Corrosion Failure Analysis And Mechanism Of Fluorine Chemical Stainless Steel Materials

With the development of fluorine chemical industry in our country, equipment failure problem increase prominent which cause by material corrosion. The corrosion failure make the equipment frequent replacement, as a result it not only affect the economic efficiency of enterprise, but also increased the environment load. To investigation and analysis the corrosion failure of the stainless steel material in fluorine chemical industry, can understand the mechanism and the reason of corrosion failure. Achievements of this research will have a reference value for failure prevention of the in-service equipment under similar service conditions. At the same time, the research of the corrosion failure mechanism can provide information?direction?way and method to develop and improve the equipment of the fluorine chemical industry. Detailed research contents and results are summarized as follows.1.In this study two typical case were study by a series of characterization apparatus including OES, SEM, EDS, XRD, metallographic, etc, to study the microscopic morphologies and chemical composition of the failure part. For acetylene convey stainless steel we can get the result as follows: the material of the pipe comply with the design requirements, the failure reason is the gas temperature and humidity of the pipe did not strictly control in the use. In winter the environment temperature is low, condensate can forms in the local position in the tube. With the gas convey, the Cl-which contain in the gas melting into the condensed water, cause the concentration of Cl- up to 1200 ppm. The high concentration of Cl- cans easily erosion the surface defect of the stainless steel, which produce the pitting corrosion. The H2 S accelerate the development of pitting corrosion which inclusion in the acetylene by hydrolysis in the water. The failure analysis of the reflux condenser tower can get the result as follows. Two materials of the pipe did not conform the design requirement in the three failure pipe. Especially the one of the pipe, the carbon content is uneven. Failures in the salt water exit by suffer the erosion of Cl- and form pitting corrosion. After that under the effect of tensile stress, the pit becomes a source of stress corrosion cracking initiation, stress corrosion failure has happened. The residual materials in this pipe are leaked out, in the later use process, the corrosive medium severe corrosion another two pipe, which nearby.In order to understand the material corrosion mechanism in the failure case and the key factors of the development of the corrosion under specific working conditions, the further study was perform by laboratory test. The laboratory test use electrochemical tests to study the typical medium in the two typical cases. Potentiodynamic scanning polarization technique and electrochemical impedance spectroscopy technique were applied to study the influence of media type, media concentration, and medium temperature on the corrosion sensitivity of the stainless steel. The influence factors study for case one we can get the presence of H2 S promoted the occurrence of pitting corrosion of stainless steel in the medium which containing Cl-. For case two we can get the pitting sensitivity is increase, as the temperature increase, so the failure parts are in salt water exit. The pitting corrosion are the main corrosion failure way under the condition of low concentration of HCl and HF, but with the increase of the concentration of HCl and HF, the corrosion rate of stainless steel are increase, corrosion form translate to general corrosion.