From Formation to Mitigation: Organic Acid Corrosion in Refineries– A Chemical and Metallurgical Perspective

The oil and gas industry regularly faces the challenge of corrosion, with Aqueous Organic Acid Corrosion (AOAC) being a notable threat to the integrity of refineries and petrochemical plants. Organic acids, unlike their mineral counterparts, are characterised by carbon-containing molecular structures, derived mainly from the degradation of hydrocarbons. These acids occur naturally in crude oil or form during refining processes, particularly under specific operating conditions. The presence of organic acids in aqueous environments can lead to aggressive corrosion, impacting process equipment and resulting in significant maintenance and operational costs.

Organic acids are known for their relatively weak acidic nature compared to mineral acids, yet their ability to induce corrosion is profound due to the way they interact with metal surfaces. Understanding these interactions requires both a chemical and metallurgical examination at the atomic level, which reveals the precise mechanisms by which corrosion progresses.

Chemical Structure and Corrosive Behaviour of Common Organic Acids

In the context of oil refineries and petrochemical plants, the most prevalent organic acids contributing to corrosion are formic, acetic, propionic, butyric, pentanoic, and hexanoic acids. These acids, despite sharing the carboxylic group (-COOH) as their functional group, exhibit significant differences in corrosivity, driven primarily by variations in their chemical structure, particularly the length of their carbon chains. The simplest is formic acid, containing just one carbon atom, followed by acetic (two carbons), propionic (three carbons), butyric (four carbons), pentanoic (five carbons), and hexanoic acid (six carbons).

The length of the carbon chain plays a crucial role in determining the behaviour of organic acids in aqueous environments, which is a key point of concern for corrosion engineers. The number of carbon atoms in the acid’s structure directly affects the molecular weight. Carbon atoms are heavier than hydrogen atoms, so as the number of carbon atoms in the acid increases, the overall molecular weight rises. For example, formic acid (HCOOH) has only one carbon atom and a molecular weight of 46 g/mol, whereas hexanoic acid (C?H??COOH), with six carbon atoms, has a molecular weight of 116 g/mol. The increase in molecular weight occurs because each carbon atom adds both mass and additional hydrogen atoms, contributing to the overall size and weight of the molecule.

This relationship between carbon atoms and molecular weight is important because molecular size affects how these acids interact with metals. Smaller acids, such as formic and acetic acids, have low molecular weights, making them highly mobile in aqueous solutions. Their small size allows them to easily penetrate the protective oxide layers that typically form on metal surfaces, such as the chromium oxide layer in stainless steel, enabling faster access to the underlying metal. This mobility makes smaller acids more aggressive, as they can rapidly reach the metal surface, dissociate, and contribute to the corrosion process.

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Molecular Weight and Acid Dissociation: Corrosivity of Organic Acids

In terms of dissociation, which is the process by which the acid releases hydrogen ions (H?) into solution, molecular weight also plays a role. Lower molecular weight acids like formic and acetic acid tend to dissociate more readily, because the smaller molecular structure allows for faster movement of the molecules in solution. This rapid dissociation increases the concentration of hydrogen ions, which in turn lowers the pH of the environment and accelerates corrosion. On the other hand, larger molecules such as pentanoic and hexanoic acids have higher molecular weights and are more complex, which makes their dissociation slower. These larger acids tend to release fewer hydrogen ions under the same conditions, resulting in a slower drop in pH, though their impact can still be significant over time, particularly in stagnant or low-flow environments where they can accumulate.

Thus, both the number of carbon atoms and the molecular weight influence not only the mobility of the acid in solution but also how quickly the acid dissociates, directly affecting its corrosive potential. Smaller, lighter acids tend to cause more immediate and widespread corrosion due to faster dissociation and greater mobility, while larger acids may cause more localised but persistent damage, particularly under conditions where temperature and solubility enhance their interaction with metals.

As mentioned earlier, larger organic acids such as hexanoic acid, with a molecular weight of 116 g/mol, have reduced mobility due to their higher molecular weight, which limits their ability to rapidly diffuse through solution and reach the metal surface. However, once these larger acids do reach the metal, their larger molecular structure plays a distinct role in how they interact with the metal surface.

Complex Formation, Solubility, and Localised Corrosion

Chemically, the increased carbon chain length of these acids allows them to form more stable complexes with metal ions, such as iron (Fe2?) or nickel (Ni2?). These metal ions are typically released from the metal surface as part of the corrosion process, and larger organic acids, with their more complex molecular structures, tend to form stronger bonds with these ions. This "sticking" effect results in the formation of insoluble salts or organic-metal complexes that can adhere to the metal surface, creating localised corrosion sites. This localised corrosion occurs because the acid, instead of being uniformly dispersed, concentrates in specific areas, particularly where the acid accumulates due to low flow or stagnant conditions.

The formation of these stable complexes can further exacerbate corrosion by preventing the formation of a protective oxide layer on the metal surface. In smaller acids, the dissociation process dominates the corrosion mechanism, but for larger acids, the bulkier molecular structure physically disrupts the surface, hindering the metal's ability to repair itself through passivation. This, in turn, leads to localised degradation of the metal, often in the form of pitting or crevice corrosion.

Additionally, the interaction of these larger organic acids with metal surfaces contributes to fouling, where the organic-metal complexes build up over time, forming deposits. These deposits act as physical barriers, trapping corrosive substances like water, dissolved oxygen, and more organic acids underneath, which accelerates the corrosion beneath the fouling layer. This trapped corrosive environment often becomes more aggressive due to the concentration of acids and a lack of fresh material flow to dilute or wash away the deposits, leading to a highly localised but severe form of corrosion.

In refinery systems, this effect is particularly pronounced in stagnant zones or low-flow areas, where hexanoic and pentanoic acids, with their higher molecular weights, can accumulate. The larger, heavier molecules are more likely to precipitate out of solution or adhere to surfaces, leading to persistent corrosion that can go unnoticed until significant damage occurs. This is why these heavier acids, despite their slower diffusion and lower dissociation rates compared to smaller acids, can cause long-term operational problems by creating persistent corrosive environments that are difficult to address without mechanical intervention or chemical cleaning.

The solubility of organic acids in water is another crucial factor influencing their corrosive behaviour in oil refineries and petrochemical plants. The chain length of these acids directly affects their solubility, with shorter-chain acids being more soluble in water. For example, formic and acetic acids are fully miscible with water, allowing them to remain consistently in the aqueous phase. This solubility ensures that these lighter acids have continuous access to metal surfaces, where they can rapidly dissociate and aggressively attack the metal. Their high degree of miscibility contributes to generalised corrosion because they can move freely and interact uniformly with the metal surface across a wide area.

In contrast, longer-chain acids, such as pentanoic and hexanoic acids, exhibit lower solubility in water, particularly at lower temperatures. This lower solubility can lead to phase separation, where these acids form distinct layers or droplets, reducing their immediate corrosive impact in some conditions. However, their behaviour changes significantly at elevated temperatures, which are common in refinery units. As the temperature rises, the solubility of these heavier acids increases, allowing them to dissolve more readily into the aqueous phase. This enhances their interaction with metal surfaces, increasing their corrosive potential, especially in high-temperature units like distillation towers and heat exchangers, where these acids can become more aggressive.

This ties back to the discussion on molecular weight: the higher molecular weight of larger acids not only reduces their diffusivity, as mentioned previously, but also makes them less prone to dissociation in water compared to their smaller counterparts. Acids like formic and acetic, with their lower molecular weights, dissociate more readily, releasing hydrogen ions (H?) into solution at a faster rate. This rapid dissociation produces a high concentration of hydrogen ions, which lowers the pH and accelerates the corrosion process by increasing the direct electrochemical attack on metal surfaces.

Larger molecules, such as butyric, pentanoic, and hexanoic acids, are less prone to dissociation under the same conditions. Their corrosive attack on metal surfaces may appear less intense in terms of general hydrogen ion release. However, this does not mean that these larger acids are less dangerous. On the contrary, as discussed earlier, their ability to form stable complexes with metal ions and precipitate as insoluble salts can result in more severe, localised forms of corrosion, such as under-deposit corrosion. This type of localised attack is particularly dangerous because it can remain hidden beneath deposits, making it difficult to detect until significant damage has occurred.

Thus, from a corrosion engineering perspective, while lighter organic acids typically cause generalised corrosion due to their higher solubility and faster dissociation, the heavier organic acids present a more insidious threat through localised corrosion mechanisms. These mechanisms include pitting and crevice corrosion, especially in environments where the acids accumulate or where temperature variations enhance their solubility. This makes managing corrosion caused by organic acids in refineries a multifaceted challenge, requiring an understanding not only of their chemical structure and molecular weight but also of how operating conditions like temperature and flow rates affect their behaviour in real-world scenarios.

Metallurgical Interactions of Organic Acids

The metallurgical impact of organic acids is directly related to the composition of the alloys used in refineries and petrochemical plants. Organic acids, regardless of their molecular weight, can aggressively attack alloys by breaking down their passive protective layers, which are typically formed by oxides of alloying elements like chromium, nickel, and molybdenum. The type and concentration of alloying elements become critical in determining the resistance of a material to AOAC.

Alloying elements such as chromium play a vital role in forming a stable passive oxide layer, which acts as a barrier to acid attack. For instance, in stainless steels, chromium reacts with oxygen to form a thin, adherent layer of chromium oxide (Cr?O?) that protects the underlying metal:

4 Cr + 3 O? → 2 Cr?O?

Lower molecular weight acids, such as formic and acetic acids, can penetrate this layer, especially at elevated temperatures, leading to its breakdown. This process occurs due to the reaction between organic acids and the oxide layer, which results in the reduction of the oxide and the release of chromium ions into solution. For example, acetic acid can react with chromium oxide as follows:

Cr?O? + 6 CH?COOH → 2 Cr3? + 6 CH?COO? + 3 H?O

This reaction dissolves the protective oxide layer, exposing the underlying metal to further attack. The hydrogen ions (H?) released by the dissociation of the acid contribute to the corrosion process by lowering the pH, making the environment more aggressive. Once the oxide layer is compromised, the acid can interact directly with the iron or nickel matrix, accelerating corrosion.

Nickel, as an alloying element, contributes to corrosion resistance by stabilising the austenitic structure and enhancing the overall ductility and toughness of the alloy. In nickel-based alloys like Alloy 825, the presence of both chromium and nickel provides enhanced resistance to both oxidising and reducing acids. However, these alloys are not immune to the effects of organic acids.

In the case of formic acid, nickel tends to form stable complexes with formate ions, leading to metal dissolution. The dissolution of nickel can be represented by the following reaction:

Ni + 2 HCOOH → Ni(HCOO)? + H?

Here, the formate complex (Ni(HCOO)?) is formed, which can lead to metal dissolution and under-deposit corrosion. The effectiveness of nickel alloys in resisting higher molecular weight acids like pentanoic and hexanoic acids is due to their ability to maintain a more stable oxide layer at high temperatures, where these acids are more soluble and active. Nickel's passive oxide layer is primarily composed of nickel oxide (NiO), which, like chromium oxide, can be dissolved by organic acids over time:

NiO + 2 CH?COOH → Ni2? + 2 CH?COO? + H?O

Molybdenum is another critical element in combating corrosion, particularly localised forms like pitting and crevice corrosion, which are common in the presence of organic acids. Molybdenum improves the stability of the passive layer in acidic environments and helps reduce the formation of active sites where corrosion can initiate. This is particularly important in the context of organic acids with longer chains, like butyric and hexanoic acids, which tend to cause localised attack under certain conditions.

Intergranular Corrosion

The interaction between organic acids and metal surfaces is not merely a chemical dissolution process; it involves the disruption of the metallurgical structure at a microscopic level. Organic acids can lead to intergranular corrosion, where the acids preferentially attack grain boundaries in alloys with insufficient stabilisation. This is often seen in stainless steels that lack stabilising elements like titanium or niobium. The grain boundaries contain segregated alloying elements or impurities, making them more reactive with organic acids.

Stress Corrosion Cracking (SCC) and Hydrogen Embrittlement

Moreover, organic acids can lead to stress corrosion cracking (SCC) in metals under tensile stress, especially in high-temperature environments. The dissociation of acids and the subsequent release of hydrogen ions facilitate hydrogen embrittlement, particularly in steels and nickel alloys. The smaller acids, like formic and acetic acids, dissociate readily into corrosive ions that can penetrate metal lattices, causing cracks to propagate. The hydrogen ions released in these environments can enter the metal lattice, forming atomic hydrogen (H), which then leads to embrittlement and cracking under stress:

2 H? + 2 e? → H?

This phenomenon is exacerbated by the smaller acids, which more readily dissociate into hydrogen ions, facilitating SCC and hydrogen embrittlement.

Formation of Organic Acids and Refining Scenarios

Organic acids in refineries can originate from crude oil or form as a result of various processes during refining. Understanding the formation of these acids is crucial for mitigating their corrosive effects. Crude oil naturally contains a variety of organic compounds, including fatty acids and their derivatives. During the refining process, these compounds can break down into organic acids due to thermal and catalytic degradation. For instance, in crude distillation units, the high temperatures can decompose hydrocarbons into simpler organic acids such as formic, acetic, and propionic acids. These acids often form as a result of thermal cracking and hydrolysis of larger organic molecules.

In crude distillation towers, the process involves heating the crude oil to separate it into different fractions based on boiling points. At high temperatures, reactions can occur that lead to the formation of organic acids. For example, the thermal cracking of fatty acids or esters can yield formic and acetic acids:

R-COOR' + H?O → R-COOH + R'-OH

Here, an ester (R-COOR') undergoes hydrolysis to form an organic acid (R-COOH) and an alcohol (R'-OH).

Organic Acids in Vacuum and Coker Units

In vacuum distillation and coker units, the formation of organic acids is similarly influenced by the presence of water and elevated temperatures. These units operate under high temperatures and pressures to process heavier fractions of crude oil. The presence of water, especially in the form of steam or condensation, can facilitate the formation of organic acids through reactions such as hydrolysis.

If sufficient organic acid is present, AOAC is likely to occur at the acid dew point and in areas where condensation occurs, such as the first water condenses and downstream of cold reflux areas. For instance, in coker units, where the cracking of heavier hydrocarbons occurs, the resultant organic acids can accumulate, particularly at lower temperatures where condensation of these acids can lead to corrosive conditions. The breakdown of heavier hydrocarbons can produce acids like butyric and pentanoic acids, which are less volatile but still corrosive:

C??H?? + 2 H?O → C?H?O? + C?H??O

Here, a heavier hydrocarbon (C??H??) breaks down into butyric acid (C?H?O?) and other by-products.

Effect of Operating Temperature on Organic Acid Formation

Operating temperature plays a pivotal role in both the formation and corrosivity of organic acids. High temperatures can accelerate the dissociation of these acids, thereby increasing their corrosive potential. Elevated temperatures also enhance the solubility of metal ions in the acidic solution, leading to more severe corrosion. For instance, the formation of acetic acid is more pronounced at higher temperatures, which can lead to increased concentrations in refinery units:

CH?COOH (gas) ? CH?COOH (liquid)

While it is true that higher temperatures generally reduce the solubility of gases in liquids, the behaviour of organic acids like acetic acid is different. Elevated temperatures can actually increase the solubility of acetic acid in water and enhance its reactivity. This increase in solubility, combined with accelerated dissociation into hydrogen ions (H?) and acetate ions (CH?COO?), can significantly intensify the acid’s corrosive effects on metal surfaces.

Impact on Specific Refinery Units

In crude distillation, vacuum, and coker units, organic acids form as a result of high temperatures and the presence of water. AOAC is most likely to occur at or below the acid dew point and in areas where water condenses. For example, in the overhead systems where water first condenses and downstream from there, corrosion can be severe if not mitigated by appropriate alloy selection or corrosion inhibitors. Corrosion often becomes a concern when a previously effective corrosion inhibitor system fails, which may coincide with the introduction of a new crude oil with a different acid profile.

In visbreaker units, which operate at elevated temperatures to break down heavy hydrocarbons, organic acids can still cause corrosion even when temperatures exceed the calculated water dew point. While organic acids like acetic acid can remain in the gas phase at these higher temperatures, their corrosive potential does not rely solely on the presence of liquid water. Instead, the acids can condense and interact with metal surfaces directly, even in their gaseous phase.

Organic acids, such as acetic acid, can react with metal surfaces through a process where they don’t require liquid water to cause corrosion. At elevated temperatures, acetic acid may evaporate and then re-condense on cooler surfaces, forming an acidic film that remains corrosive. The process can be described as follows:

CH?COOH (gas) ? CH?COOH (liquid) at high temperature

Here, even if the condensation of acetic acid occurs at temperatures higher than the water dew point, it can still create an acidic environment that corrodes metal surfaces. The formation of acidic films or residues from these acids can be enough to maintain corrosive conditions, leading to significant metal degradation.

Alloy Selection for Resistance to Organic Acids

In the quest to combat aqueous organic acid corrosion (AOAC) in refineries and petrochemical plants, the selection of appropriate alloys is crucial. The alloys chosen must be robust enough to withstand the aggressive nature of organic acids while meeting the demands of operating conditions.

Stainless Steels

Stainless steels, such as 316L, are commonly employed due to their inherent resistance to corrosion. The presence of chromium in these alloys is pivotal, as it forms a passive oxide layer on the metal surface. This chromium oxide (Cr?O?) layer acts as a protective barrier against corrosive agents. The addition of molybdenum in 316L stainless steel further enhances its resistance, particularly against chloride-induced pitting, which is a frequent issue in acidic environments. The fine-grained structure of 316L provides improved toughness and resistance to localised attack, making it well-suited for environments where organic acids are present.

Nickel Alloys

Nickel-based alloys, such as Alloy 825, are another excellent choice for resisting AOAC. These alloys are rich in nickel, chromium, and molybdenum. Nickel imparts exceptional resistance to reducing environments, while chromium offers protection against oxidizing conditions. The face-centred cubic (FCC) crystal structure of nickel alloys allows for high ductility and resistance to stress-induced cracking. The alloying elements work together to promote the formation of a stable, protective oxide layer that resists breakdown by organic acids, including those with higher molecular weights.

Operational Considerations

Despite the robust properties of these alloys, managing corrosion effectively often requires more than just material selection. The use of corrosion inhibitors is a common method for controlling corrosion in overhead systems. Even with corrosion-resistant alloys, inhibitors are necessary to protect equipment downstream. These inhibitors are primarily designed to control inorganic acid corrosion but can also be effective against AOAC. As the presence of aqueous organic acids increases, there may be a need to adjust the inhibitor programme. This might involve increasing dosage rates or modifying the chemicals used, as the demand for neutralising chemicals grows with higher concentrations of organic acids.

Filming Amines

When considering inhibitors, it is important to use them judiciously. Filming amines, while effective, should be used only when absolutely necessary. They have the potential to cause fouling and other operational problems within the system. Their application requires careful evaluation to avoid complications that might arise from their use.

Conclusion

Aqueous Organic Acid Corrosion (AOAC) poses significant challenges in the oil and gas industry, particularly within refineries and petrochemical plants. A deep understanding of the atomic-level chemical and metallurgical interactions between organic acids and metals is essential for mitigating these corrosive effects. Selecting the right alloys, informed by the specific chemical nature of organic acids and their behaviour under various conditions, is crucial for ensuring the longevity and safety of refinery infrastructure. Alongside alloy selection, implementing a well-considered inhibitor programme and managing the use of filming amines can further enhance corrosion control.

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