Corrosion is a common and costly problem in many industrial processes that involve the removal of acid gases, such as carbon dioxide (CO2) and hydrogen sulfide (H2S), from gas streams using amines or sour water. Amines are organic compounds that can react with acid gases to form soluble salts, while sour water is water that contains dissolved acid gases and other impurities. Both amines and sour water are alkaline solutions that can corrode metal equipment and piping if not properly controlled.
Causes of corrosion in amine and sour water systems
The main causes of corrosion in amine and sour water systems are the presence of acids, heat, fluid velocity, solids, and metallurgy. These factors can interact in complex ways to affect the corrosion rate and mechanism of different metal surfaces.
Acids
The acids that cause corrosion in amine and sour water systems are mainly the dissolved acid gases, such as CO2 and H2S, and their ionic forms, such as bicarbonate (HCO3-), carbonate (CO32-), bisulfide (HS-), and sulfide (S2-). These acids can react with the metal surface to form iron compounds, such as iron carbonate (FeCO3) and iron sulfide (FeS), that can either protect or damage the metal depending on their stability and adherence.
In general, H2S is more corrosive than CO2 because it can form a more robust and protective FeS layer that can prevent further corrosion. However, if the FeS layer is disturbed by high fluid velocity, temperature, or pH, it can expose the metal surface to more corrosion. On the other hand, CO2 is less corrosive than H2S because it forms a more fragile and less protective FeCO3 layer that can easily dissolve or detach from the metal surface. However, if the CO2 concentration is high enough, it can lower the pH of the solution and increase the corrosion rate.
Other acids that can cause corrosion in amine and sour water systems are heat stable salts (HSS), which are organic or inorganic salts that are formed by the reaction of amines with other acidic impurities, such as hydrogen cyanide (HCN), formic acid (HCOOH), acetic acid (CH3COOH), and sulfuric acid (H2SO4). HSS can accumulate in the system and lower the pH of the solution, especially in the reboiler section where the temperature is high and the amine is regenerated. HSS can also increase the conductivity and corrosivity of the solution by forming complexes with metal ions.
Heat
Heat can increase the corrosion rate in amine and sour water systems by several mechanisms. First, heat can increase the solubility and dissociation of acid gases, which can increase the concentration and activity of the corrosive ions. Second, heat can increase the reaction rate between the metal surface and the corrosive ions, which can accelerate the formation and dissolution of the iron compounds. Third, heat can increase the evaporation rate of water, which can increase the concentration and acidity of the solution. Fourth, heat can degrade the amines and form more HSS, which can lower the pH and increase the corrosivity of the solution.
Fluid velocity
Fluid velocity can affect the corrosion rate in amine and sour water systems by influencing the mass transfer and the mechanical stress on the metal surface. High fluid velocity can increase the mass transfer of the corrosive ions to the metal surface, which can increase the corrosion rate. High fluid velocity can also increase the mechanical stress on the metal surface, which can damage or detach the protective iron compounds and expose the metal to more corrosion. Low fluid velocity can decrease the mass transfer of the corrosive ions to the metal surface, which can decrease the corrosion rate. Low fluid velocity can also decrease the mechanical stress on the metal surface, which can allow the protective iron compounds to form and adhere better.
Solids
Solids can affect the corrosion rate in amine and sour water systems by acting as abrasives, catalysts, or inhibitors. Abrasive solids, such as sand, rust, or scale, can erode or scratch the metal surface, which can damage or remove the protective iron compounds and increase the corrosion rate. Catalytic solids, such as copper, nickel, or chromium, can enhance the reaction rate between the metal surface and the corrosive ions, which can increase the corrosion rate. Inhibitive solids, such as zinc, molybdenum, or phosphate, can form a protective layer on the metal surface, which can decrease the corrosion rate.
Metallurgy
Metallurgy can affect the corrosion rate in amine and sour water systems by determining the chemical and physical properties of the metal surface. Different metals have different corrosion potentials, which can affect the electrochemical reactions between the metal surface and the corrosive ions. Different metals also have different mechanical strengths, which can affect the resistance of the metal surface to the mechanical stress caused by the fluid velocity and the solids. Different metals also have different affinities to form iron compounds, which can affect the stability and adherence of the protective iron compounds.
Corrosion control methods in amine and sour water systems
The corrosion control methods in amine and sour water systems can be classified into four categories: chemical, physical, mechanical, and metallurgical. These methods can be used individually or in combination to reduce the corrosion rate and extend the service life of the equipment and piping.
Chemical methods
Chemical methods involve the addition of chemicals to the amine or sour water solution to modify the pH, the concentration, or the activity of the corrosive ions. The most common chemical methods are:
- pH control: The pH of the solution can be adjusted by adding acid or base to increase or decrease the acidity of the solution. The optimal pH range for corrosion control depends on the type and concentration of the acid gases, the type and concentration of the amines, the temperature and pressure of the system, and the metallurgy of the equipment and piping. In general, a higher pH can reduce the corrosion rate by decreasing the activity of the corrosive ions and increasing the stability of the protective iron compounds. However, a too high pH can also increase the corrosion rate by increasing the solubility and dissociation of the acid gases and the formation of HSS. A lower pH can reduce the corrosion rate by decreasing the solubility and dissociation of the acid gases and the formation of HSS. However, a too low pH can also increase the corrosion rate by increasing the activity of the corrosive ions and decreasing the stability of the protective iron compounds.
- Corrosion inhibitors: Corrosion inhibitors are chemicals that can form a protective film on the metal surface, either by adsorption or by precipitation, to prevent or reduce the contact between the metal surface and the corrosive ions. Corrosion inhibitors can be classified into two types: anodic and cathodic. Anodic inhibitors, such as chromates, molybdates, or phosphates, can protect the metal surface by forming a passive oxide layer that can reduce the anodic reaction of metal dissolution. Cathodic inhibitors, such as amines, sulfites, or nitrites, can protect the metal surface by forming a complex with the corrosive ions that can reduce the cathodic reaction of hydrogen evolution. Corrosion inhibitors can be added to the amine or sour water solution in different locations, such as the absorber, the regenerator, the reboiler, or the overhead system, depending on the corrosion mechanism and the system configuration. The effectiveness of corrosion inhibitors depends on several factors, such as the type and concentration of the inhibitor, the type and concentration of the acid gases, the type and concentration of the amines, the temperature and pressure of the system, the fluid velocity, the metallurgy of the equipment and piping, and the presence of solids or other contaminants.
- Oxygen scavengers: Oxygen scavengers are chemicals that can react with dissolved oxygen in the amine or sour water solution to reduce the oxidation potential and the corrosion rate. Oxygen scavengers can be classified into two types: inorganic and organic. Inorganic oxygen scavengers, such as sulfites, bisulfites, or hydrazine, can react with oxygen to form sulfates, bisulfates, or nitrogen, respectively. Organic oxygen scavengers, such as hydroquinone, carbohydrazide, or methylethylketoxime, can react with oxygen to form quinones, carboxylic acids, or ketones, respectively. Oxygen scavengers can be added to the amine or sour water solution in different locations, such as the feed gas, the lean amine, the sour water, or the overhead system, depending on the source and the level of oxygen ingress. The effectiveness of oxygen scavengers depends on several factors, such as the type and concentration of the scavenger, the type and concentration of the acid gases, the type and concentration of the amines, the temperature and pressure of the system, the fluid velocity, the metallurgy of the equipment and piping, and the presence of solids or other contaminants.
Physical methods
Physical methods involve the modification of the physical conditions of the amine or sour water solution to reduce the solubility, dissociation, or activity of the corrosive ions. The most common physical methods are:
- Temperature control: The temperature of the solution can be controlled by adjusting the heat input or output of the system, such as the reboiler, the condenser, the cooler, or the heater. The optimal temperature range for corrosion control depends on the type and concentration of the acid gases, the type and concentration of the amines, the pressure and pH of the system, and the metallurgy of the equipment and piping. In general, a lower temperature can reduce the corrosion rate by decreasing the solubility and dissociation of the acid gases and the formation and dissolution of the iron compounds. However, a too low temperature can also increase the corrosion rate by increasing the viscosity and the fluid velocity of the solution. A higher temperature can reduce the corrosion rate by decreasing the viscosity and the fluid velocity of the solution. However, a too high temperature can also increase the corrosion rate by increasing the solubility and dissociation of the acid gases and the formation and dissolution of the iron compounds.
- Pressure control: The pressure of the solution can be controlled by adjusting the valves, the pumps, the compressors, or the blowers of the system. The optimal pressure range for corrosion control depends on the type and concentration of the acid gases, the type and concentration of the amines, the temperature and pH of the system, and the metallurgy of the equipment and piping. In general, a lower pressure can reduce the corrosion rate by decreasing the partial pressure and the solubility of the acid gases. However, a too low pressure can also increase the corrosion rate by increasing the evaporation and the concentration of the solution. A higher pressure can reduce the corrosion rate by decreasing the evaporation and the concentration of the solution. However, a too high pressure can also increase the corrosion rate by increasing the partial pressure and the solubility of the acid gases.
- Concentration control: The concentration of the solution can be controlled by adjusting the flow rate, the level, or the ratio of the amine or sour water solution. The optimal concentration range for corrosion control depends on the type and concentration of the acid gases, the type and concentration of the amines, the temperature and pressure of the system, and the metallurgy of the equipment and piping. In general, a lower concentration can reduce the corrosion rate by decreasing the acidity and the activity of the corrosive ions. However, a too low concentration can also increase the corrosion rate by decreasing the buffering capacity and the stability of the protective iron compounds. A higher concentration can reduce the corrosion rate by increasing the buffering capacity and the stability of the protective iron compounds. However, a too high concentration can also increase the corrosion rate by increasing the acidity and the activity of the corrosive ions.
Mechanical methods
Mechanical methods involve the modification of the mechanical design or operation of the equipment and piping to reduce the mechanical stress or damage on the metal surface. The most common mechanical methods are:
- Erosion control: Erosion control involves the reduction of the fluid velocity, the solids content, or the turbulence of the amine or sour water solution to prevent or minimize the erosion or abrasion of the metal surface. Erosion control can be achieved by using proper pipe sizing, routing, and layout, by installing flow control devices, such as orifices, nozzles, or valves, by using erosion-resistant materials, such as ceramic, rubber, or plastic, by using erosion inhibitors, such as polymers, or by using filtration or separation devices, such as cyclones, filters, or settlers, to remove the solids from the solution.
- Stress control: Stress control involves the reduction of the thermal, mechanical, or chemical stress on the metal surface to prevent or minimize the cracking or fatigue of the metal. Stress control can be achieved by using proper material selection, fabrication, and welding, by using stress relief or post-weld heat treatment, by using corrosion-resistant coatings or claddings, by using cathodic protection or anodic protection, by using stress corrosion cracking inhibitors, such as amines, or by using proper inspection and maintenance procedures.
Metallurgical methods
Metallurgical methods involve the selection of the appropriate metal or alloy for the equipment and piping to resist the corrosion in the amine or sour water system. The most common metallurgical methods are:
- Carbon steel: Carbon steel is the most widely used metal for amine and sour water systems because of its low cost, high availability, and good mechanical properties. Carbon steel can resist corrosion by forming a protective FeCO3 or FeS layer on the metal surface. However, carbon steel can also suffer from corrosion if the FeCO3 or FeS layer is unstable or damaged by high temperature, high pressure, high fluid velocity, high acid gas concentration, low pH, or high HSS concentration. Carbon steel can also suffer from stress corrosion cracking if exposed to high tensile stress and high H2S concentration. Carbon steel can be improved by adding alloying elements, such as chromium, molybdenum, or nickel, to increase the corrosion resistance, the mechanical strength, or the stability of the protective layer.
- Stainless steel: Stainless steel is a metal that contains at least 10.5% chromium, which can form a passive oxide layer on the metal surface that can resist corrosion by most acids. Stainless steel can be classified into different types, such as austenitic, ferritic, martensitic, or duplex, depending on the microstructure, the composition, and the properties of the metal. Stainless steel can resist corrosion by CO2 and H2S better than carbon steel, especially at high temperature, high pressure, high acid gas concentration, or low pH. However, stainless steel can also suffer from corrosion if the passive layer is damaged by chloride, oxygen, or HSS. Stainless steel can also suffer from stress corrosion cracking if exposed to high tensile stress and high chloride concentration. Stainless steel can be improved by adding alloying elements, such as molybdenum, nickel, or nitrogen, to increase the corrosion resistance, the mechanical strength, or the stability of the passive layer.
- Other metals: Other metals that can be used for amine and sour water systems include copper, nickel, titanium, or zirconium, which have high corrosion resistance, high mechanical strength, or high thermal conductivity. However, these metals are also more expensive, less available, or more difficult to fabricate than carbon steel or stainless steel. These metals can also suffer from corrosion if exposed to certain acids, such as sulfuric acid, nitric acid, or hydrochloric acid. These metals can also suffer from stress corrosion cracking if exposed to high tensile stress and high acid concentration. These metals can be improved by adding alloying elements, such as chromium, molybdenum, or iron, to increase the corrosion resistance, the mechanical strength, or the stability of the protective layer.
