Corrosion and protection of inter-stage cooler housings in petrochemical hydrogen compressors
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The inter-cooler in the hydrogen compressor section of a chlor-alkali plant belonging to Sinopec Group is made of 0Cr18Ni9 material; the medium flowing through it is H2, with small amounts of impurities such as ammonium chloride (NH4Cl), chlorine (Cl2), sulfur dioxide (SO2), and carbon dioxide (CO2). The operating temperature is 140°C, while the operating pressures are 0.7 MPa and 1.6 MPa respectively. After operating for several cycles, corrosion and perforation occurred at the top of the device’s wall, leading to leakage and the shutdown of the entire unit, which resulted in significant economic losses. Through inspection and testing of the failed shell of the interstage cooler in the hydrogen compressor, it was concluded that the main causes of failure are pitting corrosion (crevice corrosion) and stress corrosion cracking (SCC) induced by ammonium chloride (NH4Cl) and chlorine gas (Cl2). With the promotion of Cl-, crack propagation-type stress corrosion cracking (SCC) caused by a small amount of hydrogen sulfide (H2S) may also occur. In the cooler, intergranular corrosion still exists due to the dispersed precipitated secondary phases present in the original austenitic structure. The original design of these two coolers used inappropriate materials, as 0Cr18Ni9 (304L) is not resistant to corrosion by boiling ammonium chloride; even 316L stainless steel is susceptible to pitting and stress corrosion. Therefore, a duplex stainless steel resistant to pitting and stress corrosion should be selected. From a process perspective, it is necessary to minimize the concentration of halide ions in the environment, increase the pH value, and add an appropriate amount of corrosion inhibitor to prevent corrosion caused by the adsorption of chloride ions. By implementing the above measures, the corrosion of the shell of the inter-stage cooler in the chlorine compressor section was effectively slowed down, ensuring the stable operation of the chemical processing plant.
Due to improper design and material selection, the inter-stage cooler of this hydrogen compressor uses 0Cr18Ni9 material, which is prone to pitting and stress corrosion when operating in an environment containing chlorides, leading to corrosion-induced perforation and leakage. It is recommended to use corrosion-resistant duplex stainless steel, and to reduce the concentration of halide ions, increase the pH value, and add corrosion inhibitors in the manufacturing process, in order to slow down the corrosion process and ensure the safe and stable operation of the equipment. .