Research On The Surface Enhancement Of Sucker Rod Coupling For Improving Wear-corrosion Resistance And Self-lubricating Performance By Plasma Cladding Composite Coating

According to statistics,more than 90%of the world’s crude oil is produced by rod pumping system.The oilfields which are in the middle and late stage of exploitation,the tubing deforms along with the casing due to the formation creep.The tubing or sucker rod coupling deviates from its normal position,resulting in friction between the sucker rod coupling and the inner wall of the tubing during the sucker rod’s periodic reciprocating motion.In addition,the produced fluid has high water content and high salinity,which making the sucker rod coupling and the oil tube inner surface long under the harsh environment of high contact stress,strong wear and corrosion.In view of the above specific situation,the in-situ wear-resisting,anti-corrosion and self-lubricating composite coating was prepared on the outer surface of sucker rod coupling by plasma cladding technology in this paper.In order to effectively improve the sucker rod coupling and wear parts of the tubing service life.The five kinds of composite coatings with diffident mixed powders were fabricated by plasma cladding technology on the surface of N80 steel(coupling material).Their chemical composition(wt.%)are Ni-40Cr-16 nickel-coated graphite,Ni-50Cr-20 nickel-coated graphite,Ni-60Cr-24 nickel-coated graphite,Ni-52.8Cr-21.2 nickel-coated graphite-10 nickel-coated MoS2 and Ni-45.7Cr-18.3 nickel-coated graphite-20 nickel-coated MoS2.The wear-resistant reinforcing phase and self-lubricating phase were synthesized in situ on the surface of N80 steel by plasma cladding technology.The microstructure and phase composition of the composite coating were determined by OM,XRD,SEM,and EDS.The results showed that the micro structure of the coatings prepared from Ni,Cr,nickel-coated graphite and nickel-coated MoS2 powder were composed of Cr7C3,CrxSy,Mo2C,y-(Ni,Fe)phases.The phases of Cr7C3/Ni composite coatings are mainly Cr7C3 and y-(Ni,Fe),and the reinforcing phase Cr7C3 is dispersed in irregular block and strip shape in Cr7C3/y-(Ni,Fe)eutectic matrix;The phases of the Cr7C3/MoS2/Ni composite coatings are mainly Cr7C3,CrxSy,y-(Ni,Fe)and a small amount of hard phase Mo2C.The reinforcing phase Cr7C3 is in the form of blocks,strips and needles,and the lubricating phase CrxSy is in the form of oval granular dispersion,these reinforcing phases are dispersed in the Cr7C3/y-(Ni,Fe)eutectic matrix.The microhardness of the composite coatings were tested by microhardness tester.The results show that the hardness of the coatings increase with the increase of Cr-C mass fraction in the Cr7C3/Ni composite coatings.In the Cr7C3/MoS2/Ni composite coatings,the hardness of the coating decreases as the MoS2 content increases.The wear resistance of the composite coating under three load conditions was tested by a ring-block friction and wear tester.The results show that the friction coefficient of the composite coatings decrease with the increase of the load,but the wear loss increases with the load increases.The friction coefficient and grinding loss of Cr7C3/Ni composite coatings decrease with the increase of Cr7C3 content in the coating.The friction coefficient of Cr7C3/MoS2/Ni composite coatings decrease with the increase of MoS2 content in the coating,but the grinding loss increases with the MoS2 content increases.The corrosion resistance of the composite coatings were tested by electrochemical workstation and immersion test.The results show that the corrosion resistance of the composite coatings are improved compared to the N80 steel substrate.The corrosion resistance of Cr7C3/Ni composite coatings decrease with the increase of Cr7C3 content in the coating.The corrosion resistance of Cr7C3/MoS2/Ni composite coatings decrease with the increase of MoS2 content in the coating.The corrosion and spalling phenomenon on the N80 steel surface is serious,there are a lot of corrosion pits and corrosion products,but on the surface of the composite coatings,a layer of CaCO3 scale with higher density is formed,which reduces the corrosion area of the coating and improves the overall corrosion resistance of the coating.