An In-depth Analysis of H2S Sour Water Corrosion: From A Silent Killer to Destruction of Carbon Steel.

Acidic sour water, a seemingly innocuous by-product in petroleum refining processes, harbours a destructive secret – its relentless attack on carbon steel equipment. This article examines into the intricate interplay between chemistry, metallurgy, and thermodynamics that dictates the fate of steel in this hostile environment. We embark on a journey to explore the initiation and progression of sour water corrosion, analysing the factors that influence its severity, and unveiling the enigmatic role of the protective iron sulfide (FeS) layer.

The Initiation: A Step-by-Step Descent into Sourness

The saga begins with the presence of hydrogen sulphide (H?S) dissolved in water. As H?S encounters water molecules, a dissociation reaction occurs:

H?S (aq) + H?O (l) ? H?O? (aq) + HS? (aq)? ??(Ka ≈ 1 × 10?? at 25 °C)

This reaction establishes a mildly acidic environment (pH between 4.5 and 7.0) due to the formation of hydronium ions (H?O?). Carbon dioxide (CO?) may also be present, contributing further to the acidity:

CO? (aq) + H?O (l) ? H?CO? (aq) ? H? (aq) + HCO?? (aq)? ?(Ka? ≈ 4.45 × 10?? at 25 °C)

Temperature and pH: Key Players in Sour Water Corrosion Rates

The interplay between temperature and pH significantly influences sour water corrosion rates. At lower temperatures (around 30°C), the increased solubility of H?S in water might seem detrimental. However, the sluggish movement of H?S molecules towards the steel surface (mass transfer limitations) often outweighs the higher concentration, leading to relatively low corrosion rates.

As temperature rises towards 90°C, reaction kinetics accelerate, and H?S reacts with steel much faster, promoting the formation of iron sulfide (FeS). While H?S solubility actually decreases at higher temperatures, the enhanced reaction rates dominate, causing corrosion rates for carbon steel, low-alloy steels, and copper alloys to increase rapidly.

Beyond temperature, pH also plays a crucial role. Within the pH range of 4.5 to 7, the formation of a thin FeS layer is more likely. However, the effectiveness of this layer in slowing corrosion depends on various factors beyond its mere presence. Extremely acidic environments (pH < 4.5) often indicate the presence of stronger acids like hydrochloric acid (HCl) that can disrupt FeS formation and accelerate corrosion. Conversely, highly alkaline conditions (pH > 7) might signify elevated ammonia (NH?) levels, leading to corrosion through the formation of aggressive ammonium salts.

In essence, sour water corrosion rates are a complex interplay between temperature, pH, and various other factors. Understanding this delicate balance is crucial for mitigating corrosion and preventing metal failure in these environments. The influence of these factors on the formation and stability of protective films, such as FeS, will be discussed in greater detail in the following sections.

How FeS Forms to Combat Sour Water Corrosion

The acidic nature weakens the protective oxide layer naturally present on the steel surface. This vulnerability allows H?S to react directly with the iron (Fe) in the steel, initiating the corrosion process:

Fe (s) + H?S (aq) → FeS (s) + 2H? (aq)? ??(E° = -0.41 V)

The formation of a FeS layer on the steel surface presents a double-edged sword. Here's why:

Formation: The reaction between iron (Fe) and hydrogen sulfide (H?S), as mentioned earlier, leads to the formation of iron sulfide (FeS). This reaction is favoured from a thermodynamic perspective due to its negative standard potential (E° = -0.41 V). In electrochemistry, the standard potential (E°) represents the electrical potential of a cell under standard conditions (specific temperature, pressure, and concentrations). A negative E° value indicates that the reaction proceeds spontaneously in the forward direction, favouring the formation of products (in this case, FeS) over the reactants (Fe and H?S). Essentially, the negative E° signifies a natural tendency for the system to release energy by converting Fe and H?S into FeS.

Types and Properties: There are various types of FeS films, each with distinct properties influencing their protectiveness:

Mackinawite (FeS?-x): This iron-deficient FeS layer forms at lower temperatures (< 90 °C) and is relatively porous, offering limited corrosion protection. Mackinawite's chemical formula, FeS?-x, indicates a non-stoichiometric composition. This means it doesn't have a fixed ratio of iron (Fe) to sulfur (S) atoms. The "x" in the formula represents the vacancy of iron sites within the crystal lattice. This vacancy arises due to several factors:

§? Kinetic Limitations: At lower temperatures (< 90 °C), the reaction between Fe and H?S may not reach completion. Suppose a cold room where furniture (iron and sulfur atoms) moves sluggishly. This sluggishness, due to limited atomic mobility, hinders the complete filling of all available iron sites in the FeS crystal structure. This results in a vacancy-rich, iron-deficient mackinawite layer (FeS?-x). Interestingly, from a thermodynamic perspective, the presence of these vacancies can actually favour the formation of mackinawite under these conditions.

§? Free Energy and the Trade-Off: A perfectly stoichiometric FeS (FeS?) represents a very ordered state, like a meticulously arranged room with all the furniture (atoms) in their designated spots. This orderliness corresponds to lower entropy. However, achieving this perfect order at lower temperatures might require additional energy input (like meticulously arranging furniture in a cold room) to overcome the sluggish atomic mobility. This energy corresponds to enthalpy. The presence of vacancies in mackinawite (FeS?-x) is like having a room with some empty spaces where furniture should be. It's less organised than the perfectly arranged room (higher entropy). However, forming this structure might require less effort (energy) compared to the perfectly ordered one, especially at lower temperatures. Here, the vacancies act like shortcuts, avoiding the extra effort needed to force all the atoms into their exact positions at low temperatures. In essence, the system "chooses" the pathway (mackinawite formation with vacancies) that minimises its overall free energy (G = H - TS), where G is free energy, H is enthalpy (internal energy), and S is entropy. At lower temperatures, the gain in disorder (increased entropy) due to vacancies in mackinawite might outweigh the slight increase in effort (enthalpy) needed to form this non-stoichiometric structure. In simpler terms, the system favours the easier path (mackinawite formation) that minimizes its overall effort required for the process under these specific conditions.

Greigite (Fe?S?): Greigite exists at moderate temperatures (around 150-300 °C). At these temperatures, the free energy landscape favors the formation of a more ordered and densely packed greigite structure (Fe?S?) compared to the iron-deficient mackinawite (FeS?-x). This allows for a more complete reaction between Fe and H?S, facilitating the filling of iron vacancies and leading to a denser crystal structure.

greigite packs its iron and sulfur atoms more efficiently in a cubic crystal structure. This denser packing results in fewer voids and a more tortuous path for aggressive sour water components to reach the underlying steel. This increased compactness translates to a more effective barrier against corrosion. Greigite also exhibits higher chemical stability in sour water environments due to its stoichiometric composition minimizing reactive sites within the crystal lattice. Additionally, it might adhere more strongly to the steel surface, providing a more robust protective layer.

Greigite exhibits higher chemical stability in sour water environments compared to mackinawite. This is because the stoichiometric Fe?S? composition minimises the presence of reactive sites within the crystal lattice. Moreover, greigite may adhere more tenaciously to the steel surface compared to mackinawite. This stronger adhesion provides a more robust and long-lasting protective layer against corrosion.

It's important to note that while greigite formation is typically discussed in the context of sour water environments, similar conditions can exist in high-temperature hydrocarbon streams containing H?S, such as naphtha. In these environments, the presence of H?S and the elevated temperatures can promote the formation of greigite films, offering enhanced protection against corrosion.

Pyrite (FeS?): Stable at higher temperatures (> 300 °C), pyrite offers the most effective corrosion resistance due to its highly compact and impermeable nature. As temperature increases, the free energy landscape undergoes a significant shift. Pyrite's highly compact and ordered FeS? structure becomes the most thermodynamically stable phase compared to mackinawite (FeS?-x) and greigite (Fe?S?). This means that at high temperatures, the system naturally favours the formation of pyrite, minimizing its overall free energy.

High temperatures significantly increase the mobility of iron and sulfur atoms. This enhanced mobility allows for a more complete and rapid reaction between Fe and H?S, facilitating the formation of the perfectly stoichiometric FeS? structure. Lower temperatures, on the other hand, hinder this complete reaction, leading to the formation of iron-deficient phases like mackinawite. Additionally, the strong covalent bonds between iron and sulfur atoms within the pyrite structure contribute significantly to its chemical stability. These bonds require a significant amount of energy to break, making pyrite highly resistant to attack by H?S.

Pyrite formation is most prevalent in high temperature sulfidation environments, where temperatures surpass the limits for liquid water to exist. These environments can include high-pressure gas streams containing H?S or the vapor phase above hot oil or water with dissolved H?S.

• Impact of External Parameters:

Temperature: As temperature increases, the type of FeS formed transitions from mackinawite to greigite and eventually pyrite. While this progression generally enhances protectiveness, higher temperatures can also accelerate the overall corrosion rate (thoroughly explained above ??)

pH: A more alkaline (higher pH) environment favours the formation of a more stable and protective FeS layer. Conversely, acidic conditions promote the breakdown of FeS, diminishing its effectiveness.

The formation of an FeS layer involves a chemical reaction between iron (Fe) from the steel and hydrogen sulfide (H?S) dissolved in water. This reaction can be represented by the following equilibrium:

Fe (s) + H?S (aq) ? FeS (s) + 2H? (aq)?????

The position of this equilibrium, and therefore the amount of FeS formed, is influenced by the concentration of hydrogen ions (H?). In a more alkaline (higher pH) environment, the concentration of H? ions are lower. This lower concentration shifts the equilibrium towards the formation of more FeS products, favouring a thicker and more robust layer.

In an alkaline environment (higher pH), the dissociation of H?S is more favourable, leading to a higher concentration of sulfide ions (S2?) available for FeS formation:

H?S (aq) ? H? (aq) + S2? (aq)

A higher concentration of S2? ions further promote the formation of a completer and more protective FeS layer.

Conversely, acidic environments (lower pH) with a higher concentration of H? ions push the equilibrium towards the breakdown of FeS. This can lead to the dissolution of the existing FeS layer, exposing the underlying steel to further attack by H?S and other corrosive elements.

Flow Conditions: Flow conditions play a crucial role in the formation and stability of FeS layers. While a stagnant or slow-moving environment might favour FeS formation, turbulent flow can act as a double-edged sword: hindering its formation through mass transfer limitations and potentially removing existing layers through mechanical forces.

Mass Transfer and Limited Growth:

• Mass Transfer Barrier: Turbulent flow creates a dynamic environment with rapid movement of water molecules. This rapid movement can create a thin layer (boundary layer) around the steel surface where the flow velocity is relatively low. The diffusion of H?S towards the steel surface to react and form FeS must occur through this boundary layer.
• Limited Supply: Turbulent flow can hinder the efficient transport of H?S from the bulk solution to the steel surface due to the boundary layer effect. This limitation in mass transfer can restrict the availability of H?S for FeS formation, potentially leading to a thinner and less complete layer.

Mechanical Removal: The Shearing Force:

• Fluid Shear Stress: Turbulent flow is characterised by eddies and fluctuations in velocity. These eddies exert a shearing force on the FeS layer, particularly when it's newly formed and still weak. This shearing force can physically remove or damage the developing FeS layer, exposing the underlying steel.
• Vulnerability of Weak Layers: Newly formed FeS layers are often more fragile and susceptible to mechanical removal by the shear stress of turbulent flow compared to thicker and more established layers.

Combined Effects:

The combined effect of mass transfer limitations and mechanical removal by shear stress in turbulent flow can significantly hinder the formation and stability of FeS layers. This can leave the underlying steel more vulnerable to attack by H?S and other corrosive species in sour water.

A Dense FeS: Thick Layers at Low Temperatures (<90°C) in Controlled Environments

While the formation of denser and more protective FeS layers typically occurs at higher temperatures, it's possible to observe thicker FeS layers forming in relatively low flow velocity and moderate temperatures (< 90 °C) under controlled laboratory conditions. Here's why this might happen:

Enhanced Reaction Kinetics:

• Temperature Effect: At lower temperatures (less than 90°C), while atomic mobility is still limited compared to higher temperatures, it's significantly improved compared to very low temperatures. This allows for a more complete reaction between iron (Fe) and hydrogen sulfide (H?S) to occur. This complete reaction facilitates the formation of a thicker and more stoichiometric FeS layer.
• Reduced Diffusion Limitations: In a low flow velocity environment, there's less disruption from turbulent flow. This allows for more efficient diffusion of H?S molecules towards the steel surface, enabling them to participate in the reaction and contribute to FeS layer growth.

Thermodynamic Favourability:

• Kinetic Control: At lower temperatures, while the overall thermodynamic equilibrium might favour the formation of denser iron sulfide phases (like greigite) at higher temperatures, kinetic limitations can play a more significant role. The sluggish movement of atoms at these temperatures can hinder the complete transformation of the initially formed FeS layer into denser phases. This allows for the FeS layer to continue growing, albeit as a less-dense structure, within the timeframe of the experiment.

Lab-Controlled Environment:

• Controlled Conditions: In a controlled lab environment, factors like temperature, flow velocity, and H?S concentration can be carefully monitored and adjusted. This allows for optimised conditions to promote the formation of a thicker FeS layer. Additionally, the absence of real-world complexities like contaminants and variations in flow patterns can further contribute to a more consistent and potentially thicker layer growth.

It’s important to mention that, at low temperatures (below 90°C) present a double obstacle for greigite and pyrite formation. Firstly, these complex structures are not energetically favourable at these temperatures. Secondly, sluggish atomic movement hinders the precise arrangement of atoms needed for their formation. In simpler terms, it's too cold and the "building blocks" move too slowly to create these specific structures. Therefore, in a controlled environment with a temperature below 90 °C, the most probable FeS layer to form is a stoichiometric iron sulfide (FeS) or a phase very close to it. The controlled parameters can promote a more complete reaction compared to uncontrolled environments at similar temperatures, minimising vacancies and leading to a more robust FeS layer.

The Iron Sulphide vs Iron Carbonate Mystery

Iron carbonate (FeCO?) (discussed thoroughly in your previous article), another corrosion product that can form in CO?-containing environments, presents a point of comparison with FeS. While FeCO? can offer some protection, it's generally less effective than FeS due to its higher solubility in acidic environments. In a mixed-film scenario, the relative abundance of FeS and FeCO? determines the overall corrosion rate. A dominant FeS layer offers superior protection, while a high concentration of FeCO? indicates potential for accelerated corrosion.

Unveiling the Weakness: Why Iron Carbonate Dissolves Easier

The increased susceptibility of iron carbonate to acids compared to iron sulfide boils down to the fundamental differences in their chemical bond strengths:

• Ionic Bonding in FeCO?: Iron carbonate features ionic bonding between the iron (Fe2?) cation and the carbonate (CO?2?) anion. Ionic bonds involve electrostatic attraction between oppositely charged ions. While these bonds hold the structure together, they are generally weaker than the covalent bonds found in FeS.
• Predominantly Covalent Bonding in FeS: In iron sulfide, the primary bonding character between iron (Fe) and sulfur (S) atoms leans heavily towards covalent bonding. This involves the sharing of electrons between the atoms, resulting in a strong and directional attraction that holds the structure together. While there might be a minor ionic contribution in certain FeS phases with slight iron deficiencies, the overall bonding character remains predominantly covalent.

The Impact on Acid Solubility:

When exposed to acidic environments, the weaker ionic bonds in FeCO? are more susceptible to being disrupted by the acidic protons (H?). These protons can essentially "pry apart" the Fe2? and CO?2? ions, dissolving the iron carbonate layer. In contrast, the predominantly covalent bonds in FeS offer greater resistance to attack by acids, making the FeS layer more stable and persistent.

The Influence of the Gaseous Phase: The Oxygen Factor

The presence of air or dissolved oxygen (O?) in sour water introduces a complex dimension to the corrosion drama. Here's a breakdown of its influence:

Initial Protection: The presence of air or dissolved oxygen (O?) in sour water introduces a complex dimension to the corrosion drama. Here's a breakdown of its influence:

While oxygen can initially promote the formation of an iron oxide layer (Fe?O?) on the steel surface, this layer's effectiveness as a protective barrier in sour water environments is limited.

There are two key reasons for this limited protection:

Susceptibility to H?S: Fe?O? is not entirely resistant to attack by H?S. Over time, H?S can react with and break down the Fe?O? layer, exposing the underlying steel to further corrosion.

Competition for Iron: Oxygen has a higher mobility compared to iron. This means oxygen can readily react with iron at the steel surface to form oxides and hydroxides (like FeOOH) before iron can react with H?S to form the more desirable FeS layer. These iron oxides and hydroxides generally offer less protection against sour water corrosion compared to FeS.

In essence, the presence of oxygen introduces a double-edged sword effect. While it might initially form a thin oxide layer, its high mobility, and the susceptibility of Fe?O? to H?S attack limit its long-term effectiveness as a protective barrier. Additionally, oxygen competes with H?S for iron, potentially hindering the formation of the more desirable FeS layer.

Furthermore, Oxygen can also react with H?S in a detrimental way. This reaction, as shown below, consumes oxygen and generates acidic by-products like sulphuric acid (H?SO?):

4H?S (aq) + 4O? (aq) + 4H?O (l) → 4H?SO? (aq) + 8H? (aq) + 8e?

This newly formed sulphuric acid is highly corrosive and can dissolve the initial protective Fe?O? layer, exposing the underlying steel to further attack by H?S and other aggressive components in sour water.

Disruption of FeS Film: The presence of oxygen can complicate the delicate balance on the steel surface. While it can initially promote the formation of a protective Fe?O? layer, its reaction with H?S leads to the formation of elemental sulfur (S°) through the following reaction:

2H?S (aq) + O? (aq) → 2H?O (l) + 2S° (s)

While elemental sulfur itself isn't necessarily detrimental to FeS formation, an excessive amount can disrupt the stability of the FeS film, the key protective layer against sour water corrosion. Here's how:

The accumulation of elemental sulfur particles within the FeS layer can act like unwelcome guests at a party. These particles can disrupt the compact structure of the FeS film, creating voids and increasing its porosity. This porous structure becomes a weak barrier, allowing aggressive species like H?S and acidic components in sour water to reach the underlying steel surface and accelerate corrosion.

Conclusion: A Delicate Balance

Sour water corrosion in carbon steel is a complex interplay between numerous factors. Understanding the formation and properties of the FeS layer, the influence of external parameters like temperature, pH, and flow, and the role of the gaseous phase is crucial for mitigating this destructive process. By carefully selecting materials like high-alloy steels or nickel-based alloys and implementing operational practices that minimise H?S concentration and oxygen ingress, engineers can strive to achieve a delicate balance – ensuring the integrity of equipment while optimizing the efficiency of petroleum refining processes.

Take Care ??

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