissolved Oxygen (O2)                         Water Content (H2O)
  - Mandatory driver (>1 to 2 ppmw)             - Aggressive window: 0.1 to 4.0 vol%
  - Operates as cathodic depolarizer            - Water > 5.0 vol% passivates steel
  - Deaerated ethanol does not crack              and inhibits SCC
                     │                                       │
                     └───────────────────┬───────────────────┘
                                         ▼
                  Aqueous Fuel-Grade Ethanol Electrolyte (ASTM D4806)
                                         ▲
                     ┌───────────────────┴───────────────────┐
                     │                                       │
       Trace Chlorides (Cl-)                         Acidic pHe & Organic Acids
  - Synergistic film depassivator               - Acidity measured as apparent pH (pHe)
  - Concentrations of 1 to 5 ppm                - Acetic acid / pHe < 6.0 drastically
    accelerate crack velocity                     accelerates localized attack

2. Environmental Factors Driving Ethanol SCC

Ethanol SCC is an electrochemical cracking mode driven by a specific combination of chemical constituents within fuel ethanol:

1. Dissolved Oxygen ($O_2$):
   • The single most essential chemical driver. Dissolved oxygen acts as the primary cathodic reactant supporting anodic dissolution at the crack tip.
   • Cracking requires dissolved oxygen concentrations exceeding 1 to 2 ppmw. In completely deaerated fuel-grade ethanol, cracking does not occur.
2. Water Concentration:
   • Ethanol SCC exhibits a distinct water susceptibility envelope. Cracking occurs predominantly in dry to intermediate water contents (0.1 to 4.0 vol% water).
   • Counterintuitively, higher water concentrations ($>5.0\text{ vol}\%$) inhibit cracking by promoting the formation of a continuous, protective iron oxide passive film across the entire metal surface.
3. Trace Chlorides:
   • Chloride ions ($Cl^-$) in concentrations as low as 1 to 5 ppmw act synergistically with dissolved oxygen to disrupt the passive film, concentrating stress-assisted dissolution.
4. Apparent Acidity (pHe):
   • The apparent pH of ethanol solutions, designated as pHe (measured per ASTM D6423), governs cracking severity. Acidic fuel ethanol with $\text{pHe} < 6.0$ (resulting from organic acid breakdown or acetic acid carryover) significantly accelerates cracking rates.
5. Mechanical Residual Stress:
   • Residual tensile stresses from welding or cold forming are the primary mechanical driver. Cracking is concentrated in non-PWHT weld heat-affected zones.

3. Morphology and Mitigation of Ethanol SCC

• Morphology: Intergranular cracking initiating on internal wetted surfaces in the weld HAZ and adjacent base metal, propagating parallel to the weld seam. Cracks appear as parallel arrays of tight, branched fissures.
• Prevention & Mitigation:
  1. Post-Weld Heat Treatment (PWHT): Reduces residual welding stresses below the threshold driving force.
  2. Corrosion Inhibitors: Dosing fuel ethanol with qualified commercial filming amine corrosion inhibitors formulated for renewable fuels.
  3. Internal Protective Linings: Coating tank interiors and railcars with ethanol-resistant novolac epoxy or phenolic linings.
  4. Oxygen Minimization: Implementing vapor recovery systems and nitrogen blanketing during bulk storage and offloading to prevent aeration.

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Comparison of Section 12.3 Environmental Cracking Mechanisms