| The fracture failure of downhole tools caused by corrosion is a serious threat to the the safe operation of China's petroleum industry.The electroless composite coating has excellent corrosion resistance and wear resistance,deposit uniform and no edge effect,it is widely used in corrosion protection of metal materials.Aiming at the corrosion problem of downhole tools,Ni-W-P-nSiO2 electroless composite coating was prepared on the surface of L245 steel,and the process optimization was carried out by orthogonal test and single factor analysis.The microstructure and phase composition of the coating were analyzed by optical microscopy(OM),scanning electron microscopy(SEM),X-ray diffraction(XRD),transmission electron microscopy(TEM)and selected area electron diffraction(SAED).The crystallization behavior of the coatings were studied by differential seanning ealorimetry(DSC).The structure and properties of the coating at different temperatures(300,350,400,450,500?)were studied by heat treatment experiments.The corrosion resistance of the coatings to chloride ion were studied by polarization curve(Tafel),alternating current impedance(EIS),immersion corrosion and scanning electrochemical microscopy(SECM).The main findings are as follows:(1)The optimum process conditions of Ni-W-P-nSiO2 electroless composite plating are:temperature?90 ?,PH?8.5,SiO2 addition amount?5 g/L.The best surfactant for plating is sodium dodecyl sulfate(SDS).The Ni-W-P-nSiO2 coating with the surfactant of SDS has the lowest corrosion current density(1.0755E-06 A/cm2),the highest corrosion potential(-0.33229 V),the largest capacitive arc radius,and the charge transfer resistance is 40280 Qn.The corrosion resistance of the SDS coating is the best,which is related to the uniform density of the surface of the SDS and high W content.The typical surface of Ni-W-P-nSiO2 electroless composite coating is cellular shape,the thickness is about 20 ?m,and the binding force is 30 N.(2)XRD and TEM analysis showed that the coating was a mixed structure of amorlphous and Ni nanocrystals,and the preferred orientation is Ni(111)crystal plane.The nanocrystalline size calculated by Scherrer formula is about 10 nm,which is consistent with the local grain size in the transmission electron microscope high-resolution image.(3)The coating grows at the edge or scratch of the activated substrate preferentially,and expansion along the substrate surface.The second and third layer grows on the autocatalytically active surface of nickel-based plating layer.The thickness of the plating layer is gradually increased,and the hardness increased first and then stabilized(729.2 HV).According to the crystallization Onset and peak temperature of coatings at different heating rates(5,10m 15,20?/min),the crystallization activation energy of Ni-W-P-nSiO2 electroless composite coatings was deduced by using Kissinger equation was Ep=279.737 kJ/mol and E0=306.50384 kJ/mol.(4)The heat treatment test showed that the corrosion resistance of the Ni-W-P-nSiO2 electroless composite coating is the best after the heat treatment of 350?.The corrosion current density(icorr)of the coating is 4.157E-07 A/cm2,self-corrosion potential(Ecorr)is-0.36806 V,charge transfer resistance Ret is 98240 Q.At 400?,P atoms begin to segregate and Ni3P phase precipitated,the hardness reaches the highest(950.7 HV).The amorphous structure of the coating gradually crystallizes,and the corrosion resistance is reduced.After 450? heat treatment,the oxide film was formed on the surface of the coating,and the corrosion resistance increased.While at 500?,the grain grew and the corrosion resistance decreased.Therefore,as the heat treatment temperature changes from low to high,the evolution of the coating structure:amorphous and Ni nanocrystals(300?)?amorphous and nanocrystalline aggregation(350?)?amorphous partial crystallization,Ni nanocrystals and Ni3P crystal phase(400?)?Ni3P phase increased(450?)?grain growth(500?).(5)The addition of SiO2 does not change the the morphology of cellular structure and phase structure of the Ni-W-P coating,but the surface particles of the coating are refined,the porosity is reduced,and the contact angle of the surface is increased.The corrosion rate of Ni-W-P-nSiO2 electroless composite coating,Ni-W-P electroless coating and L245 steel was 0.0552 mm/a,0.1075 mm/a,and 0.1784 mm/a,respectively.The simulated high temperature and high pressure corrosion test showed that the corrosion resistance of Ni-W-P-nSiO2 electroless composite coating was the best.In the simulated oil field corrosion environment,the Ni3S2 corrosion products are formed by uniform corrosion on the surface of the coating,and there is obvious local corrosion on the surface of the coating,but the local corrosion is not pierced,and the coating can still protect the substrate.(6)In the results of SECM test on the surface of Ni-W-P-nSiO2 coating,there was no local activation point on the test eara,indicates the preparation of the coating is uniform.The SECM analysis of the coating cross-section showed that the current of the coating is obviously lower than that of the substrate,and the corrosion resistance is increased. |
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