Corrosion Mechanism and Countermeasures in Oil Refineries - Comprehensive Review

Due to the international economic growth reliance on petroleum, corrosion is a critical problem for refineries and it has attracted considerable attention in recent times. There is a plethora of knowledge on the prevention of corrosion in petroleum refineries, but it is distributed among several scholarly studies. Therefore, a comprehensive and current analysis of corrosion prevention in refineries is required. Corrosion issues at several refinery units are examined in this paper. In addition, the foundations of the corrosion issue and modern mitigation techniques, like refinery design, cathode safeguard, inhibitors, and covering protection, were investigated. Study concludes by pointing out knowledge gaps, collecting adequate data on refinery facility corrosion, and offering suggestions for future studies.

corrosion, chloride stress cracking, under-insulation corrosion, and high temperature corrosion are just a few of the kinds of corrosion that may affect refinery equipment if the proper precautions are not followed.The explosion at Venezuela's largest oil refinery in 2012, which killed 26 people, injured 37 others, and shut down production for a long time, was caused by corrosion [5][6], [7].
There is an important need for study and in-depth laboratory investigations to determine the most significant causes and types of corrosion in oil refineries' different operating units.Additionally, research the most effective treatments for decreasing sensitivity to corrosion.The overall amount of research articles on corrosion in refineries demonstrates the scarcity of study in this field.The purpose of this review article is to present the latest information on the concerns and causes of corrosion in oil refineries.One of the future goals of the research is to find ways to stop corrosion and the economic and human losses that come with it.

Petroleum Refinery
The refinery for petroleum products is a large industrial complex within the confines of which continuous processing equipment is put to work in order to transform crude oil into consumerfriendly goods such as kerosene, gasoline, and diesel fuel [8].Refineries usually process between 104 -106 barrels of crude oil every day to make a wide variety of petroleum products.Where, desalters, top evaporation and catalyst fluid allow basic refineries to transform crude oil into marketable products and more valuable [9].Refineries use polymerization, catalytic reforming, and hydrocracking units as shown in Figure (1).Most oil refinery activities usually are susceptible to corrosion because they include the use of corrosive chemicals, high rates of flow, and high temperatures [10].

Fig. (1): Crude Schematic flow diagram in petroleum crude oil
demulsifies or pH adjusters [20].The desalter's water effluent must be maintained acidic to be effective in counteracting the role that sodium naphthenates play in creating disturbances.High crude viscosity and density complicate separation; high salt content and the probability of producing a rag layer raise the risk of fouling, which reduces desalter efficiency.The refinery's crude oil distillation unit processes crude oil by breaking it down into useful product fractions.The most important components for the distillation systems in the oil refinery are pre-flashed drums, heat exchanger systems and atmospheric distillation units [21].Hydrogen sulphide may damage the tower and condenser if dissolved in the aqueous condensation from the top refluxed barrels of the main fractionation column and water from the vacuum distillation unit.At temperatures ranging from 250 °C to 450 °C, corrosion is often likely to occur in the faster area of the crude distillation unit.Figure (3) shown how these extremely acidic salts may develop a coating on the surface of the crude distillation unit, one of the numerous ways in which corrosion might manifest itself in this apparatus [22].

Hydrotreatment units
Sulphur and nitrogen removal from petroleum refineries is critical for fraction stability and corrosion avoidance [23].These impurities, if not eliminated throughout the refining process, have the potential to harm equipment, degrade catalysts, and impair product quality [24].The fundamental goal of any hydrotreating unit is to remove the Sulphur atoms that are covalently linked to the hydrocarbon molecule.Hydrogen combines with Sulphur and Nitrogen at high temperatures (325 o C -450 o C) and pressures 70 bar in hydrotreating to form H2S and NH3 [25].The

B. Corrosion at high temperatures
High-temperature corrosion occurs when a material is subjected to a chemical assault at temperatures greater than 400 o C, generally from gases, solid or molten salts, or molten metals.
Corrosion resistance is not a sufficient criterion for selecting materials for use in high-temperature service [30].In addition to that, you have to take into consideration their creep strength and structural stability.High temperature corrosion issues are significant in refineries.Breakdowns in machinery may have catastrophic repercussions since high-temperature operations sometimes entail high pressures.There is always the danger of fire and toxic material discharges when a hydrocarbon stream ruptures [31].Besides having a significant negative effect on the environment, they might cause serious harm to persons, severe property damage, and even death [31].
Fortunately, many of the Sulphur compounds contained in crude oil are also major contributors to high-temperature refinery corrosion.Over the years, a lot of research has been done to figure out how different kinds of high-temperature sulphide corrosion work.Because of the link between corrosion rates and how long things last, it is possible to make very accurate predictions about how long they will last [30].

C. Corrosion caused by naphthalene
Naphthenic acid, one of the most frequent corrosive agents in crude oil, is one of the most aggressive and has a variety of additional impacts on refinery equipment [32].To combat the erosion caused by naphthenic acid, it is necessary to first comprehend the structure and behavior of these compounds.As an oil field ages, biodegradation sets in, and with it, a changed naphthenic acid structure.However, as they progress in sophistication, they grow more belligerent.Naphthenic acid causes corrosion in carbon steel between 210oC -240°C, with the maximal activity temperature occurring at 370°C [33].Because naphthenic acid decomposes at high temperatures, corrosion is limited to a maximum of 430oC.Naphthenic acid corrosion has been shown to take place solely in the liquid state [34].It has also been suggested that the amount of Sulphur and naphthenic acid in crude oil has an effect on the rate of corrosion.It has been the subject of extensive research in a number of studies, as shown in Figure (4) [35].Predicting how naphthenic acid will corrode is hard because of the complex interactions between the type of crude oil, temperature, flow rate, type of alloy, surface state and Sulphur contented [36].

Fig. (4): Example parts of naphthalene corrosion in refinery units [35] D. Corrosion caused by H2S
Despite the fact that both CO2 and H2S are dissolved in water inside natural gas, the quantity and concentrations of both chemicals may vary substantially depending on the sample.When carbon dioxide is the source of corrosion, a condition identified as " uniform corrosion" develops.Acid corrosion is the condition that occurs when H2S causes corrosion [37].H2S corrosion induces a breach in the metal, releasing H2S gas into the surrounding surroundings and inflicting substantial environmental harm as well as health dangers to site employees.Temperatures and pressures in equipment are crucial in avoiding corrosion [38].High temperatures accelerate hydrogen sulphide corrosion; when the distillation unit reaches 210 °C or higher, the quality of the distillate starts to degrade, and the consequences are particularly visible in drilling pipelines used in the oil industry.
Pressure lowers the acidity of H2S, improves its solubility in water, and lessens the danger of H2S corrosion by slowing the rate of heat loss [39].Temperature, flow rate, exposure length, H2S partial pressure, salt levels, metallurgy, and the kind of deposits on the mineral surface have all been identified as factors that inspiration rate and performance of H2S corrosion in refinery.Figure (5) shown H2S corrosion in several of the components.Because the different components that influence hydrogen sulphide corrosion are interrelated, it is impossible to isolate the effect of any one component on this occurrence [40].However, the oil and gas industry has conducted extensive studies on H2S corrosion [41].

E. CO2 corrosion
Carbon dioxide corrosion is common form corrosion in the petroleum industry, and is responsible for more than 62% of failures.Investments in CO2 erosion prevention represent 10%-30% of the total budgets of refiners and fossil fuels [42].The CO2 corrosion process includes the following stages that have been extensively studied in order to gain a comprehensive understanding of it.
The first stage is the dilution of CO2 with the water content of the crude oil producing carbonic acid.As for the second stage, it is dissolving the metal surface, releasing iron ions and creating hydrogen gas, and it is called (anodic and cathodic reactions) [43].Changes in temperature, CO2 partial pressure, and type of flow affect the corrosion properties of carbon-dioxide and the carbonate layer in it [43].

F. Ammonium disulfide corrosion (NH4HS)
If nitrogen is present in the feed phases, NH3 can be given as neutralizer or generated in the reactors.Since NH3 reacts with H2S to form NH4HS, it must be added to the mixture to neutralize the product [44].Carbon steel has prevalent metal in oil refinery, and is also the most displayed to NH4HS corrosion, which can cause significant corrosion at many points around the refinery, but will primarily damage corrosion and water treatment tools [45].It has been seen that every 1% increase in the concentration of NH4HS makes carbon steels more likely to rust badly.Figure (6) shows that increasing the speed from 3.4 m/s -6.4 m/s increased the wear rate of carbon steel by 40% -64%, which poses a threat to worker safety.Bubbles on the tape surface are one of the most visible indicators of NH4HS [46].
Carbon steel can only be taken out of NH4HS after it has rusted and broken up.At that point, water must be forced through the exit because there is no solid chemical treatment available at the moment.Once the protective coating is stripped away, there are a few things that need to be rectified before any more treatment can be done to lessen the environmental damage caused by NH4HS [47].

G. Ammonium chloride-induced corrosion (NH4Cl)
One of the most common reasons for the breakdown of machinery and pipes in the modern petroleum refining sector is corrosion caused by ammonium chloride.Corrosion caused by NH4Cl has a catastrophic impact on refineries because of the associated expenses of equipment, operation, and maintenance [48].Corrosion can also mess up the refinery's structure and make it harder for equipment to work as it should.Corrosion caused by NH4Cl may be seen on pipes and other metal components in places like drainage and water treatment plants, ore distillation tower devices, and fractionation stages for heat exchange units [49].Under the combined action of chlorides and sulfides, the usage of steel materials has already had inconsistent results, with some studies indicating that stress corrosion cracking and breaking have occurred.So far, only nickel alloys have shown good results in these areas because ammonium chloride contains hydrochloric acid and a weak base (NH3).It is classified as acid salt, and its corrosion is worse near the condensation point of water, when NHCl concentrations are very high [50].An indication of corrosion cracking brought on by the NH4Cl layer is viewed in Figure (7), where, carbon steel formed after 12 hours of immersion in (10%, 20%) NH4Cl resolutions, respectively [51].The main key elements must be addressed for NH4Cl corrosion:  The temperature and concentration of NH3 and HCl.
 Based on NH3 and HCl concentrations, NH4Cl salts may condense from increasing streams as they cool and harm tubes and devices above the water's boiling point.
 Due to the fact that NH4Cl salts are very acidic and rapidly absorb water, even minute quantities of water may result in severe corrosion.

Corrosion Treatment Methods
Implementing corrosion mitigation measures, such as coating or injecting corrosion inhibitors, into refinery design is essential to safeguard metallic equipment from corrosion in a harsh environment [52].Especially as the market continues to push up demand for oil and as many oil fields approach the middle and late phases of development, the materials and composition of crude oil have a direct impact on the types of corrosion that may occur and the wear that can occur on oil refining equipment.Deteriorations in crude oil quality, Sulphur content, salt content, heavy metal content, and acid value have all contributed to a rise in equipment corrosion.When you include in evident and observable material losses, it's easy to see why metal corrosion is such a concern throughout the globe [53].Furthermore, due to metal corrosion's impact on the human element, which may result in severe injuries and even death, cost-effective and efficient protective solutions have become an absolute need.Corrosion prevention methods may be broadly categorized as follows:

The refinery designs
Corrosion-causing elements should be taken into account during strainer design.By analyzing the geometry of the apparatus, one can regulate the flow rate of the fluid and prevent buildups of acidic water.Additionally, it is crucial to choose a construction material that can withstand the severe conditions inherent to refinery operations.Polymeric materials and compounds are widely used in the refining industry.The strong chemical resistance of these materials is seen in their ability to withstand exposure to many different gases and solvents.It is often believed that polymers lack both the heat resistance and mechanical characteristics.Polymers chemically breakdown at high temperatures and become brittle when cooled to low ones.As a consequence, the goals of the refinery's processes dictate the kind of polymeric materials and groups that are used.A corrosion monitoring system, such as electrical resistance sensors, must be deployed at various points around the refinery to show the corrosion condition in real-time [54].

Coating
Metals that have been painted or coated with organic compounds may be more resistant to corrosion from the effects of atmospheric gases [55].Coatings may be sorted into several categories depending on the polymer that was used to make it.Corrosion is stopped dead in its tracks by the coating's ability to regulate electrochemical properties or establish a buffer zone between the metal part and the environment.Since it is resistant to heat, wear, pitting, and erosion, it may be employed under figure (8) shown crude oil conditions [55].

Fig. (8): Corrosion protection by coatings
The following are examples of organic coatings that are often used:  Coatings consisting of alkyds and epoxy esters that, once dry, encourage the formation of an insulating layer.
 Coatings made of two components urethane.
 Rubber coatings consisting of polymer compositions of vinyl or polystyrene.
 Water soluble coatings.
 Coatings with high solid content.

Cathodic
During the cathodic protection process, the surface of the metal is changed so that it becomes the negative electrode of the electrochemical cell [56].This eliminates the possibility of corrosion of the metal in the first place.This technology is used more often in petroleum refineries so that pipelines or underground tanks can be protected from potential hazards [57].Carbon steel, stainless steel, and aluminum are just some of the metallic materials that might benefit from cathodic protection.Refineries often employ above-ground storage tanks to hold petroleum products, and these tanks are coated with a cathodic protective coating to prevent corrosion as shown in figure (9).Pure hydrocarbon liquids, as a rule, do not contribute to corrosion, which means that the internal surfaces do not need protection against corrosion [58].Corrosion inhibitors prevent a metal's decomposition when they come in touch with corrosive metal or air [59].It is possible that the inhibitors work by absorbing themselves on the surface of the metal and creating a protective layer there.Dispersion methods can be used to apply these compounds either as a solution or as a protective layer, both applications are possible [60].The ability of the inhibitor to reduce corrosion is contingent upon the following factors:  Altering the conductivity of the anodic or cathodic polarization state.
 Reducing the amount of ionization that occurs at the metal's surface is the goal.
 Bringing about an increase in the external resistance value of the metal.