6.3 Hydrofluoric (HF) Acid Corrosion

6.3 Hydrofluoric (HF) Acid Corrosion

Hydrofluoric (HF) Acid Corrosion, categorized under API RP 571 Section 3.38, is the primary materials degradation mechanism in HF alkylation units. HF alkylation combines light olefins (primarily propylene and butylene) with isobutane in the presence of an anhydrous hydrofluoric acid catalyst to synthesize high-octane, low-vapor-pressure motor alkylate blendstock.

While anhydrous HF can be safely contained in carbon steel under tightly controlled operational parameters, small excursions in acid water content, temperature, fluid velocity, or steel trace metallurgy produce devastating metal loss rates exceeding 100 to 500 mils per year (mpy) (2.5 to 12.7 mm/yr). Furthermore, because hydrofluoric acid is an acutely toxic, volatile inhalation hazard that causes deep tissue and bone destruction via systemic calcium depletion, preventing loss of primary containment (LOPC) is of paramount importance to mechanical integrity teams.

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Electrochemical Mechanism: The Iron Fluoride ($\text{FeF}_2$) Passivation Film

Unlike aqueous acids that rapidly dissolve iron, pure anhydrous hydrofluoric acid reacts with carbon steel to generate an insoluble, protective salt barrier layer composed of ferrous fluoride / iron fluoride ($\text{FeF}_2$):

$\text{Fe} + 2\text{HF} \longrightarrow \text{FeF}_2\text{ (insoluble solid)} + \text{H}_2\text{ (gas)}$

                                  PROTECTIVE FLUORIDE SCALE
                    Anhydrous HF Acid Phase (Water < 1% - 2%)
               ----------------------------------------------------
                    FeF2 Protective Passivating Scale (~1-10 um)
               ====================================================
                    Carbon Steel Base Metal (Controlled Residual Elements)

The Nature of the $\text{FeF}_2$ Barrier

In dry, anhydrous HF (< 1% water), the solubility of iron fluoride is virtually zero. The precipitated $\text{FeF}_2$ forms a thin, tightly adherent, microcrystalline scale that physically passivates the underlying steel, suppressing the baseline corrosion rate to an acceptable < 2 to 5 mpy (0.05 to 0.13 mm/yr).

However, this protective passivity is fragile. If the operational equilibrium is disrupted—by water contamination, excessive velocity, or elevated temperature—the $\text{FeF}_2$ scale dissolves or strips away, exposing bare metal to violent chemical attack.

Critical Operating Variables Governing HF Acid Corrosion

Corrosion kinetics in HF alkylation service are dictated by three primary process parameters:

1. Acid Water Content: The Primary Kinetic Driver

API RP 571 states simply that corrosion increases as HF concentration decreases (water content increases) and as temperature increases. The bands below reflect common HF alkylation operating practice rather than RP 571 limits.

Acid water content is universally recognized as the single most critical process variable in an HF alkylation plant:

• Target Operating Envelope (Anhydrous Acid): Commercial HF catalyst is typically maintained at < 1.0 wt% to 1.5 wt% water (with 85% to 95% HF purity, remainder acid-soluble oil / ASO). In this regime, the $\text{FeF}_2$ scale remains stable and insoluble.
• The Critical Dissolution Threshold (2% to 3% Water): As acid water content rises above 2.0 wt%, the solubility of $\text{FeF}_2$ in the acid phase increases dramatically. The protective scale thins, and active dissolution of steel accelerates.
• Catastrophic Wet HF Regime (> 3% to 5% Water): When water content exceeds 3.0 wt%, the $\text{FeF}_2$ scale completely dissolves. The environment behaves as aggressive aqueous hydrofluoric acid. Corrosion rates escalate to > 100 to 300 mpy, resulting in rapid equipment perforation.

  Corrosion Rate on Carbon Steel (mpy)
      ^
  300 |                                         Catastrophic Wet HF
      |                                         Acid Dissolution
  200 |                                              / 
      |                                             /
  100 |                                            /  
      |                        Critical           /   
   50 |                       Threshold          /    
   10 |   Safe Anhydrous Zone     |             /     
    0 +------------+--------------+------------+---------->
      0%          1.0%           2.0%         3.0%    4.0%
                        Acid Water Content (wt% H2O)

2. Operating Temperature Limits

Corrosion rates in HF acid increase exponentially with operating temperature:

• Continuous Operating Ceiling: Carbon steel should be maintained below 150 °F (66 °C) in all wetted HF services.
• Preferred Operating Zone: The highest equipment reliability is achieved when temperatures are maintained between 70 °F and 100 °F (21 °C and 38 °C) (such as in reactor acid chillers and settler drums).
• High-Temperature Excursions (> 150 °F / 66 °C): In fractionation towers (deisobutanizers, acid rerun columns, and reboilers) where process temperatures reach 200 °F to 300 °F (93 °C to 149 °C), carbon steel corrodes at unacceptable rates regardless of water content. Upgraded metallurgy (Alloy 400) is strictly mandatory.

3. Turbulence and Phase Change

• Turbulence: high velocity and turbulence downstream of control valves, pumps, and orifices can disrupt the fluoride scale and cause localized scalloping; owners set velocity limits in their HF design standards.
• Phase change of rich HF: RP 571 highlights that HF-rich streams undergoing vaporization and condensation (for example, in exchangers that heat or condense HF-rich streams) cause significant localized corrosion of carbon steel.
• Contaminants: oxygen and sulfur compounds in the feed increase corrosion, which is why careful operation to minimize water, oxygen, sulfur, and other contaminants is a primary RP 571 mitigation.

Residual Elements in Carbon Steel: A Core HF Concept

Carbon steel pipe and fittings that all meet the same ASTM specification can behave very differently in HF service. Operating experience summarized in API RP 751 (Safe Operation of Hydrofluoric Acid Alkylation Units), and referenced in API RP 571, shows that carbon steels with elevated residual elements (RE), namely copper, nickel, and chromium, can suffer accelerated and non-uniform (localized) corrosion in HF service. These residuals come mainly from scrap-based steelmaking and are not limited by ordinary pipe specifications.

• Why it matters: a single high-RE elbow or fitting can thin much faster than the surrounding piping, so a circuit's average corrosion rate can hide a component that is close to failure.
• Real-world lesson: the U.S. Chemical Safety Board's investigation of the 2019 Philadelphia Energy Solutions HF alkylation fire found that the ruptured elbow had thinned severely and that its high residual-element content had not been identified by the inspection program.
• What RP 751 and RP 571 expect: purchase specifications for HF-service carbon steel that limit residual elements, positive material identification that actually measures copper, nickel, and chromium, and special-emphasis inspection programs that track residual-element components along with small-bore piping, deadlegs, and flange faces.

Materials Selection: Modified Carbon Steel vs. Alloy 400 vs. Stainless Steels

Material selection in HF alkylation units follows rigorous engineering standards governed by process temperature, acid water content, and fluid dynamics.

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Damage Morphology and Secondary Degradation Modes

Hydrofluoric acid corrosion presents several distinct morphological signatures, along with severe secondary hydrogen and mechanical failure modes:

1. General Thinning vs. Localized Scalloping

• In low-velocity vessels and piping, metal loss is smooth and uniform, coated with a dull brown to dark reddish-gray $\text{FeF}_2$ scale.
• In high-velocity or boiling services, damage manifests as deep, localized, saucer-shaped scallops and sharp-edged grooves where the scale has been stripped.

2. "Scale Jacking" in Crevices and Flanges

Because iron fluoride corrosion products possess a significantly larger specific molar volume than the underlying iron from which they form, the scale exerts tremendous mechanical expansion force as it accumulates inside tight clearances:

• Flange Deflection: $\text{FeF}_2$ scale packs between flange faces, bowing flange bolts, separating gasket surfaces, and causing catastrophic flange leaks.
• Valve Binding: Scale accumulation inside plug valve bodies and gate valve guide tracks seizes moving internals, rendering emergency isolation valves inoperable.

3. Hydrogen Charging and Environmental Cracking

The cathodic reaction of HF corrosion generates prolific quantities of nascent atomic hydrogen ($H^0$):

$\text{Fe} + 2\text{HF} \longrightarrow \text{FeF}_2 + 2\text{H}^0$

Because fluoride ions act as a surface recombination poison (hindering the recombination of atomic $H^0$ into gaseous $H_2$), atomic hydrogen diffuses freely into the steel wall. This drives severe secondary hydrogen degradation:

• Hydrogen Blistering in vessel shell plates
• Hydrogen-Induced Cracking (HIC) and Stress-Oriented HIC (SOHIC) in dirty, non-inclusion-controlled steels
• Hydrogen Stress Cracking (HSC) in hard welds and heat-affected zones (HAZs), which is why HF-service carbon steel welds are hardness-controlled (commonly 200 HBW maximum) and often post-weld heat treated.

Affected Equipment Across the HF Alkylation Unit

Every major processing section within an HF alkylation plant presents distinct corrosion risks:

                         HF ALKYLATION PROCESS FLOW SCHEMATIC
                                    +-----------------------+
                                    |   Main Fractionator   |
                                    |   (Deisobutanizer)    |
                                    | Top: Isobutane Recycle|
                                    +-----------------------+
                                                ^
                                                |
[ Acid Settler ]                                |
+----------------------+            +-----------------------+
| Acid / HC Separation | ---------> | Acid Rerun Column     |
| Interface Level      |            | Separates ASO & Acid  | ===> High-T Bottoms (> 250 °F)
+----------------------+            | Hot Reboiler (300 °F) |      Requires Solid Alloy 400
           |                        +-----------------------+
           v (Acid Return)
+----------------------+
| Reactor Chiller /    | <=== High Velocity Tube Bundles
| Contactor Loop       |      Requires Low-Si Modified CS or Alloy 400
+----------------------+

1. Reactor / Contactor and Acid Cooler Loop

• High acid concentration (85% to 92% HF), low temperature (70 °F to 100 °F / 21 °C to 38 °C).
• Chiller tube bundles subject to high fluid velocity and turbulent mixing; requires low-Si modified carbon steel or solid Alloy 400 tubes.

2. Acid Settler Drum

• Wetted carbon steel vessel shell subject to broad uniform corrosion and potential hydrogen blistering at the hydrocarbon/acid phase interface.
• Level gauge bridles and interface instrumentation prone to plugging by $\text{FeF}_2$ scale.

3. Acid Rerun Column and Deisobutanizer (DIB) Bottoms

• Regenerates active HF by distilling off heavy Acid-Soluble Oil (ASO) polymer byproduct.
• Operating temperatures exceed 250 °F to 300 °F (121 °C to 149 °C) with concentrated acid and water carryover.
• Carbon steel corrodes uncontrollably; solid Alloy 400 or Alloy 400 cladding is mandatory for column bottoms, reboiler bundles, and transfer piping.

4. Potassium Hydroxide (KOH) Relief and Neutralization Scrubbers

• Neutralizes acid relief gas and effluent streams prior to flaring.
• Why KOH is used instead of NaOH: Potassium hydroxide neutralizes HF to form potassium fluoride (KF), which is highly soluble in water ($> 90\text{ g}/100\text{ mL}$ at 20 °C). Conversely, sodium hydroxide (NaOH) produces sodium fluoride (NaF), which is virtually insoluble ($< 4\text{ g}/100\text{ mL}$) and precipitates out as an unyielding, rock-hard solid cake that rapidly chokes scrubber packing, spray nozzles, and drain lines.

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Inspection, Non-Destructive Examination (NDE), and Monitoring Strategies

Integrity management in HF alkylation service requires specialized inspection programs designed to detect thinning, scale jacking, and residual metallurgical non-conformances.

1. Positive Material Identification (PMI) for Residual Elements

• Measure what matters: handheld XRF confirms alloy grade but is not reliable for low residual levels in carbon steel, so facilities use methods that can quantify copper, nickel, and chromium at residual levels (such as optical emission spectrometry or laboratory analysis) on HF-service carbon steel components.
• Track and prioritize: components with elevated residual elements are flagged for closer thickness monitoring or replacement, as described in API RP 751 (API RP 578 also discusses residual elements in HF alkylation units).

2. Ultrasonic Thickness (UT) Grid Scanning

• Automated UT Scans: Executed systematically across carbon steel piping spools, vessel nozzles, and settler bottom shells to monitor uniform and localized metal loss.
• High-density thickness grids on elbows, reducers, and piping downstream of acid injection points.

3. Profile Radiography (PRT)

• Internal Geometry Sizing: Indispensable for evaluating valve bodies, thermowells, small-bore piping (< 2 in. NPS), and flange gaps.
• PRT clearly identifies localized scalloping beneath the $\text{FeF}_2$ scale, detects scale accumulation inside valve throats, and measures scale jacking displacement between flange faces.

4. Process and Analytical Water Monitoring

• Continuous online water-in-acid analyzers and routine laboratory Karl Fischer titration sampling of circulating acid to ensure water content remains safely below the critical 1.5% to 2.0% threshold.