Study On The Influence Mechanism Of Al,Cr And Mo Additions On Wear And Corrosion Performances Of Laser Cladded FeSiB Coatings

Offshore platform is the infrastructure required for offshore operations,oil drilling and production.S355 steel has excellent comprehensive mechanical properties,which becomes one of the main steels for the applications on offshore platform.S355 steel easily fails due to corrosion and wear in marine environment,and its long-term anti-corrosion and wear resistance have become one of the hotspots in the field.In this paper,laser cladding was used to prepare FeSiB,FeSiBAl,FeSiBCr and FeSiBCrMo coatings on the S355 steel,and the aim was to investigate the effects of Al,Cr and Mo addition on microstructure,corrosive wear,immersion corrosion and electrochemical performances of FeSiB coatings.The main research works and achievements were shown as follows:（1）The microstructure and mechanical properties of coatings are analyzed,and the results show that the grain shape of FeSiB coating is coarse rod-like grains and dendrites,while the FeSiBAl and FeSiBCr coatings with the Al and Cr additions are fine-grained dendrites,and the FeSiBCrMo coating with the Mo addition is further fine-grained non-directionally equiaxed crystals and dendrites.The hardness of FeSiB,FeSiBAl,FeSiBCr and FeSiBCrMo coatings is 531±31,660±24,620±28,and 584±14 HV_0.2,respectively.This shows that the additions of Al and Cr have the effects of dispersion strengthening and fine-grain strengthening,which improve the hardness and residual compressive stress of coatings.Accrrding to the Marangoni convection in the molten pool,the difference in flow rate between the Mo and other fluids forms segregation due to the large atomic mass of Mo and the high melting point,which leads to the uneven distribution of coating phases,and produces an adverse effect on the compactness and hardness of FeSiBCrMo coating.（2）The corrosive wear test of FeSiB,FeSiBAl,FeSiBCr and FeSiBCrMo coatings in 3.5%Na Cl solution shows that the wear plays a dominant role,the corrosion has an auxiliary acceleration effect,and the wear mechanism is abrasive wear.The coefficients of friction（COFs）of FeSiB,FeSiBAl,FeSiBCr and FeSiBCrMo coatings are 0.68,0.49,0.47,and 0.65,respectively;and the corresponding wear rates are 82.8,43.3,57.5,and 79.9mm³·N^-1·mm^-1,respectively.The FeSiBCr coating with the Cr addition increases the hardness of substrate and reduces the COFs.The formation of Cr₂O₃film in the friction process further improves the corrosion resistance of coating and reduces the wear rate.The corrosion resistance of FeSiBCrMo coating is the best among the four kinds of coatings,but its wear resistance is declined due to its uneven phase distribution and low hardness.The addition of Al has the most significant strengthening effect on the hardness of FeSiBAl coating to lead to the lowest COFs.At the same time,the Al₂O₃produced in the friction process has the strong corrosion resistance.Therefore,the FeSiBAl coating has the best corrosive wear resistance.（3）The immersion corrosion results of FeSiB,FeSiBAl,FeSiBCr and FeSiBCrMo coatings in 720 h show that the FeSiB coating occurs severe pitting corrosion and filiform corrosion,while the other three coatings occur only slight pitting corrosion.The Al₂O₃and Cr₂O₃films produced by the Al and Cr additions to FeSiB coating inhibit the corrosion progress,and the Mo addition further increases the thickness and stability of Cr₂O₃film.The corrosion rate of coatings is calculated by the weight loss method shows that the corrosion rate of FeSiB coating is 1/4 of S355 steel,moreover,the FeSiBCrMo coating has the best immersion corrosion resistance,and its corrosion rate is only 1/9 of S355 steel.（4）The electrochemical corrosion performance of coatings is investigated,and the results showed that the corrosion potentials of substrate,FeSiB,FeSiBAl,FeSiBCr and FeSiBCrMo coatings are-0.96,-0.71,-0.51,-0.45,and-0.43V,respectively,corresponding to the polarization resistances are 3624,5464,9701,12645,and 19016W·cm²,respectively.At the same time,through data fitting,corresponding to the the charge transfer resistances are 464,1567,4597,5546,and 9478W·cm²,respectively.This also shows that the charge transfer resistance of FeSiB increases with the addition of Al,Cr and Mo,and the electrochemical corrosion resistance of FeSiB coating with the addition of Cr and Mo at the same time is the best.