Ammonium Bisulfide Corrosion: From Bulk Behaviour to Localised Attack

Ammonium bisulfide (NH?HS) corrosion poses a significant threat to the integrity of equipment in various refinery units. This article delves into the phenomenon, analysing it from a comprehensive perspective, encompassing macro effects down to the atomic level.

Chemical Underpinnings of the Threat

The formation of NH?HS arises from the reaction between ammonia (NH?) and hydrogen sulfide (H?S) within the processing stream. This reaction is governed by the following equilibrium:

NH? (g) + H?S (g) <=> NH?HS (s) ΔH < 0 (1)

As indicated by the negative enthalpy change (ΔH), the reaction is exothermic, favouring NH?HS formation at lower temperatures. This explains why ammonium bisulfide corrosion becomes more prevalent in cooler sections of refinery units (How? ??)

In the NH?HS formation case (Equation 1), some ammonia and hydrogen sulfide molecules must possess enough energy (reach the activation energy) to collide effectively and form NH?HS.

However, if the system temperature is lowered (external change), the equilibrium will shift in a direction that counteracts this change. In this case, lowering the temperature disrupts the equilibrium. To restore stability and compensate for the cooler environment, the system favours the reaction that releases heat (NH?HS formation). However, the story doesn't end there. Solubility also plays a critical role. NH?HS has a specific solubility limit in the process stream, defining the maximum amount that can remain dissolved at a given temperature. When the NH?HS formation, driven by the lower temperature equilibrium shift, outpaces its solubility limit, the excess NH?HS precipitates as a solid salt. This solid NH?HS then accumulates on equipment surfaces, posing a significant corrosion threat.

The presence of water (H?O) further influences this equilibrium. The following reaction depicts the dissociation of NH?HS in water:

NH?HS (s) + H?O (l) <=> NH?? (aq) + HS? (aq) + H? (aq) (2)

The dissolution of NH?HS according to equation (2) generates corrosive elements in two ways:

Formation of Hydrosulfide Ion (HS?):

• The bisulfide ion (HS?) formed during dissociation is a weak acid (HS? has a much lower tendency to donate a proton (H?) compared to strong acids) according to the equilibrium:

HS? (aq) + H?O (l) <=> H?S (aq) + OH? (aq)

Then again, the equilibrium for its dissociation lies heavily in favour of the undissociated form (HS?) and water (H?O). While some H?S remains as an undissociated molecule (H?S), the released proton (H?) contributes to the overall acidity of the solution. The limited dissociation, the presence of even a small amount of H?S contributes to the overall acidity of the solution.

Increased Electrolytic Conductivity:

• The dissociation of NH?HS leads to the formation of charged ions (NH?? and HS?). These ions contribute to the electrical conductivity of the solution. This increased conductivity facilitates the flow of corrosion currents between different areas of the metal surface with varying electrode potentials. These corrosion currents can accelerate the anodic dissolution of the metal (oxidation) at specific locations, leading to localised corrosion attacks.

Unlike the bisulfide ion (HS?), NH?? doesn't directly participate in acidic reactions that attack the metal surface. It remains a positively charged cation in solution. However, in some specific scenarios, depending on the presence of other ions and the overall solution chemistry, NH?? might have a more pronounced indirect effect on corrosion through its buffering capacity. However, for NH?HS corrosion in refinery units, the focus remains on the acidic nature generated by HS? dissociation and the combined effect of both ions on conductivity.

Since the corrosion caused by NH?HS primarily involves an acidic attack from H?S formed during the limited dissociation of HS?, under-deposit corrosion (UDC) is a highly likely mechanism.

Metal Sulphide complexes: A Key Driver of Ammonium Bisulfide Corrosion

The formation of metal sulphide complexes is a critical factor in ammonium bisulphide corrosion, particularly affecting carbon steel and higher grades of steel. In this process, ammonium bisulphide decomposes to release sulphide ions (HS-), which interact with the metal surface, leading to the formation of metal sulphide compounds. The chemical reaction underlying this involves a redox process, as illustrated when iron (Fe) reacts with sulphide ions, resulting in the following reaction:

Fe(s)?+?2HS-?(aq)?→?FeS(s)?+?H2(g)

In this reaction, iron metal is oxidised to iron(II) ions, while sulphide ions are reduced to form iron sulphide (FeS). This iron sulphide is often insoluble and can form a protective layer on the steel surface. However, the properties and effectiveness of this layer in preventing further corrosion can vary greatly, depending on the specific conditions.

The formation and characteristics of metal sulphide layers in the context of ammonium bisulphide corrosion are influenced by several factors. The composition of the steel is crucial, as different grades of steel exhibit varying reactivities with sulphide ions released from ammonium bisulphide. Carbon steel, in particular, may form reactive or porous sulphide layers that may not offer sufficient protection against ongoing corrosion. The concentration of ammonium bisulphide in the environment is another key factor, as higher concentrations lead to an increased availability of sulphide ions, which can accelerate the formation of metal sulphides and thus increase the corrosion rate. The pH of the solution also plays a significant role, especially in environments where ammonium bisulphide is present, as lower pH levels can enhance the availability of sulphide ions for reaction. Temperature is an important factor as well; elevated temperatures can increase the rate at which metal sulphides form, further accelerating corrosion. Additionally, the presence of other ions in the solution can influence the formation and properties of metal sulphide complexes by competing with sulphide ions for bonding with metal ions.

In the context of ammonium bisulphide corrosion, metal sulphide layers can have both protective and detrimental effects on steel. it can facilitate further corrosion by allowing corrosive agents to penetrate and reach the underlying steel. This can lead to localised corrosion, such as pitting or crevice corrosion.

In summary, the formation of metal sulphide complexes is a critical aspect of ammonium bisulphide corrosion. The nature and properties of these complexes, influenced by various factors, can significantly impact the overall corrosion behaviour of carbon steel and higher grades of steel in environments where ammonium bisulphide is present.

Temperature Dependence and NH?HS Precipitation

The susceptibility of equipment to NH?HS corrosion hinges on a critical factor – temperature. For corrosion to occur, the process temperature must fall below a threshold value known as the NH?HS deposition temperature. This temperature represents the point at which NH?HS condenses or precipitates from the gas phase.

There are two main scenarios for NH?HS precipitation:

1. Solid Salt Formation: If the process stream lacks a liquid water phase, NH?HS precipitates directly as a solid salt onto equipment surfaces. This solid salt accumulation is highly detrimental and accelerates corrosion.
2. Dissolution in Water: The absence of bulk water or visible moisture condensation leads to a larger amount of NH?HS remaining as a solid salt on the metal surface. This concentrated presence of NH?HS does indeed create a zone for potential acidic attack. However, even without readily visible moisture condensation, a thin film of adsorbed moisture can still be present on the equipment surface due to the hygroscopic nature of most materials (their tendency to attract and hold water vapor). This thin film might not be enough for bulk water dissolution but can play a crucial role:

·???????? Limited Dissociation: In this thin adsorbed moisture film, some dissociation of NH?HS to HS? can still occur.

·???????? Localised Acidity: While limited compared to a scenario with bulk water, this dissociation creates a localised acidic environment around the salt particle due to the presence of HS? ions. This acidic environment can directly attack the metal surface, leading to localised corrosion pits or more specifically (UDC) Under-deposit corrosion.

The NH?HS deposition temperature isn't constant and varies depending on the specific process conditions, such as pressure, gas composition (NH? and H?S content), and the presence of other components. Generally, this temperature can range from below ambient to as high as 150°C.

Material Susceptibility and the Role of Alloying Elements

Carbon Steel: Widely used due to its cost-effectiveness, carbon steel exhibits severe susceptibility to NH?HS corrosion. The aggressive nature of the bisulfide ion (HS?) readily attacks the iron (Fe) in the steel, leading to the formation of iron sulfide (FeS). This reaction can be represented as:

Fe (s) + HS? (aq) <=> FeS (s) + H? (aq) (3)

The loss of metallic iron weakens the material and compromises its structural integrity.

Stainless Steels: Offering superior corrosion resistance compared to carbon steel, various grades of stainless steels exhibit varying degrees of effectiveness against NH?HS.

• 300 Series Stainless Steel: This commonly used austenitic stainless steel (e.g., AISI 304) provides moderate resistance. The presence of chromium (Cr) forms a passive oxide layer (Cr?O?) that hinders corrosion. However, the relatively low Cr content (around 18%) can be overwhelmed under severe NH?HS conditions.
• Duplex Stainless Steel: These steels, containing a higher Cr content (typically 22-25%) and a balanced ferritic-austenitic microstructure, offer improved resistance. The higher Cr content strengthens the passive layer, while the nitrogen (N) addition enhances pitting resistance.

Nickel-based Alloys and Titanium Alloys: These materials represent the most resistant options for NH?HS environments.

• Nickel-based alloys: High nickel (Ni) content promotes the formation of a stable nickel sulfide layer (NiS) that effectively protects the underlying metal. Where nickel (Ni) readily reacts with sulfur (S) from H?S to form nickel sulfide (NiS). This reaction consumes the corrosive H?S, reducing its availability to attack the underlying metal. Additionally, NiS has a lower solubility compared to iron sulfide (FeS), a common corrosion product in steels without high nickel content. This lower solubility means less NiS dissolves in the solution, further minimizing its impact on the underlying metal. Also, NiS layer acts as a physical barrier between the corrosive environment (H?S and acidic species) and the underlying metal. Additionally, molybdenum (Mo) additions further enhance resistance by promoting the formation of a passive layer rich in molybdenum sulfides. (How? ??) Molybdenum (Mo) has a strong affinity for sulfur and readily forms molybdenum sulfides (MoS?). When Mo is present in the steel, these molybdenum sulfides can form a duplex layer with the underlying NiS layer. This duplex layer offers superior protection compared to NiS alone. MoS? has a lamellar structure with weakly bonded layers. This allows for self-healing if the layer is damaged, as new MoS? can form within the existing structure. MoS? is generally considered a better diffusion barrier for corrosive species compared to NiS.

·???????? Titanium Alloys: Exhibiting exceptional resistance, titanium (Ti) readily forms a tenacious oxide layer (TiO?) that is highly resistant to NH?HS attack.

Impact of Process Conditions

Ammonium Bisulfide Concentration: A direct correlation exists between NH?HS concentration and the severity of corrosion. Higher concentrations lead to a more aggressive environment, accelerating the corrosion rate.

Flow Conditions: Fluid flow plays a critical role in NH?HS corrosion. Turbulent flow conditions, particularly at high velocities, can erode the protective layers formed on the metal surface, exposing fresh metal to attack. This phenomenon is known as erosion-corrosion. In multiphase flow scenarios (where gas and liquid phases coexist), the presence of gas bubbles can further exacerbate erosion by impinging on the metal surface.

Effect of Oxygen: While a trace of oxygen can theoretically form a weak barrier against NH?HS attack, it's generally considered detrimental in most refinery processes. In wash water systems, maintaining oxygen levels below 15 ppb is crucial to minimize corrosion risks. High oxygen levels (> 15 ppb) can promote the formation of less protective oxides and potentially exacerbate localised corrosion attacks.

Cyanide Content: The presence of cyanide (CN?) in the process stream can significantly accelerate NH?HS corrosion. Cyanide ions can complex with iron, weakening the passive layer and facilitating further attack by the bisulfide ion. The reaction can be simplified as:

Fe2? (aq) + CN? (aq) <=> Fe(CN)? (aq) (4)

In scenarios with cyanide is presence, the reaction: H?S (aq) + CN? (aq) <=> HSCN? (aq) + H? (aq) can become more prominent. However, this reaction consumes H?S, potentially reducing its direct involvement in corrosion. However, the released proton (H?) can contribute to the overall acidity of the solution, potentially exacerbating corrosion differently.

CO? Effect: Carbon dioxide (CO?) can indirectly influence NH?HS corrosion through its effect on solution pH. The dissolution of CO? in water forms carbonic acid (H?CO?), which dissociates to release protons (H?):

CO? (g) + H?O (l) <=> H?CO? (aq) <=> H? (aq) + HCO?? (aq) (5)

The increased acidity can enhance the solubility of NH?HS, potentially leading to a higher concentration of aggressive bisulfide ions available for attack.

Vulnerable Units in Refineries

Several refinery units are particularly susceptible to NH?HS corrosion:

• Hydrocracking Units: These units convert heavier hydrocarbons into lighter products using a catalyst and hydrogen. The presence of nitrogen in the feedstock can lead to NH?HS formation, particularly in cooler sections like the reactor effluent air cooler (REAC).
• Fluid Catalytic Cracking (FCC) Units: FCC units convert heavier fractions into gasoline using a fluidised catalyst bed. NH?HS corrosion can occur in areas where sour water (water containing H?S and NH?) accumulates.
• Amine Units: These units employ amine solutions to remove acidic components (like H?S and CO?) from hydrocarbon streams. Amine degradation products can react with H?S to form NH?HS, posing a threat to equipment integrity.

Mitigation Strategies

Combating NH?HS corrosion requires a multi-pronged approach:

• Material Selection: Selecting the most appropriate material for the specific process conditions is crucial. Nickel-based alloys and titanium alloys offer superior resistance but come at a higher cost. Duplex stainless steels can be a cost-effective option for moderately severe environments.
• Process Control: Maintaining optimal process conditions can minimize NH?HS formation and mitigate its effects. This includes controlling operating temperatures (favouring higher temperatures), managing NH? and H?S levels, and maintaining a controlled flow regime to avoid excessive turbulence.
• Water Wash Injection: Introducing a water wash stream strategically can help dilute NH?HS concentration and promote its dissociation, reducing its aggressiveness.
• Monitoring and Inspection: Regular monitoring of equipment condition through techniques like ultrasonic thickness measurement is essential for early detection of corrosion and timely intervention.

Last words

Ammonium bisulfide corrosion presents a significant challenge in refinery operations. Understanding the underlying chemical and metallurgical factors, along with the impact of process conditions, is crucial for selecting appropriate mitigation strategies. Through careful material selection, process optimisation, and effective monitoring, refineries can ensure the integrity of their equipment and maintain safe and reliable operations.

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