6.1 Naphthenic Acid Corrosion (NAC)

6.1 Naphthenic Acid Corrosion (NAC)

Naphthenic Acid Corrosion (NAC) is a form of high-temperature, localized or velocity-accelerated organic acid attack that primarily afflicts crude distillation units (CDUs) and vacuum distillation units (VDUs) processing heavy, acidic crude oils. Documented under API RP 571 Section 3.46, NAC represents one of the most complex damage mechanisms in petroleum refining due to its intricate interplay with high-temperature sulfidation, fluid hydrodynamics, phase-change vaporization, and crude oil distillation chemistry.

Unlike low-temperature aqueous organic acid corrosion (which requires free liquid water to ionize), naphthenic acid corrosion occurs in hot, non-aqueous liquid hydrocarbon streams. It is characterized by high localized metal loss rates that can exceed 100 to 500 mils per year (mpy) (2.5 to 12.7 mm/yr) in severe, turbulent environments, leading to rapid breach of primary containment if not systematically monitored and mitigated.

---

Chemical Nature and Molecular Architecture of Naphthenic Acids

Naphthenic acids are naturally occurring cycloaliphatic monocarboxylic acids found in unrefined petroleum crudes, formed over geological time through the in-reservoir microbial degradation of petroleum hydrocarbons. They possess the general stoichiometric formula:

$\text{R}-(\text{CH}_2)_n-\text{COOH}$

Where R represents one or more cyclopentyl or cyclohexyl saturated ring structures, and n typically ranges from 1 to 12. The carboxylic acid functional group (-COOH) is attached either directly to the alicyclic ring or through an aliphatic paraffinic side chain.

          Cyclopentyl Core                     Cyclohexyl Core
              CH2 - CH2                            CH2 - CH2
             /         \                          /         \
       CH2  -  CH - (CH2)n - COOH           CH2  -  CH - (CH2)n - COOH
             \         /                          \         /
              CH2 - CH2                        CH2 - CH2 - CH2

Molecular Size, Boiling Range, and Corrosivity

The molecular weight of naphthenic acids ranges predominantly between 200 and 600 g/mol, placing their boiling points squarely between 400 °F and 850 °F (204 °C and 454 °C). Because they boil within the exact range of intermediate and heavy gas oils, they do not remain evenly dispersed across the crude barrel during distillation. Instead, they concentrate heavily in specific side cuts—most notably:

• Heavy Atmospheric Gas Oil (HAGO)
• Light Vacuum Gas Oil (LVGO)
• Heavy Vacuum Gas Oil (HVGO)
• Atmospheric Tower Bottoms (Reduced Crude)
• Vacuum Tower Flash Zones and Wash Oil Beds

The corrosivity of an individual naphthenic acid molecule is governed by its steric hindrance, ring structure, and molecular weight. Low-molecular-weight acids (carbon number $C_{10}$ to $C_{18}$) are significantly more mobile, chemically reactive, and corrosive than very large, highly hindered polycyclic acids ($C_{30}+$).

Critical Operating Variables and Degradation Kinetics

Naphthenic acid corrosion is governed by a tightly bounded combination of process variables, environmental thresholds, and hydrodynamic forces.

1. The Critical Temperature Envelope: 425 °F to 750 °F (218 °C to 400 °C)

NAC occurs almost exclusively within a well-defined thermal window:

2. Total Acid Number (TAN) as a Screening Tool and Its Pitfalls

Refiners evaluate the potential acidity of crude oils using the Total Acid Number (TAN), defined as the quantity of potassium hydroxide (KOH) in milligrams required to neutralize the organic acids present in one gram of oil (mg KOH/g), measured via titration according to ASTM D664 (potentiometric) or ASTM D974 (colorimetric).

• Crude Oil Screening Benchmark: Whole crude with a TAN > 0.5 mg KOH/g is historically classified as an "opportunity crude" or "acidic crude" requiring corrosion evaluation.
• Distillation Side-Cut Benchmark: Process side streams (such as HVGO or vacuum tower wash oil) with a localized cut TAN > 1.5 to 2.0 mg KOH/g are considered aggressive to carbon and low-alloy steels.

Why Whole-Crude TAN is Insufficient

Relying solely on whole-crude TAN can result in catastrophic underestimation of corrosion risk for three critical reasons:

1. Side-Cut Concentration Factor: A whole crude with a seemingly benign TAN of 0.3 mg KOH/g can yield a vacuum gas oil or wash oil stream with a localized cut TAN exceeding 3.0 to 5.0 mg KOH/g due to atmospheric and vacuum fractionation.
2. Boiling Point Distribution: Crude TAN represents the sum of all acidic species. If the acidity consists of light acids boiling below 425 °F or heavy asphaltenic acids boiling above 800 °F, the stream will produce minimal NAC. If the acids boil precisely between 550 °F and 700 °F, extreme localized attack will occur.
3. Steric and Structural Variance: Two crude oils with identical TAN values of 1.5 mg KOH/g can exhibit corrosion rates differing by an order of magnitude depending on whether the acids are low-molecular-weight single-ring compounds or bulky polycyclic structures.

Competitive Thermodynamic Balance: NAC vs. Sulfidation

In crude and vacuum distillation units, naphthenic acid corrosion never occurs in isolation. It operates in constant, dynamic competition with high-temperature sulfidation (driven by reactive sulfur species, hydrogen sulfide $\text{H}_2\text{S}$, and mercaptans).

The Fundamental Chemical Reactions

Three interconnected chemical reactions dictate whether protective scale accumulates or bare metal actively dissolves:

1. Sulfidation (Protective Film Formation): $\text{Fe} + \text{H}_2\text{S} \longrightarrow \text{FeS (solid scale)} + \text{H}_2$ Reactive sulfur reacts directly with the iron lattice to generate an iron sulfide (FeS) scale. Under quiescent conditions, this semi-adherent FeS scale acts as a physical diffusion barrier, decreasing the sulfidation rate over time.

2. Direct Naphthenic Acid Attack: $\text{Fe} + 2\text{RCOOH} \longrightarrow \text{Fe}(\text{RCOO})_2\text{ (oil-soluble)} + \text{H}_2$ Naphthenic acids react with metallic iron to yield iron naphthenate, an organometallic compound that is completely soluble in hot oil. Because the corrosion product dissolves into the flowing hydrocarbon, no protective barrier forms on the metal.

3. Scale Dissolution (The Destructive Synergism): $\text{FeS} + 2\text{RCOOH} \longrightarrow \text{Fe}(\text{RCOO})_2\text{ (oil-soluble)} + \text{H}_2\text{S}$ Naphthenic acids actively attack and dissolve the protective FeS scale previously deposited by sulfidation, regenerating gaseous $\text{H}_2\text{S}$ and leaving the underlying steel completely bare and exposed to renewed attack.

                               +-------------------------+
                               |     Metallic Iron       |
                               +-------------------------+
                                 /                     \ 
      + H2S (Sulfidation)       /                       \  + 2 RCOOH (Direct NAC)
      Forms Protective Scale   /                         \ Dissolves into Oil
                              v                           v
                 +-----------------------+      +---------------------------+
                 |  Solid FeS Scale      |      | Iron Naphthenate in Oil   |
                 | (Diffusion Barrier)   |      |   Fe(RCOO)2 + H2          |
                 +-----------------------+      +---------------------------+
                              |                               ^
                              |  + 2 RCOOH                    |
                              |  (Acid Dissolves Scale)       |
                              +-------------------------------+

The Inhibitive Role of Reactive Sulfur

Because Reaction 1 forms FeS scale and Reaction 3 dissolves it, the ratio of reactive sulfur to TAN determines the steady-state corrosion rate. If the crude oil contains high concentrations of reactive sulfur (such as active mercaptans or dissolved $\text{H}_2\text{S}$) relative to naphthenic acids, FeS scale forms faster than the acids can dissolve it. The FeS layer shields the base metal, suppressing NAC.

Conversely, when processing low-sulfur, high-TAN crudes (e.g., certain West African, North Sea, or South American crudes), there is insufficient reactive sulfur to maintain the FeS barrier. Naphthenic acid dissolution dominates, producing catastrophic metal loss.

Fluid Hydrodynamics, Shear Stress, and Phase Change Acceleration

Naphthenic acid corrosion is acutely sensitive to fluid velocity, mechanical shear stress, and two-phase flow phenomena. In stagnant or low-velocity laminar liquid flow (< 10 ft/s or < 3 m/s), corrosion rates are often moderate because a loose boundary layer forms.

Mechanisms of Velocity Acceleration

When fluid velocity and turbulence exceed critical thresholds, metal loss rates accelerate by one to two orders of magnitude due to three distinct physical processes:

1. Mass Transfer Enhancement: High velocity compresses the hydrodynamic boundary layer, maximizing the diffusion rate of fresh unreacted acid molecules to the pipe wall.
2. Mechanical Scale Stripping: Dynamic fluid shear stress physically shears away any fragile, partially formed iron sulfide or iron naphthenate films, exposing bare nascent metal.
3. Phase Transformation & Acid Droplet Concentration: In furnace tubes, vacuum transfer lines (VTL), and flash zones, liquid hydrocarbon partially flashes into high-velocity vapor. The lighter hydrocarbon fractions vaporize, leaving the high-boiling naphthenic acids concentrated in the remaining liquid phase. The expanding vapor accelerates to velocities between 100 and 300 ft/s (30 to 90 m/s), driving these concentrated acid micro-droplets directly against pipe walls, elbows, and internal vessel structures in annular-mist or slug flow regimes.

---

Affected Materials and The Failure of Low-Alloy Steels

A critical, heavily tested principle in API RP 571 is the complete failure of conventional low-alloy Cr-Mo steels to resist naphthenic acid corrosion.

Damage Morphology and Physical Appearance

The macroscopic and microscopic appearance of naphthenic acid corrosion is distinct from almost all other refining damage mechanisms, providing clear forensic signatures during internal turnaround inspections.

Key Morphological Characteristics

1. Clean, Bare, Shiny Metal Surfaces: Because the primary reaction product (iron naphthenate) is oil-soluble, and because naphthenic acids actively dissolve iron sulfide scales, affected metal surfaces are completely devoid of the heavy, dark gray or black FeS scale deposits typical of sulfidation. The steel presents a clean, bright, or bare "electropolished" appearance.
2. Flow-Induced Grooving and Riverbed Channels: Attack manifests as localized, directional grooving, sharp-edged channels, wave-like ripples, and smooth, scallop-shaped troughs oriented parallel to the fluid flow path. In severe cases, the surface resembles an eroded dry riverbed or glacial canyon.
3. Cusp-Shaped Depressions and Gouges: At points of flow impingement or high turbulence (e.g., downstream of weld roots, thermowells, or piping reducers), NAC forms deep, cusp-shaped craters with knife-sharp leading edges.
4. Absence of Pitting or Intergranular Cracking: Under microscopic examination, NAC is strictly a surface-thinning phenomenon. It does not produce sub-surface microfissuring, intergranular penetration, or stress-corrosion cracks.

---

Affected Equipment Across Refinery Process Units

Naphthenic acid corrosion is concentrated in high-temperature crude fractionation and vacuum distillation systems:

                                    [ Vacuum Distillation Column ]
                                    +----------------------------+
                                    | Top Temp: ~150 - 250 °F    |
                                    | LVGO Drawoff (Low NAC)     |
                                    |----------------------------|
                                    | HVGO Drawoff               | ===> HVGO Pump-Around Piping
                                    | Cut TAN Concentrates Here  |      (Severe NAC: 600 - 680 °F)
                                    |----------------------------|
[ Vacuum Furnace ]                  | Wash Oil Bed Packing       |
+----------------+                  | & Support Grids            |
| Tubes: 700 °F  |                  |----------------------------|
| 9Cr Fails      | ===> Transfer    | FLASH ZONE:                |
| 317L Required  |      Line        | Tangential Inlet Horn,     | ===> Slugs & Acid Droplets
+----------------+      (150-250    | Deflector Vanes & Shell    |      Impinge on Internals
                        ft/s)       |----------------------------|
                                    | Vacuum Bottoms: > 720 °F   | ===> Quench System
                                    +----------------------------+

1. Crude Distillation Units (CDU)

• Atmospheric Furnace Tubes & Transfer Lines: Peak metal temperatures between 650 °F and 720 °F (343 °C and 382 °C).
• Atmospheric Column Flash Zone & Lower Trays: Feed nozzles, stripping section internals, and column walls opposite the feed inlet.
• HAGO Pumparound Circuits: Heat exchanger tubes, pump casings, impellers, and control valve manifolds.

2. Vacuum Distillation Units (VDU)

• Vacuum Furnace Fired Tubes: High-heat-flux zones where oil film temperatures reach 680 °F to 750 °F (360 °C to 400 °C).
• Vacuum Transfer Lines (VTL): Large-diameter lines operating under vacuum at velocities of 150 to 300 ft/s (45 to 90 m/s) with extreme two-phase annular/mist turbulence; highly vulnerable at mitered elbows and reducers.
• Vacuum Tower Flash Zones: Tangential feed horns, vapor distribution deflector plates, internal shell cladding, and vacuum column stripping trays.
• Wash Oil Beds and HVGO Circuits: Structured packing grids, collector trays, pump-around reboilers, and suction piping.

---

Mitigation, Materials Selection, and Operational Controls

Managing naphthenic acid corrosion requires an integrated strategy combining crude logistics, chemical treatment, and metallurgical upgrading.

1. Crude Blending and Opportunity Crude Management

The primary operational control is crude blending. High-TAN crude shipments (TAN > 1.0 to 3.0 mg KOH/g) are systematically blended in storage tanks with high-sulfur, low-TAN paraffinic crudes to achieve two operational objectives:

• Dilute the overall CDU/VDU feed TAN to a safe operational baseline (typically maintaining crude TAN < 0.5 mg KOH/g and individual side-cut TAN < 1.5 mg KOH/g).
• Maintain an adequate ratio of reactive sulfur to naphthenic acid, ensuring that protective FeS scale formation outpaces acid dissolution kinetics.

2. Chemical Corrosion Inhibitors

High-temperature chemical inhibitors (primarily organic phosphate esters and polysulfide formulations) can be injected into furnace transfer lines and tower pumparounds:

• Mechanism: Phosphate esters react with hot steel surfaces to deposit an iron phosphate barrier film that resists both acid dissolution and sulfidation.
• Downstream Penalties: Phosphate-based inhibitors carry significant operational risks. Phosphorus decomposes in downstream hydrotreaters, catalytic reformers, and hydrocrackers, permanently poisoning noble-metal catalysts (platinum/palladium) and causing bed fouling.

3. Metallurgical Upgrading Guidelines

When process economics justify processing high-TAN crudes continuously, facilities upgrade equipment metallurgy according to strict metallurgical criteria:

• Specify Type 317L Stainless Steel: Standard Type 316L (2.0 to 2.5% Mo) provides inadequate protection in high-shear zones. Specifications mandate Type 317L with a minimum molybdenum content of 3.0 wt% to 3.5 wt% for furnace tubes, transfer piping, and tower cladding.
• Nickel-Base Alloys for Severe Impingement: For vacuum flash zone tangential horns, deflector plates, and transfer line elbows subject to velocities > 150 ft/s, solid or clad Alloy 625 (UNS N06625) or 6% Mo super-austenitic alloys are utilized.
• Avoid Relying on Cr-Mo Alloys: 1.25Cr, 5Cr, and 9Cr alloys provide little incremental protection over carbon steel against naphthenic acid.

---

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

Because naphthenic acid corrosion produces highly localized, flow-directed grooving rather than uniform metal loss, conventional spot UT thickness measurements often miss severe localized degradation.

Recommended Inspection Techniques

• Automated Ultrasonic Thickness Scanning (AUT): High-density ultrasonic grid mapping or phased array UT (PAUT) over 100% of high-risk pipe elbows, miter bends, reducers, and vessel shell areas opposite feed nozzles to capture narrow, sharp-edged riverbed grooves.
• Profile Radiography (PRT): Essential for piping bends, small-bore nozzles, thermowells, and piping downstream of orifice plates. PRT provides an unambiguous cross-sectional view of internal grooving and scallop formation without removing insulation.
• High-Temperature Non-Intrusive UT Sensors: Permanently mounted waveguide UT transducers installed at known impingement points on vacuum transfer lines and furnace crossover piping, providing real-time wall thickness telemetry during operation.
• Process and Analytical Monitoring: Routine laboratory distillation testing of CDU and VDU side streams via ASTM D664 to monitor cut TAN, combined with atomic absorption spectroscopy to track dissolved iron content (iron counting) in HVGO and atmospheric residue streams.