4.2 Ammonium Chloride and Amine Hydrochloride Corrosion

Ammonium Chloride and Amine Hydrochloride Corrosion — API RP 571 Section 3.6

1. Mechanism and Crystallization Thermodynamics

Ammonium Chloride ($NH_4Cl$) and Amine Hydrochloride Corrosion is an extremely aggressive form of localized underdeposit corrosion that occurs across multiple refinery units, including crude distillation overhead systems, hydroprocessing reactor effluent trains, catalytic reformer stabilizer overheads, and FCC fractionator overheads.

The damage mechanism originates from the gas-phase reaction between volatile bases—ammonia ($NH_3$) or organic neutralizing/scavenger amines ($R\text{-}NH_2$)—and acidic hydrogen chloride ($HCl$) vapor to form solid chloride salts: $NH_3(g) + HCl(g) \rightleftharpoons NH_4Cl(s)$ $R\text{-}NH_2(g) + HCl(g) \rightleftharpoons R\text{-}NH_3Cl(s)$

This reversible reaction is governed by thermodynamic equilibrium. The deposition temperature—frequently designated as the salt crystallization temperature ($T_{\text{salt}}$)—is a direct function of the partial pressure product of the basic and acidic gas constituents: $K_p = p_{NH_3} \times p_{HCl} \quad \text{or} \quad K_p = p_{\text{Amine}} \times p_{HCl}$

Hot Hydrocarbon & Gas Stream (Above Tsalt: NH3, Amine, HCl exist as dry vapors)
       │
       ▼  Cooling through Heat Exchangers / Tower Trays
       │  - Temperature drops below Tsalt (typically 180 °F - 350 °F / 82 °C - 177 °C)
       │  - NOTE: Tsalt is tens or hundreds of degrees ABOVE the water dew point!
       ▼
Solid Salt Crystals Precipitate Directly on Metal Walls (NH4Cl, Amine-HCl)
       │
       ▼  Hygroscopic Deliquescence (Absorption of trace moisture from vapor)
       ▼
Concentrated, Stagnant Acidic Chloride Brine Layer (pH 2.0 - 4.0)
       │
   ┌───┴───────────────────────────────────────────┐
   ▼                                               ▼
Carbon Steel: Deep localized gouging,           300-Series SS: Severe pitting &
scalloping & thinning (>100-500 mpy)           Chloride Stress Corrosion Cracking (Cl- SCC)

Crucially, salt deposition occurs at temperatures significantly above the water dew point. In many process streams, the water dew point may be between 100 °F and 160 °F (38 °C to 71 °C), whereas the salt crystallization temperature ($T_{\text{salt}}$) can exceed 300 °F (149 °C) depending on the NH3 (or amine) and HCl partial pressures. Consequently, dry solid crystals precipitate directly onto metal surfaces in what operators perceive to be a completely "dry" vapor stream.

2. Hygroscopic Deliquescence and Corrosion Chemistry

While completely dry solid $NH_4Cl$ crystals are relatively non-corrosive to steel, ammonium chloride and amine hydrochloride salts are profoundly hygroscopic. They possess a low deliquescence relative humidity (DRH):

• The deposited salt crystals exert a vapor pressure significantly lower than pure water, enabling them to extract and absorb trace moisture from hydrocarbon gases even when the relative humidity of the stream is as low as 15% to 20%.
• As the salt absorbs water, it dissolves into its own water of hydration, transforming into a stagnant, concentrated, saturated aqueous chloride brine trapped directly beneath the solid deposit.

Within this stagnant underdeposit brine layer, the ammonium ion undergoes partial hydrolysis, releasing hydronium ions: $NH_4^+ + Cl^- + H_2O \rightleftharpoons NH_4OH + H^+ + Cl^-$

Because ammonia ($NH_3$) is volatile and readily flashes into the passing gas stream, the underdeposit solution becomes severely depleted of base, causing the interfacial pH to plunge into the intensely acidic regime of pH 2.0 to 4.0.

This combination of hyper-concentrated chloride ions ($Cl^-$) and low pH generates astronomical corrosion rates: $Fe + 2HCl \rightarrow FeCl_2 + H_2$

In carbon steel and low-alloy steels, localized penetration rates beneath wet salt mounds routinely exceed 100 to 500+ mpy (2.5 to 12.7+ mm/yr), leading to through-wall perforation in a matter of weeks.

3. The Tramp Amine Threat

In modern refining, the introduction of tramp amines has emerged as a premier cause of unexpected overhead failures. Tramp amines enter crude unit feedstocks through multiple unintended routes:

1. Unreacted or excess neutralizing amines (e.g., monoethanolamine [MEA], diethanolamine [DEA], methoxypropylamine [MOPA], morpholine) recycled from upstream gas treating units or slop oil systems.
2. Triazine-based $H_2S$ scavenger byproducts used in oilfield production and marine transit that break down in refinery preheat trains into volatile alkylamines.

Comparison of Salt Crystallization Temperatures:

Lower Temperature (Overhead Piping / Condensers)
  ▲  Ammonium Chloride (NH4Cl): Tsalt ~ 180 °F - 230 °F (82 °C - 110 °C)
  │  Morpholine Hydrochloride: Tsalt ~ 230 °F - 270 °F (110 °C - 132 °C)
  │  MOPA Hydrochloride: Tsalt ~ 250 °F - 300 °F (121 °C - 149 °C)
  ▼  Ethanolamine Hydrochloride (MEA-HCl): Tsalt ~ 300 °F - 360 °F (149 °C - 182 °C)
Higher Temperature (Fractionation Trays / Pumparound Loops)

Unlike ammonia ($NH_3$), which forms $NH_4Cl$ that deposits in cooler overhead piping and condensers, organic amine hydrochlorides possess significantly higher molecular weights and stability, resulting in much higher crystallization temperatures ($T_{\text{salt}}$ can exceed 300 °F to 360 °F / 149 °C to 182 °C). As a consequence:

• Amine salts precipitate deep inside the distillation column itself—on the top 5 to 15 fractionation trays, bubble caps, downcomers, and pumparound circuits.
• Because wash water cannot be injected into the internal fractionation zone of a distillation column without disrupting distillation hydraulics, these internal deposits accumulate unabated, destroying stainless steel internal hardware via rapid pitting and cracking.

4. Susceptible Metallurgy and Damage Morphology

API RP 571 states that all commonly used materials are susceptible and lists them in order of increasing resistance: carbon steel, low alloy steels, 300 series SS, Alloys 400, duplex SS, 800, and 825, Alloys 625 and C-276, and titanium. The table below follows that ranking.

5. Prevention, Mitigation and Water Wash Engineering

The 25% Free Water Rule for Continuous Wash Water Injection

The primary defense against ammonium chloride deposition in overhead piping and heat exchanger trains is continuous injection of oxygen-free wash water upstream of the calculated salt crystallization point ($T_{\text{salt}}$).

Continuous Wash Water Injection Point
  │ (Upstream of Tsalt: 100% of injected water enters hot stream)
  ▼
Process Pipe / Exchanger Train
  │ Water partially vaporizes (flashes) due to sensible heat of process gas
  ▼
Downstream Accumulator / Separator Drum
  ┌────────────────────────────────────────────────────────────────────────┐
  │ CRITICAL DESIGN REQUIREMENT:                                           │
  │ Minimum 25% of the total injected wash water MUST remain as free       │
  │ liquid water at the downstream separator!                              │
  └────────────────────────────────────────────────────────────────────────┘
  │
  ├─► If >25% Free Water: Salts dissolve completely into dilute, safe brine.
  │
  └─► If <25% Free Water (or complete dry-out): Water flashes completely;
      chlorides re-concentrate and re-crystallize immediately downstream,
      triggering catastrophic localized gouging (Wash Water Dry-Out Zone)!

• Thermodynamic Flash Calculation: Because the process vapor stream is hot, a substantial portion of the injected liquid water immediately flashes into steam. To prevent complete vaporization, the wash water injection rate must be calculated using rigorous process simulation to guarantee that at least 25% of the injected water volume remains as free liquid water in the vapor-liquid separator drum.
• Wash Water Quality: Injected water must be polished steam condensate or stripped sour water with <10 to 20 ppb dissolved oxygen and <10 to 20 ppm chlorides. Oxygen contamination in hot wash water initiates rapid pitting of carbon steel and accelerates chloride cracking in stainless steel.
• Quench vs. Slip-Stream Injection: Continuous slip-stream injection via specially designed hydraulic atomizing spray nozzles (co-current with flow, centered in the pipe) ensures full wetting of the pipe circumference without impingement erosion.

Upstream Desalter Optimization

Minimizing the entry of chloride into the fractionation system represents the most cost-effective mitigation:

• Optimize electrostatic desalters to achieve >95% to 99% salt removal efficiency, targeting an effluent crude salt content below 1 to 2 ptb (pounds per thousand barrels).
• Inject dilute sodium hydroxide (caustic soda, $NaOH$) into the desalted crude line upstream of the crude furnace (typically 1 to 3 lb NaOH per 1,000 bbl) to chemically convert hydrolyzable magnesium chloride ($MgCl_2$) and calcium chloride ($CaCl_2$) into thermally stable, non-hydrolyzable sodium chloride ($NaCl$), preventing the generation of overhead $HCl$ gas: $MgCl_2 + 2NaOH \rightarrow Mg(OH)_2\downarrow + 2NaCl$

Eliminating Tramp Amines

• Ban the use of unapproved oilfield H2S scavengers and crude tanker biocides.
• Segregate and treat slop oil streams containing residual neutralizing amines before re-introducing them into the crude slate.

6. Inspection, Non-Destructive Examination & Monitoring Methodologies

• Profile Radiography (PRT): The premier NDE technique for identifying internal salt crusts, underdeposit pitting, and localized wall thinning in overhead piping elbows, tees, and bypass lines without shutting down or removing insulation.
• Ultrasonic Thickness Grid Scanning (UT / PAUT): Automated and manual high-density grid thickness mapping on overhead piping, especially immediately downstream of wash water injection points and around cooler shell nozzles.
• Process Modeling and $T_{\text{salt}}$ Calculation: Real-time computerized tracking comparing actual operating stream temperatures to calculated $T_{\text{salt}}$ based on laboratory-analyzed $NH_3$, $HCl$, and amine concentrations. If stream temperature approaches within 20 °F to 30 °F (11 °C to 17 °C) of $T_{\text{salt}}$, salt deposition is imminent.
• Infrared (IR) Thermography: Scanning overhead piping runs to identify localized cold spots on pipe exteriors. A cold spot often indicates an internal accumulation of wet salt or a localized zone where wash water is flashing off.
• Boot Water Analytical Tracking: Routine laboratory analysis of accumulator boot water for chloride ion concentration (target <20 to 50 ppm with proper wash water), pH (target 6.0 to 7.0 with neutralizers), and dissolved iron counts (surges in iron indicate active underdeposit attack).