Ammonium Chloride Corrosion

General Information

NH₄Cl corrosion is virtually present at any refinery process unit where gaseous HCl and NH₃ are stream components. Table 1 shows most common areas affected by NH₄Cl corrosion.

Table 1 Potential locations for NH₄Cl corrosion in process units.^{1 2 3 4 5}

The primary effect of NH₄Cl is deposition, which leads to fouling or plugging. This issue significantly impacts process operations by increasing pressure drops across the exchangers and disrupting heat flux. The secondary problem arises as a consequence of the first; when the solid deposit (which is virtually noncorrosive - if dry) becomes wet. In this scenario, the area beneath the deposit becomes “enriched” with Cl^- and H⁺ from the dissociation of NH₄Cl and water. As a result, under-deposit HCl corrosion is initiated, leading to the rapid degradation not only of carbon steel but also a wide range of corrosion resistant alloys (CRAs).

Surprisingly, despite the significance of NH₄Cl corrosion (under-deposit), there is limited published data detailing the mechanism, parametric impacts, or predictive approaches. The primary challenge lies in effectively simulating deposit formation and controlling or measuring under-deposit corrosion under dynamic flow conditions. Some recent publications have attempted to address this issue; however, they do not efficiently account for the impact of flow, especially in multiphase systems like partially condensed overhead (OVHD) environment.^{6 7}

It is important to note that aside from ammonium chloride, the formation of amine hydrochlorides may also occur if amine-based neutralizers are used (as shown in Reactions 1 and 2).

\(\ce{R1NH2 + HCl <=> R1NH2HCl}\) (1 - gas phase)

\(\ce{R1NH2HCl <=> R1NH3 + Cl-}\) (2 - liquid phase)

This further complicates the overall assessment of corrosion in the OVHD system because corrosion under deposits of amine hydrochlorides is even less understood than that caused by ammonium chloride. However, the corrosion resulting from amine hydrochlorides falls beyond the scope of this chapter.

Mechanism

Chemically, NH₄Cl formation is a simple salt creation resulting from the reaction of an acid (HCl) with a base (NH₃). This reaction can take place in both gas and liquid phases, following straightforward reactions 3-6.

\(\ce{HClg + NH3g -> NH4Clsolid v}\) (3 - gas phase)

\(\ce{HCl + H2O <=> H3O+ + Cl-}\) (4 - liquid phase)

\(\ce{NH3 + H2O <=> NH4+ + OH-}\) (5 - liquid phase)

\(\ce{NH4Cl <=> NH4+ + Cl-}\) (6 - liquid phase)

It is important to note that each reaction reaches its equilibrium based on factors like temperature, concentrations, phase makeup, and pressure. In a simple scenario with only HCl and NH₃ in the vapor phase, any NH₄Cl formed stays in that phase until the temperature surpasses a specific equilibrium point for reaction (3) shown in Figure 1.

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The dissociation constant, Kp, for NH₄Cl is directly calculated from NH₃ and HCl concentrations in the vapor phase, expressed as partial pressures using Equation 1.

\(\ce{Kp = [NH3pp] * [HClpp]}\) (Equation 1)

The partial pressures of individual components are determined as per Equations 2-3:

[NH₃pp] = (nNH₃ vapor phase) / (n total vapor phase) × P (Equation 2)

[HClpp] = (nHCl vapor phase) / (n total vapor phase) × P (Equation 3)

Here, nNH₃ – represents the mole fraction of ammonia in the vapor phase; nHCl – represents the mole fraction of HCl in the vapor phase; ntotal – indicates the total moles in the vapor phase, and P denotes the total pressure.

For NH₄Cl salt point calculator, please see Calculation Tool section (for subscribers only).

It is important to note that while thermodynamic equilibrium describes changes in equilibrium states, it does not provide insights into reaction kinetics (the speed of reaction). Reaction kinetics are generally influenced by the concentration of components and temperature.

Once conditions favor the formation of solid, undissociated NH₄Cl, it tends to deposit on the metal surface, causing fouling or plugging in the OVHD exchanger’s tubes. The exact deposition spot is challenging to predict, influenced by several factors like flow dynamics, phase composition (e.g., liquid hydrocarbon, water, gas hydrocarbon), or localized temperature changes. Usually, fouling in the exchanger becomes evident through increased pressure drop across the equipment and a decline in heat exchange efficiency.

The gases in the overhead process stream, especially from the distillation unit, can contain as much as 2-3% volume of water. When the operating temperature hits the water dew point, water droplets form and get absorbed by the highly hygroscopic NH₄Cl deposit. Under the wet NH₄Cl deposit the concentration of chloride ions rises, prompting the generation of H⁺ (or H₃O⁺) from the dissociation of water, in line with Le Chatelier’s Principle. Consequently, as the concentration of hydronium ions increases, the pH drops to extremely low levels (<2), triggering rapid HCl attack. Given that temperatures in the OVHD system typically range from 90-120°C (194-248°F) before cooling to 20-30°C (68-86°F), the HCl attack can cause rapid degradation in virtually any of the commonly used corrosion-resistant alloys

Key Variables

The fundamentals of ammonium chloride corrosion hinge on factors like the concentration of corrosive elements (HCl, ammonia), temperature influencing salt formation, water presence impacting the dew point and wash rate, and flow affecting phase separation and areas prone to deposition. Other parameters that contribute to NH₄Cl corrosion include the presence of H₂S, which might offer some surface protection by forming a FeS layer, the type of hydrocarbon phase (heavier fractions potentially enhancing the surface’s hydrophobic nature), and the use of amine neutralizers instead of ammonia, which eliminates NH₄Cl corrosion from one side but triggers potential amine hydrochlorides corrosion. Given this multifaceted complexity, accurately predicting NH₄Cl corrosion remains a challenging endeavor.

Temperature and Concentration

The relationship between the NH₄Cl dissociation constant and temperature, as depicted in Figure 1, serves as the primary guiding principle for assessing the propensity for NH₄Cl corrosion. Generally, operating temperatures should exceed the salt formation temperature (as shown in Figure 2) to ensure that the NH₄Cl formed remains in the vapor phase. However, in practical processes, maintaining a consistently higher temperature than the salt point can be challenging—such as when there are fluctuations in HCl and NH₃ concentrations, thereby affecting the specific salt point - as shown in Figure 3.¹¹

Hence, an industry rule of thumb advises keeping the operating temperature (T_op) above the salt formation temperature (T_salt) by a certain margin, typically a few degrees (around 10-20°).¹² This approach helps accommodate potential fluctuations in the salt point due to variations in HCl and NH₃ concentrations. Various process modelling software are commonly employed to calculate salt points under different operating scenarios.¹³

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For NH₄Cl salt point calculator based on partial pressures or gas-phase concentrations of HCl and NH₃, please refer to the Calculation Tool section.

Controlling and maintaining the process temperature above the salt point is a relatively straightforward task. However, accurately determining HCl and NH₃ concentrations in the vapor phase – and, consequently, calculating the salt point as a reference to the operating temperature – is complex. The typical source for NH₃ and HCl concentrations is a heat-material flowsheet usually prepared by a process designer or licensor for various operating scenarios (shutdown/start-up/normal operation/winter-summer operating regime, etc.). In many older units, where several process modifications have taken place and under fluctuating feed parameters, the heat-material flowsheets may not be available or may not be relevant to the current operating scenario.

Some operators resort to back-calculation of vapor phase concentrations of these species based on their concentration in OVHD separator boot-water (CDU, FCC fractionation, etc.) or from water leaving pressure separators in hydroprocessing units. However, this is a very complex exercise that requires sophisticated ionic modeling, making it susceptible to errors, such as if one phase component is omitted or under/over-estimated.

Flow

Flow plays an important role in NH₄Cl corrosion. While the flow does not directly impact the reaction of NH₄Cl formation, as this is governed by thermodynamics, it does play a crucial role once salt particles are formed in the vapor phase. The accumulation, settlement, and formation of deposits are influenced by the flow pattern. This becomes particularly significant after water injection. In a multiphase environment, with growing water and hydrocarbon droplets, as well as salt particles, proper flow distribution (flow pattern) may either enhance or diminish NH₄Cl settlement and, consequently, under-deposit corrosion.^{14 15 16}

There is no strict rule governing specific flow rates to prevent NH₄Cl deposition in overhead (OVHD) condensers or REAC (reactor effluent air cooler) systems. The industry-established limits primarily stem from operational experience and may vary depending on the process. This variability is crucial as it can impact not only NH₄Cl deposition but also other elements such as deposition of FeS or/and coke, etc.

Typically, for CDU OVHD vapor lines, the recommended velocity falls within the range of 12-15 m/s (40-50 ft/s). Velocities exceeding 20 m/s (65 ft/s) are generally considered unacceptable, as they may significantly accelerate flow-induced corrosion or corrosion-erosion.^{12 17} It’s essential to note that linear velocities might not fully capture the flow impact. During discussions involving other damage mechanisms like NH₄HS and sulfidation, it has been emphasized that Wall Shear Stress (WSS) is a preferable parameter over velocity for describing flow impact.

Water Wash

Water wash is one of the most common methods for mitigating NH₄Cl deposition and under-deposit corrosion.^{17 18} Water is injected continuously (most commonly) or intermittently (infrequently). The specific locations of injection points may vary based on the type of process unit.²⁰ In the Crude Distillation Unit (CDU) overhead (OVHD), where primary stress is typically placed on reducing HCl_aq corrosion, wash water commonly contains the relevant amine-type neutralizer. Injection points are located, for example, in the OVHD line (vertical or horizontal sections) or close to the inlet of the OVHD exchangers.^{17 19 20} Table 2 summarizes typical wash water injection systems for different refinery process units.

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Materials

In the case of ammonium chloride corrosion, the selection of resistant materials becomes very challenging. Under-deposit corrosion caused by NH₄Cl, ultimately driven by Cl^- ions (HCl), can degrade almost any common metallic material, ranging from carbon steel up to sophisticated high Cr/Ni/Mo alloys. The very basic, qualitative ranking of materials (from the least resistant—carbon steels—to the most resistant—titanium) provided by API norms should be used with caution.¹

carbon steel/low-alloy steels < 300-type ss < duplex steels/alloys 400/800/825 < 625/C-276 < Ti

The presence of Cl^- ions at high concentrations and temperatures above 100°C (212°F) necessitates the use of corrosion-resistant alloys (CRAs) with a minimum Pitting Resistance Equivalent Index (PREN) of 38-40.^{2 5} This requirement limits the choice of suitable materials to super duplex stainless steels such as 2507 (PREN=38), alloy 625 (PREN=46.5), C-276 (PREN=64), or C-22 (PREN=61). However, it does not guarantee immunity to NH₄Cl corrosion (or chloride-stress corrosion cracking “Cl-SCC” as secondary damage mechanism).²² Failures of super duplex and super-austenitic 6%Mo alloys were reported due to Cl-SCC.²

The cost of exotic Cr-Ni-Mo alloys, even when applied as an overlay, still encourages the utilization of carbon steel in overhead systems susceptible to NH₄Cl corrosion. This is viable with properly implemented mitigation measures such as water wash, salt point and dew point/relative humidity control, inhibitors, and overall process control

Minimizing NH₄Cl corrosion - guidelines

To effectively address NH₄Cl corrosion, the industry employs a range of rules and approaches, each varying in relevance and applicability. The following section will succinctly outline some of the most pertinent methods, as highlighted by several authors. These approaches prove instrumental in controlling the ammonium chloride corrosion phenomenon. Table 3 presents a list of selected mitigation measures, accompanied by generic comments and references from the literature.

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Tools

Below is a user-friendly calculator to estimate the NH₄Cl salt point temperature by considering the concentrations of HCl and NH₃.^after10

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References

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