| Heat exchangers are widely used in various industrial production processes,carbon steel and stainless steel are the most widely used basic materials for heat exchangers,corrosion is one of the important factors in the service failure of heat exchangers.Heat exchange tube is the core component of the heat exchanger,heat transfer processes are caused by hot and cold fluid medium on the metal wall of heat exchange tube.On the one hand,heat transfer process changes the temperature of the metal wall of the heat exchange tube,and on the other hand,heat transfer process forms a heat transfer interface on the metal wall of heat transfer tube.Our previous research found that heat transfer effect of the metal interface will affect the effect of the corrosion inhibitors,but it is still unclear whether it affects the corrosion behavior of the metal itself.The numerical theoretical calculation of the heat transfer process is complicated and cumbersome,involving a large number of empirical formulas,the wall temperature of the metal heat transfer interface is difficult to solve and the calculation efficiency is low.Therefore,the finite element analysis software COMSOL Multiphysics 5.3a is used to calculate the self-made simulated low temperature heat transfer surface experimental device and the self-designed and modified high temperature heat transfer surface experimental device in the autoclave.Based on the numerical calculation,the electrochemical test methods and surface morphology analysis are used,the effects of low temperature?<100°C?and high temperature heat transfer?170240°C?on the corrosion behavior of Q235 carbon steel and304 stainless steel in ammonia?3 wt.%?medium were studied.The main research results are as follows:?1?The setting parameters of the ammonia water heat transfer medium temperature?T1?and ammonia solution temperature?T2?in the test system of Q235 carbon steel and 304stainless steel low temperature heat transfer surface were determined.Including three heat transfer conditions,namely heat balance condition?T1=T2?,endothermic condition?T1>T2?and exothermic condition?T1<T2?.Under three heat transfer conditions,the temperature of Q235 carbon steel and 304 stainless steel test surfaces is 50.0±0.5°C.The T1-T2 setting parameters of ammonia solution temperature under three heat transfer conditions in Q235carbon steel low temperature heat transfer surface test system are:51.5°C-51.5°C?heat balance condition?,66.0°C-38.0°C?endothermic condition?,36.0°C-66.0°C?exothermic condition?.The T1-T2 setting parameters of ammonia solution temperature under three heat transfer conditions in the 304 stainless steel low temperature heat transfer surface test system are:51.7°C-51.7°C?heat balance condition?,66.0°C-39.0°C?endothermic condition?,and35.0°C-66.0°C?exothermic condition?.?2?Electrochemical impedance spectroscopy and polarization curve electrochemical tests were carried out on Q235 carbon steel and 304 stainless steel of low temperature heat transfer surface system.The results show that in the system of Q235 carbon steel low temperature heat transfer surface,exothermic condition inhibits the corrosion of Q235 carbon steel,endothermic condition promotes the corrosion of Q235 carbon steel.The order of corrosion current density icorr is:icorr?exothermic condition?<icorr?heat balance condition?<icorr?endothermic condition?.The corrosion current density of endothermic condition is increased by about 71.3%compared with the heat balance condition,and the corrosion current density of exothermic condition is decreased by about 28.8%compared with the heat balance condition.In the system of 304 stainless steel low temperature heat transfer surface,the order of corrosion current density icorr is:icorr?exothermic condition?<icorr?heat balance condition?<icorr?endothermic condition?.The corrosion current density of endothermic condition is increased by about 23.0%compared with the heat balance condition,and the corrosion current density of exothermic condition is decreased by about 16.9%compared with the heat balance condition.The effect of low temperature heat transfer on the corrosion behavior of Q235 carbon steel and 304 stainless steel is the same.?3?The actual value of the external surface temperature?Tw?of Q235 carbon steel and304 stainless steel in the high temperature heat transfer surface system of autoclave were determined.The temperature?Ts?of ammonia solution and the temperature?Th?at the top of the heating tube were measured by thermocouples.Under the condition of high temperature heat transfer surface 1#?Ts=170°C,Th=200°C?,the value of Tw of Q235 carbon steel is226°C,and the value of Tw of 304 stainless steel is 218°C.Under the condition of high temperature heat transfer surface 2#?Ts=200°C,Th=240°C?,the value of Tw of Q235 carbon steel is 263°C,and the value of Tw of 304 stainless steel is 261°C.?4?Electrochemical impedance spectroscopy and polarization curve electrochemical tests were carried out on Q235 carbon steel and 304 stainless steel of high temperature heat transfer surface system.The results show that the existence of high temperature heat transfer surface reduces the corrosion resistance of Q235 carbon steel and 304 stainless steel.When there is a high temperature heat transfer surface 1#,the corrosion current density of Q235carbon steel is increased by about 58.5%compared with the absence of heat transfer surface,and the corrosion current density of 304 stainless steel is increased by about 97.4%.When there is a high temperature heat transfer surface 2#,the corrosion current density of Q235carbon steel is increased by 88.5%compared with the absence of heat transfer surface,and the corrosion current density of 304 stainless steel is increased by about 79.7%.High temperature heat transfer has the same effect on the corrosion behavior of Q235 carbon steel and 304 stainless steel. |
PDF Full Text Request