【Weekly Topic】1.4~1.10 Why do areas where phase changes occur corrode easily?

Firstly, the phase transition mentioned here refers to the vapor-liquid phase transition. When the liquid phase transforms into a vapor phase, some components that were originally dissolved in the liquid phase do not evaporate along with the main portion of the liquid due to their lower volatility; as a result, their concentration in the remaining liquid increases. When this concentration exceeds their corresponding solubility limit, these components may even precipitate out. If such components are corrosive, then the phase transition **increases their concentration at the site of phase transition, thereby enhancing corrosion there. A common example is the top of the desorption tower in hydrodesulfurization units, where acid water is carried back into the tower by reflux. When the vapor phase turns into a liquid phase, if highly corrosive substances are present in the vapor and can dissolve easily in the liquid, the high concentration of these substances in the liquid leads to an accelerated rate of corrosion. Another common example is the condensation of water in the pipelines at the top of atmospheric pressure towers; this is why additives are used at the top of such towers. There are many similar situations

It is not the areas where the medium undergoes phase change that are prone to corrosion; rather, when the gas phase transforms into a liquid phase, corrosion shifts from being primarily physical in nature to being primarily chemical in nature. Chemical corrosion is more severe than physical corrosion, and all chemical corrosion reactions are ionic reactions. Once the gas phase turns into a liquid phase, these ionic reactions intensify, which can result in more severe corrosion.

Why does corrosion occur easily in areas where gas-liquid phase changes take place? Firstly, the phase transition mentioned here refers to the vapor-liquid phase transition. When the liquid phase transforms into a vapor phase, some components that were originally dissolved in the liquid phase do not evaporate along with the main portion of the liquid due to their lower volatility; as a result, their concentration in the remaining liquid increases. When this concentration exceeds their corresponding solubility limit, these components may even precipitate out. If such components are corrosive, then the phase transition **increases their concentration at the site of phase transition, thereby enhancing corrosion there. A common example is the top of the desorption tower in hydrogenation desulfurization units, where acid water is carried back into the tower by reflux. When the vapor phase turns into a liquid phase, if highly corrosive substances are present in the vapor and can dissolve easily in the liquid, the high concentration of these substances in the liquid leads to an accelerated rate of corrosion. Another common example is the condensation of water in the pipelines at the top of atmospheric pressure towers; this is why additives are used at the top of such towers. There are many similar situations.

When the gas phase transforms into a liquid phase, corrosion shifts from being primarily physical to being primarily chemical. Chemical corrosion is more severe than physical corrosion, and all chemical corrosion reactions are ionic reactions; as the gas phase turns into a liquid phase, these ionic reactions intensify, which may result in more severe corrosion.

When the gas phase transforms into a liquid phase, corrosion shifts from being primarily physical to being primarily chemical. Chemical corrosion is more severe than physical corrosion, and all chemical corrosion reactions are ionic reactions; as the gas phase turns into a liquid phase, these ionic reactions intensify, which may result in more severe corrosion. However, it is not enough to consider only the temperatures of the gas-phase and liquid-phase media; the corrosion caused by a high-temperature gas-phase medium can be greater than that caused by a superheated liquid-phase medium. Factors such as the material composition of the equipment also need to be taken into account.

When the liquid phase transforms into a vapor phase, some components that were originally dissolved in the liquid phase do not evaporate along with the main portion of the liquid due to their lower volatility; as a result, their concentration in the remaining liquid increases. When this concentration exceeds their corresponding solubility limit, these components may even precipitate out. If such components are corrosive, then the phase change **increases their concentration at the site of phase transition, thereby enhancing corrosion there. A common example is the top of the desorption tower in hydrogenation desulfurization units, where acid water is carried back into the tower by reflux. When the vapor phase turns into a liquid phase, if highly corrosive substances are present in the vapor and can dissolve easily in the liquid, the high concentration of these substances in the liquid leads to an accelerated rate of corrosion. Another common example is the condensation of water in the pipelines at the top of atmospheric pressure towers; this is why additives are used at the top of such towers.

Vapor-liquid phase transformation: When the liquid phase transforms into a vapor phase, some components that were originally dissolved in the liquid phase do not evaporate along with the main portion of the liquid due to their lower volatility. As a result, their concentration in the remaining liquid increases; when this concentration exceeds their corresponding solubility limit, these components may even precipitate out. If such components are corrosive, then the phase transformation increases their concentration at the site of transformation, thereby enhancing corrosion there. A common example is the top of the desorption tower in hydrogenation desulfurization units, where acid water is carried back into the tower by reflux. When the vapor phase transforms into a liquid phase, if highly corrosive substances are present in the vapor and can dissolve easily in the liquid, the low initial volume of liquid means that high concentrations of these corrosive substances will lead to an accelerated rate of corrosion. Another common example is the condensation of water in the pipelines at the top of atmospheric pressure towers; this is why additives are used at the top of such towers. There are many similar situations.

Why does corrosion occur easily in areas where gas-liquid phase changes take place? Answer: This is because the region where vapor-liquid phase change occurs is a temperature zone with rapid changes in temperature, which quickly destroys the oxide film on the metal surface, thereby accelerating corrosion and making it more likely to occur. When the phase changes from gas to liquid, corrosion shifts from being primarily physical to being primarily chemical, with chemical corrosion being more severe than physical corrosion.

Firstly, the phase transition mentioned here refers to the vapor-liquid phase transition. When the liquid phase transforms into a vapor phase, some components that were originally dissolved in the liquid phase do not evaporate along with the main portion of the liquid due to their lower volatility; as a result, their concentration in the remaining liquid increases. When this concentration exceeds their corresponding solubility limit, these components may even precipitate out. If such components are corrosive, then the phase transition **increases their concentration at the site of phase transition, thereby enhancing corrosion there. A common example is the top of the desorption tower in hydrodesulfurization units, where acid water is carried back into the tower by reflux. When the vapor phase turns into a liquid phase, if highly corrosive substances are present in the vapor and can dissolve easily in the liquid, the high concentration of these substances in the liquid leads to an accelerated rate of corrosion. Another common example is the condensation of water in the pipelines at the top of atmospheric pressure towers; this is why additives are used at the top of such towers. There are many similar situations