Seeking advice on carbon dioxide corrosion issues

This post was last edited by runisuoshi on 2011-12-31 18:02. 1. Mechanism of carbon dioxide corrosion: The destructive effects of carbon dioxide corrosion manifest differently at the cathode and the anode. At the anode, iron continuously dissolves, leading to uniform corrosion or localized corrosion, which is manifested as a gradual thinning of the wall thickness of metal structures or localized corrosion damage such as pitting and perforation； At the cathode, carbon dioxide dissolves in water to form carbonic acid, releasing hydrogen ions. Hydrogen ions are strong depolarizers; they readily capture electrons and get reduced, thereby promoting the dissolution of iron at the anode and causing corrosion. At the same time, hydrogen atoms enter the steel, leading to cracking in metal components. 2 Factors affecting carbon dioxide corrosion 2.1 Influence of temperature Temperature is a key factor influencing carbon dioxide corrosion. Below room temperature, a transparent corrosion-passivation film forms on the surface of carbon steel exposed to carbon dioxide-containing solutions; analysis shows that this film does not contain carbonate ions. Since it is not in the thermodynamically most stable state, it does not provide effective protection for the metal ； When the temperature rises to 50–60°C, although the corrosion rate increases, it also facilitates the formation of a carbonate corrosion product film. This type of corrosion product has low solubility and provides good protection; at this stage, uniform corrosion is the dominant form ； As the temperature continues to rise above 60°C, ferrous carbonate forms on the metal surface, and the corrosion rate is determined by the mass transfer process through the barrier layer, that is, by the combined effect of the permeability of the scale, the inherent solubility of the scale itself, and the flow rate ； Within the range of 60–110°C, the corrosion products are thick and loose, with coarse and uneven crystals that are prone to breaking, resulting in severe localized pitting ； When the temperature is above 150°C, the corrosion products are fine, compact, and have strong adhesion; analysis shows the presence of magnetic iron oxide, which leads to a decrease in the corrosion rate, thereby providing a certain degree of protection. Furthermore, changes in temperature affect the corrosion rate by altering the pH value of the medium. 2.2 Effect of carbon dioxide partial pressure The carbon dioxide partial pressure is a direct factor influencing carbon dioxide corrosion. It is generally believed that corrosion can be disregarded when the carbon dioxide partial pressure is below 0.021 MPa ； Corrosion will occur when the carbon dioxide partial pressure reaches 0.021 MPa ； When the carbon dioxide partial pressure is higher than 0.021 MPa, appropriate anti-corrosion measures should be taken. Generally, when the carbon dioxide partial pressure is below 0.05 MPa, damage caused by pitting does not occur. For ordinary steel and low-alloy steel, the corrosion rate can be calculated using a worst-case empirical formula: lg v = 0.67 lgPco2 + C. This equation shows the relationship between the corrosion rate (v), the carbon dioxide partial pressure (Pco2), and the temperature correction factor (C); its reliability within certain ranges has been proven. When the carbon dioxide partial pressure is below 0.2 MPa and the temperature is below 60°C, the measured values are essentially consistent with the calculated values ； Under conditions of higher carbon dioxide partial pressure and temperature, the measured corrosion rate is generally lower than the value calculated by this formula, mainly due to the effect of the corrosion product film. 2.3 Effect of ions: The presence of chloride ions disrupts the corrosion product film on the steel surface, hinders the formation of this film, and promotes pitting of the steel beneath it. These effects are particularly evident when the chloride ion concentration is above 30 g/L; the main reason for this is that chloride ions adsorb onto the corroded surface, thereby delaying the formation of the ferrous carbonate protective film ； However, under conditions of a chloride ion concentration below 1 g/L and in an anaerobic environment, it seems to be able to provide corrosion inhibition. Calcium and magnesium ions also influence the corrosion rate by forming carbonate protective films; as the concentration of calcium and magnesium ions in the solution increases, more white carbonate deposits form on the steel, resulting in better protection. 2.4 Effect of flow rate: Flow rate is also an important factor affecting carbon dioxide corrosion. High flow rates cause mechanical fatigue in the corrosion product layer, which can easily damage the corrosion product film or hinder its formation, leaving the steel exposed and thus increasing the corrosion rate. It is generally believed that at low flow rates, the corrosion rate is controlled by diffusion ； At high flow rates, the corrosion rate is controlled by charge transfer. A flow rate of 0.32 m/s is a turning point. When the flow rate is below this value, the corrosion rate increases as the flow rate rises ； When the flow rate exceeds this value, the corrosion rate is entirely controlled by charge transfer; at this point, changes in flow rate become unimportant, and the effect of temperature becomes the main influencing factor. 2.5 Effects of the corrosion product film A good corrosion product film can **reduce the corrosion rate; the composition, structure, and morphology of this film are influenced by various factors such as the composition of the medium, pH value, carbon dioxide partial pressure, temperature, and flow rate. 3 Protection measures against carbon dioxide corrosion 3.1 Selection of steel materials Numerous studies have shown that chromium-containing stainless steels exhibit excellent corrosion resistance; as the chromium content increases, the corrosion rate of the alloy decreases. Generally, when the chromium content reaches 12%, its corrosion resistance is already very good; however, in the presence of chlorides, pitting and intergranular corrosion can occur. Below are two applicable limit values for the industrial use of these three types of stainless steels: 9Cr-1Mo, 13Cr, and 22-25Cr duplex stainless steel. The first is the critical temperature limit for the use of each type of steel – 100°C for 9Cr-1Mo steel, 150°C for 13Cr steel, and 250°C for 22-25Cr duplex stainless steel℃ ； The second is the critical partial pressure limit for H2S resistance in each type of steel: for 9Cr-1Mo steel and 13Cr steel, this limit is 0.0001 MPa; that is, as long as the partial pressure of H2S does not exceed 0.0001 MPa, these two steels will not experience a significant decline in their corrosion resistance ； For 22-25Cr duplex stainless steel, it is 0.001 MPa. Nickel can also enhance the corrosion resistance of steel, though the effect is not very significant. Steel containing 9% nickel performs satisfactorily in environments with high carbon dioxide partial pressures in terms of corrosion resistance, but cracking and pitting still occur occasionally. Low-alloy steels containing manganese and nickel have similar corrosion resistance, with manganese steel seeming to be more sensitive to pitting. Furthermore, in an environment of 175°C and high chloride levels, the Monel K-500 alloy also exhibits good corrosion resistance. If carbon steel or low-alloy steel must be used due to constraints, efforts should be made to improve the uniformity of its microstructure. 3.2 Coatings inside pipelines: The main issue with internal coatings is the treatment of the pipeline joints, and practice has shown that this approach yields good results. However, due to the complex and expensive instruments required for handling pipe joints, this method has not been widely adopted in China. 3.3 Corrosion inhibitors: The addition of corrosion inhibitors is a widely used protection measure against carbon dioxide corrosion, and no further explanation is needed here. This method is much cheaper than the first two protective measures, but its protective effect is inferior to that of the former two.

Dry CO2 gas is not corrosive on its own, but it becomes corrosive when it dissolves in water. Through water, it can initiate electrochemical reactions between steel surfaces that are in contact with each other. CO2 dissolves very easily in water, forming carbonic acid; this process releases hydrogen ions. Hydrogen ions are strong depolarizers that readily take electrons away, facilitating the reduction of iron at the anode and thus causing corrosion. The corrosion caused by carbon dioxide is primarily acidic corrosion. Carbon dioxide itself is not corrosive, but when it dissolves in water, it reacts chemically with the water to form carbonic acid, which does have a corrosive effect on metals. The higher the carbon dioxide content in water, the greater its corrosiveness. The most typical characteristics of CO2 corrosion are localized pitting, ring-shaped corrosion, and platform-like pitting. Among them, the tabletop-like pitting process is the most severe form, with a corrosion rate of up to 20 mm/a.

Carbon steel is generally used; if corrosion is a concern, then 304 material can be chosen as it offers corrosion resistance.

Is there any information on this? Or literature, etc., thank you