| Friction stud welding(FSW)has the advantages of high efficiency,low material consumption and being conducive to the underwater welding.It has significant advantages in solid-phase connection of offshore working platform and submarine pipeline.However,the corrosion of welded joints have seriously affected the service life and safety of steel structure connections.In order to solve the above problems,in this project,different duty ratio Ni coatings were prepared on the surface of underwater FSW joints by pulse electrochemical deposition method to study the anti-corrosion.The surface microstructure of the coating was observed by micro spectroscopy and other testing methods.The corrosion behavior of the coating was analyzed by means of macro electrochemical testing method.The local corrosion law of joint surface and coating surface defects were innovatively explored by using micro electrochemical scanning system.The main results are as follows:The welding joint and coating characterization results:the grain size and hardness of FSW joint welded zone are the smallest and densest,and the corrosion products Fe2O3?Fe3O4 and Fe OOH were formed after the corrosion reaction;for three kinds of Ni coatings with different duty ratios,the higher the duty ratio is,the more uniform and dense the coating is,the thickness of 50%,80%and 100%duty ratio coating is 4.2,5.4 and7.8?m respectively;with the increase of duty ratio,the corrosion pits on the surface of the coating samples decreased after 7 days of immersion corrosion.The macro electrochemical test results:FSW joint and Ni coating with 50%,80%and 100%duty ratio were immersed in 3.5%Na Cl solution for 7 days,and the coating with 100%duty ratio has the most positive self-corrosion potential(-628.441m V),the lowest corrosion current density(2.413?A/cm2)and the maximum polarization resistance(6.8508k?·cm2).It can be seen that the increase of duty ratio can effectively improve the corrosion resistance of joint specimens.The micro-zone electrochemical test results:the least corrosion current density(about 60.00?A/cm2)and the highest local impedance(about 114.6k?)were found in the welded zone,which showed that the welded zone has the best corrosion resistance;with the increase of duty ratio,the corrosion current density in the scratch area was bigger than that in the coating part,the maximum corrosion current density decreased(from 32.80?A/cm2,26.20?A/cm2 to 16.10?A/cm2),the corrosion resistance of the defect area was higher than that of the coating area,and the maximum impedance value of the defect area diffuses more and more(from 1.598×105?,1.985×105?to 2.022×105?).Local corrosion propagation at defects:quantitative test of the propagation depth and width of the scratch area of the coating shows that the depth of the bottom of the coating defect from the surface of the sample is the smallest(about 21.3?m)and the propagation width is the smallest(about 106.3?m)when the duty ratio is 100%;which indicates that with the increase of the duty ratio,the degree of further corrosion damage of the substrate under the coating with defects is gradually reduced,and the corrosion protection of the coating is better higher performance. |
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