Ammonium Chloride Corrosion

Ammonium chloride corrosion, alongside ammonium bisulfide corrosion and wet-H₂S damage, constitutes the most prevalent group of low-temperature damage mechanisms in the refining industry. NH₄Cl is recognized for its dual effect: fouling and, when the deposit becomes wet, fostering severe corrosion beneath it. Due to the nature of this under-deposit attack, accurately predicting the location and rate of degradation is challenging. The subsequent chapter offers an overview of the NH₄Cl corrosion phenomenon, mitigation strategies, and guidelines for material selection.

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

Process Unit	Affected Area
Crude Distillation Unit (CDU)	Atmospheric tower top Atmospheric tower overhead (OVHD pipelines and commonly 1st stage exchangers)
Catalytic Reforming Unit (CRU)	Product separator Debutanizer section (OVHD)
Fluid Catalytic Cracking (FCC)	Main fractionator top section and OVHD system Stripping columns
Hydroprocessing Hydrotreating/Hydrocracking	REAC and surrounding pipelines (inlet, REAC tubes, outlet manifold) HP/LP separators Recycle Hydrogen lines
Delayed Coking Unit (DCU)	Fractionator OVHD section Coke drums blowdown system

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.

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References

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