Ammonium Bisulfide Corrosion

General Information

Ammonium bisulfide corrosion, also known as alkaline sour water corrosion, is a prevalent issue in hydroprocessing units, which encompass hydrocracking (HC), hydrotreating (HT), and hydrodesulfurization (HDS). Catalytic hydroprocessing involves desulfurization and denitrification reactions (see Equations 1 and 2), resulting in the production of ammonia (NH ₃) and hydrogen sulfide (H₂S). These compounds readily react to form ammonium bisulfide, or formally, ammonium hydrosulfide (NH₄HS), through a simple, reversible reaction as shown in Equation 3.

Ammonium bisulfide is thermally unstable, and at high temperatures in the hydrogenation reactor effluent (>300°C / >572°F), the equilibrium of the reaction in Equation 3 shifts to the left. Upon cooling in the reactor effluent air cooler (REAC), the formation of solid ammonium bisulfide becomes favorable. This solid salt can deposit inside the REAC tubes, leading to fouling and plugging. Water washing upstream of the REAC prevents the formation of solid deposits, as NH₄HS dissolves in water, creating the so-called alkaline sour water (see Figure 1). The phenomenon of ammonium bisulfide corrosion can be triggered in two ways: by the sour water itself and by wet, incompletely dissolved ammonium bisulfide deposits. The latter scenario is the most severe, capable of rapidly degrading many Corrosion Resistant Alloys (CRAs).

Ammonium bisulfide corrosion is not exclusive to hydroprocessing units; it also plays a significant role in amine units (specifically the regenerator overhead) and sour water strippers (the entire unit). Table 1 lists the most common areas where alkaline sour water corrosion may occur.

Table 1 Potential locations for ammonium bisulfide corrosion in refinery units.^{1 2}

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Tools

Below are a user-friendly calculators and integrity tools to estimate sour water corrosion of carbon steel.

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NH₄HS-Corrology®

NH₄HS-Corrology® is designed to provide rapid corrosion rate estimates for carbon steel in alkaline sour water service. It allows you to evaluate the impact of key process parameters like H₂S partial pressure, NH₃ concentration and velocity, on corrosion rates.

Note: Calculation results are currently hidden. Register for Free Trial to unlock Corrosion Rate and Service Life Results.

Integrity Risk Indicator (Corrology®-IRI: Sour Water)

The Challenge: Moving Beyond Linear Estimates

In alkaline sour water service, traditional linear corrosion tracking often fails to capture the rapid transition to accelerated damage. As wall thickness decreases, risk does not grow steadily - it accelerates exponentially. Small changes in NH₄HS severity, H₂S partial pressure, NH₃ concentration, or velocity can also shift equipment from a stable operating window into a severe-corrosion regime.

The Solution: The NH₄HS Integrity Risk Indicator (IRI-NH₄HS)

The IRI-NH₄HS quantifies asset risk by calculating corrosion rates, service-life context, and remaining-life indicators through a margin-aware, API 581-inspired engine. This tool functions as a sensitivity analysis engine, allowing you to evaluate how shifting NH₄HS-related process variables, velocity, and inspection evidence impact Carbon Steel across both Predictive and Inspection workflows.

This score should be interpreted as an integrity / probability-of-failure-style indicator. It is not a full API RBI risk result in the sense of likelihood × consequence, because consequence-of-failure is not part of this score.

The Advantage

Access a high-speed, agile modeling engine designed for rapid, defensible engineering decisions:

• Non-Linear Risk Scaling: Utilize a logarithmic-exponential transformation that reflects the true physical reality of risk escalation in late-life degradation states.

• Dynamic Inspection Credit: Input your Inspection Effectiveness (Class A-E) to see how higher-quality inspection evidence reduces conservatism in the Bayesian risk calculation.

• Auto-Derived Prior Confidence: Prior confidence is automatically computed from the uninspected time interval (t_uncertainty): <3 years = High, 3-5 years = Medium, >=5 years = Low. This applies in both Predictive and Inspection modes and remains read-only in the standard UI workflow; API and integration callers may pass priorConfidenceOverride or priorConfidence when an explicit override is required.

• Precision Structural Floors: Take control of your model by setting Manual T-Min structural floors to match your equipment’s unique geometry.

• Expert Calibration: Gain confidence with a model structured against a 6-point lifecycle truth set, ensuring accuracy from low-risk early life to critical thin-wall scenarios.

• API 581-Aligned Risk Bands: Status and score-pill color are determined by the log probability of failure (log P_f) rather than the 0-100 score scale alone. For observed unrepaired wet H₂S cracking, the operator-facing Carbon Steel result is treated as an FFS hard stop and is intentionally shown without risk score or remaining service life until FFS disposition is complete.

Note: The Integrity Risk Indicator (IRI-NH₄HS) shown below is a static preview of our Professional+ engine. Register for Professional+ to unlock IRI-NH₄HS tool.

References

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