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Example: In the PVC production process of a certain company, the converter used is a tubular heat exchanger; both the shell and the tubes are made of carbon steel, with the tubes being sealed to the sheet metal via expansion joints. Acetylene gas and hydrogen chloride gas pass through the converter from bottom to top after being mixed and dehydrated, thereby undergoing an addition reaction to produce vinyl chloride gas. The reaction temperature is 120~180°C. Leaks began to occur after more than 1 year of use, with the longest duration being 18 months and the shortest being less than 13 months. The main cause of corrosion is water in the feed gas. Secondly, the quality of the hot water in the shell side is poor, with a low pH value and high chloride ion content. Analysis: Carbon steel exhibits good corrosion resistance in dry hydrogen chloride gas, with a corrosion rate of 0.05–0.5 mm/year; however, when the hydrogen chloride gas contains more than 0.02% moisture, the corrosion rate increases significantly. Similarly, carbon steel is also corrosion-resistant in chlorine with extremely low moisture content (including liquid chlorine), whereas moisture in chlorine causes severe corrosion of carbon steel. Therefore, for carbon steel equipment used to handle dry chlorine (liquid chlorine) and hydrogen chloride gas, it is extremely important to control the moisture content.