| Due to the advantages of low cost,high efficiency,good airtight and large transportation,the metal pipe as the main fluid-delivery tool has been widely used in many fields of national economy.With the development of China's petrochemical industry,the superiority of pipeline transport has become more and more recognized.However,the problem of metal corrosion has become the major factor causing pipeline damage and failure because of its laying environment,transport medium,equipment aging,increasing operation years and so on.The corrosion has brought huge losses to the national economy and caused great difficulties in production and life.In order to prevent the damage caused by the metal corrosion,it is necessary to study the principles of corrosion and the measures to deal with it.Metal corrosion affected by many factors is a complex process,and the electrochemical corrosion is the most important form of influencing its service life.Therefore,it has been an urgent problem to study the corrosion and protection technology of metal pipeline and slow down its corrosion process and perforation damage in the pipeline transportation engineering,which is of great significanze for ensuring the smooth progress of industrial production and reducing the transportation costs.On the basis of summarizing and reviewing the previous researches,this study firstly dicusses the flow electrification phenomenon in metal pipes and its effect on the corrosion process of pipe wall.Based on the electric double layer theory in the electrostatic field,the charge accumulated on the metal pipe wall has a polarization effect on the electrode potential of the electrochemical corrosion reaction,which affects the corrosion process of the metal pipes from the electric point of view.The study shows that the amount of charge generated and aggregated on the pipe wall increases as the flow rate of fluid raises,but the number and polarity of the charge and its change trend are various,because the different fluids with various nature have the difference in the double layer formation mechanisms at the fluids/metal interfaces.Therefore,it is advised to install the electric charge trapper in the metal pipes to reduce the accumulation of the charges and ultimately inhibit the metal corrosion.To solve the metal corrosion problem more effectively and comprehensively,a method of self-propagation high-temperature synthesizing ceramic coating on the inner surface of metal pipe is proposed.The prepared ceramic lining layer could completely isolate the metal pipe from the fluid medium,which greatly reduces the erosion of metal pipe by the corrosive fluid medium.Since alumina(Al2O3)has the characteristics of high melting point,high hardness and lightweight,it has become a hot topic for scholars at home and aborad to use the thermite system of Al-Fe2O3 to combustion synthesize the Al2O3 ceramic lined composite steel pipes.Different thermite systems have different heat releases,which have an important effect on the SHS rate.In this study,a new oxidant is introduced to stabilize the SHS reaction process of the single Al-Fe2O3 thermite system with high heat release and high reaction rate which will cause serious spatter.Therefore,the Al-Fe2O3/Al-Cr2O3 composite reaction system has been used to prepare the Al2O3-Fe-Cr ceramic lined composite steel pipe with better quality and performances.The results show that with the increase of Cr2O3 amount,the SHS reaction rate decreases,the thickness and hardness of prepared ceramic coating increase,and then the wear resistance of composite steel pipe is improved.In the composite reaction system,the generated Fe-Cr alloy phase instead of a single Fe phase is distributed in the Al2O3 ceramic dendrite gap,which reduces the shrinkage cavities formed during the solidification process and significantly improves the corrosion resistance and compactness of composite steel pipe.In addition,the gravitational separation-SHS process has formed a thin transition metal layer between the metal steel matrix and ceramic coating,which is of great significance to buffer the residual stress and improve the bonding strength of composite steel pipe.When the addition of Cr2O3 is 14wt%in the composite reaction system,a Al2O3-Fe-Cr ceramic lined composte steel pipe with comprehensive and excellent performance can be obtained,at this time,the optimum molar ratio of Al-Fe2O3/Al-Cr2O3 is 8:2 in this experiment.Porosity,as an important performance index of ceramic lined composite steel pipe,plays an important role in its wear resistance,corrosion resistance,thermal shock resistance and bonding strength.In order to improve the compactness of the ceramic lined composite steel pipe produced by the gravitational separation-SHS method,a wetting agent of Na2B4O7 is introduced to improve the wettability between the ceramic and the metal in the composite reaction system with the best ratio.Na2B4O7 can improve the wettability between the phases in the SHS products,increase the contact area between them,promote the bonding between each other,enhance the bonding force and increase the density,thus improving the strength of the composite steel pipe.By adding Na2B4O7 in the composite reaction system,the phase composition of ceramic coating is changed.In addition to the formation of ceramic phase Al2O3 and the metal alloy phase Fe-Cr,the solid solution phase(Al0.9Cr0.1)2O3 is also formed.The improvement of wettability resulted by Na2B4O7 makes the separation of Al2O3 and Fe/Cr incomplete,which increasing the content of Fe and Cr in the ceramic phase,and the transition structure of the ceramic coating-metal transition layer-steel pipe matrix is formed.The thickness of intermediate transition layer decreases as the Na2B4O7 increases,and the boundary between the ceramic coating and transition layer becomes more and more blurred,which helps to improve the toughness of the composite steel pipe.In addition,the uniform density of the ceramic coating prepared by the gravity separation-SHS method increases first and then decreases,and the ceramic coating with higher density is obtained when the amount of Na2B4O7 is added to 4 wt%.In the ceramic lined composite steel pipe obtained by the GS-SHS method,the binding between ceramic coating and metal pipe determines the bonding strength and thermal shock resistance of composite steel pipe,which has a great influence on its quality and service life.The structural defects of ceramic coating resulted by the high rate SHS process which is difficult to control have aroused the high attention of the researchers.In this study,a kind of diluent SiO2 is introduced in the composite reaction system with the best ratio to restrict the SHS reaction rate,thus the purpose of optimizing the SHS reaction process is achieved,By adding SiO2 in the composite reaction system,the melting point of ceramic phase can be reduced with generating the low melting point phase Al2O3 SiO2,which improving the fluidity of liquid phase melt and prolonging the time of liquid phase separation,and then the porosity of ceramic coating is reduced.The increase of SiO2 also reduces the rate of SHS reaction and initial crystallization temperature of primary phase Al2O3,and then increases the liquid phase of eutectic and peritectic reaction,thus accelerating the gas escape rate and promoting the densification of the ceramic coating.In addition,with the introduction of SiO2,the metal phase can evenly distribute in the Al2O3 ceramic phase in the form of particles or strips,resulting in a more uniform and smooth ceramic coating.In this study,the methods of inhibiting and reducing the corrosion of metal pipe are proposed from different angles.It provides an important theoretical basis and reference for the application of self-propagating synthetized ceramic lined composite steel pipes.Threrefore,it is helpful to promote the development of the in-situ synthesis technology in the field of metal corrosion and protection,which is of heavy meaning for prolonging the service life of the pipelines. |
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