Corrosion Mechanism Of External Surface Of Barrel And Laser Cladding Process

Corrosion is the main problem of Marine equipment and facilities,which can reduce the mechanical properties of the metal structure and shorten the life span,thus posing potential safety hazards.Therefore,the fundamental way to improve the corrosion resistance is to block the chemical galvanic effect.One of the metal surface anticorrosion measures is to use traditional electroplating chromium,but because of the thermal expansion coefficient of chromium and there was some difference of thermal expansion coefficient of metal matrix,some thermal components in high temperature and high pressure under cycle loading,surface chromium layer can produce micro cracks,provides a channel erosion matrix,which destroys the barrier for the chrome layer.In recent years,traditional electroplating technology has been replaced by other surface technologies in order to strengthen environmental protection and prevent the damage of+6 chromium ion.Laser cladding technology is a new type of material surface modification,it is suitable for all kinds of metal surface modification and repair,and its principle is of high energy laser beam forming a molten pool in the surface of the metal substrate,the feeding device through the conveying to the molten pool of metal powder or presets on the substrate coating melts in advance,then rapid solidification and matrix form metallurgical bonding,coating and dense,the cladding material types,thermal deformation and heat affected zone small advantages of higher efficiency.In this article,through the macroscopic and microscopi observation of the corrosion barrel and corrosion pit distribution statistical analysis of the chromium plating layer and the surface corrosion mechanism,designed to simulate the corrosion experiments,the results of the analysis and validation and carry out the laser cladding Hastelloy C-276alloy powder technology research,optimized parameters,the cladding layer,good quality is prepared on the microstructure,microhardness,corrosion resistance and mechanical properties are analyzed.In this paper,the application and development of Marine corrosion protection related research and laser cladding repair technology in Marine equipment and facility maintenance have certain theoretical significance and application value.The corrosion state of the body tube was analyzed by observation and measurement.By dividing the body tube into several regions,the number of rust pits in each region and the corrosion area and depth were calculated,and the distribution law was studied.According to the results,near the high voltage part 1¹0 areas of corrosion degree is higher,number and distribution of corrosion pit,especially in the area of 1⁴ single largest corrosion pit was reached nearly 100mm²,and maximum depth is 2mm.analysis of the reason is that the region higher firing temperature and the firing chamber pressure,and chromium layer and thermal expansion coefficient and elastic modulus of matrix exists obvious difference,the deformation at the root of the chromium plating layer and matrix,the plating chromic layer that lead to near recommend parts easier to cracking and spalling,matrix exposed to corrosion medium,the late deposit under the harsh Marine environment for a long time,make the matrix continued fast corrosion.In the straight line between area of the tank corrosion is most serious,the parts of the cooling groove section size changed significantly,when the water is relative to the body tube surface of high-speed movement,on the one hand,scour damage metal protective film,and eddy current in the local area,local compressed body and chromium layer on the surface of the pipe damage,on the other hand can increase the dissolved oxygen content,accelerating corrosion matrix.Using scanning electron microscope and EDS to study the micro-morphology of the pits,it was found that there were a certain number of penetrating microcracks in the chrome plating layer due to defects in the preparation and damages in the process of use?especially shooting?.Chloride ion radius is small,penetration ability is strong,under the action of diffusion more easily than other ions through the micro-cracks in the chromium plating layer,reach the steel substrate at the interface,and interact with it to generate soluble chloride?such as ferric chloride,etc.?Then,the micro-cavity is left behind by the matrix metal under the crack tip;at the same time,the dissolved oxygen molecules in the corrosive medium smoothly reach the steel matrix,and rust is generated at the interface between the chromium layer and the steel matrix.Accelerated laboratory simulation is a test method that accelerates corrosion by artificially controlling test conditions.It seeks to determine the tendency of a certain metal material to corrode,the relative corrosion resistance of a material,or the aggressiveness of a medium within a short period of time.After literature research,we designed two sets of simulated corrosion experiments.One was electrolytic accelerated corrosion test;the other was seawater immersion corrosion experiment.The experimental results are basically consistent with the analysis results of the rust corrosion mechanism.The electrolytic acceleration experimental results show that the corrosion process of the simulative test tube includes the bulging of the chrome layer,the increase of the bulging,the generation of micro-cracks,and the corrosion of the chromium layer.The corrosion morphology and the original Similar to pitting,the rust layer is roughly divided into two layers.The corrosion product of the outer layer is loose and the corrosion product of the inner layer is dense.The seawater immersion experimental results show that the accumulation of corrosion products causes the volume to expand,and the chromium coating on the edge of the prefabricated defect is applied.Under bending load,cracks are formed on the lower surface of the chromium layer,and the fracture section is in an angle of 45°.The fracture pattern of the chrome plating layer on the edge of the pit is basically the same as the actual corrosion process.For this project,the choice of laser cladding material should not only consider the seawater corrosion resistance of the coating,but also need to consider the matching and compatibility of the cladding material and the substrate.Through analysis and research,it was finally decided to select Hastelloy C-276 alloy,which is called"universal anti-corrosion alloy".It has extremely low silicon and carbon content and special physical,mechanical and corrosion resistance,and it is resistant to a large temperature range.The erosion of most corrosive media.For the Hastelloy C-276 alloy,the process parameters of the laser cladding technology were studied,and the cladding layer with good quality was prepared using the optimized parameters.Microstructure observation and microhardness,mechanical properties and corrosion resistance tests were performed.Orthogonal experiment was used to optimize the single-pass cladding process parameters.Through orthogonal test analysis,it was found that the scanning speed had a great influence on the macroscopic size of the cladding layer.After analysis and comparison,the optimized process parameters were finally determined:laser power 2000 W,The scanning speed was10 mm/s and the feed rate was 1.0r/min.The best overlap rate was determined by multiple laps and was 43%.The microstructure of the cladding layer is mainly composed of cellular structure?ie,primary phase?and grid-like structure.Affected by heat flow factors,the microstructure of each part is different,and the dendrite gradually becomes isometric from bottom to top.crystal.The average microhardness of the laser cladding layer is about280 HV.Since the minimum hardness of the substrate in the dilute alloying zone of the cladding layer reaches 230HV,the phase change occurs in the substrate of the heat-affected zone,the highest hardness reaches 530HV,and the average hardness of the substrate is stable at about 350HV.The results of mechanical test show that the yield strength of Hastelloy C-276 laser cladding coating reaches 600MPa,the elastic modulus of Hastelloy C-276 is 205GPa?room temperature?,the elastic modulus of gun steel is about203GPa?room temperature?,and the elasticity of chromium The modulus is 270GPa,so it can be judged that the deformation coordination ability of Hastelloy C-276 coating and gun steel is better than the deformation coordination ability of chromium plating layer and gun steel.When the tensile strength of the composite structure reached 1000 MPa and the strain reached 3%,the laser cladding layer fractured,and it also reflected that the laser repair layer and the gun steel substrate had better deformation coordination ability.After the lateral tensile test,there is no interface cracking between the laser cladding layer and the substrate,reflecting the excellent interface bonding between the laser cladding layer and the substrate.According to the electrochemical test,the corrosion potential of the Hastelloy C-276 cladding layer is-0.326 V,and the corrosion current density is 3.59×10^-6A/cm^-2.Compared with the chrome plating layer,it has the same excellent corrosion resistance performance.Instead of the chromium layer as a corrosion protective coating.The pits on the surface of the barrel can be regarded as sample damage.In the study,the pits with different degrees of corrosion can be replaced by prefabricated damage of different depths and widths of milling.By discussion,it is considered that the milling groove is the most practical method for prefabricating damage.For the barrel sample of this project,the range of milling slope is recommended to be between 60°and 70°.In the actual repair process,this can be used as a reference for repair work.The parameters can be adjusted as necessary according to the actual situation.