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Rare Earth Elements To Add Impact Studies On The Microstructure And Corrosion Resistance Of Nickel-phosphorus Coating

Posted on:2011-04-08Degree:MasterType:Thesis
Country:ChinaCandidate:X W ZhouFull Text:PDF
GTID:2191360305488469Subject:Materials Processing Engineering
Abstract/Summary:PDF Full Text Request
Electroless Ni-P and its rare-earth nano-CeO2 doped coatings were co-deposited in acidic condition, and its microstructure and properties were studied and compared, especially for coating's anti-corrosion. Under optimal experimental prescription, electroless composite coatings Ni-P/CeO2 were co-deposited on low carbon steel substrate surface. Ultrasonic made nano-CeO2 particle dispersed effectively. Environmental scanning electron microscopy (E-SEM) with energy dispersive spectrum (EDS) was used to examine the coating's surface morphology and internal chemical content,respectively. Transmission electronic microscopy (TEM) was used to analyze the microstructure. The crystal microstructure of Ni-P/CeO2 coatings was characterized by X-ray diffraction (XRD). Constant potential rectifier was used to determinate electric polarization curves.The results show that the Optimal quantity of CeO2 additive should retain (1825)g·L-1. Only this, nano-CeO2 particle can get dispersed sufficiently and effectively. Rare-earth CeO2 increase the P content(wt(P)≥12%). TEM& XRD helps to find that Ni-P coatings have partial amorphous structure mixed with Ni-nano-crystals, while the Ni-P/CeO2 coatings had perfect amorphous.And grain preferred orientation growth has changed to make grain refiner, which is mostly because it co-deposites with rare earth CeO2, increase potential and interfacial energy with metallic surfaces, increase the number and velocity of crystal nucleus .However, the rate of crystalline grain growth become lower. Corrosion test manifests that rare earth additive CeO2 makes porosity lower, the corrosion morphology of coatings has change from corrosive pitting to be general corrosion, the anti-corrosion property and passivity improve in Ni-P/CeO2 coatings. It is less liable to undergo localized corrosion and has a slower corroding rate. During the co-deposition process in acidic bath, some Cen+(n=3,4) iron might be absorbed to the metal/solution electrical doubled layer and hinder deposition. Ni-P/CeO2 coatings has perfect amorphous structure, which is probably due to hinder crystal-typed deposition of nickel while promote deposition of phosphorous. Electric polarization curves are determined by constant potential rectifier has used to study anti-corrosion resistance, the results show that potential ennoblement and corrosion current drop 1 order of magnitude. The polarization curves slightly move to positive direction and decrease corrosion current density. According to the electrochemistry theory, the corrosion property is improved. Besides, the thesis also studys the effect upon heating about microstructure, microhardness and so on. It finds that Ni3P precipitation and Ni crystallization take place at 450℃2.5h heat treatment. In addition, Ni-P/CeO2 coatings have sintered phase of NiCe2O4 spinels, precipitated phase CeO2 is priority produced at grain boundaries, micropore and other defects. So, Ni-P/CeO2 composite coatings can get diffused sufficiently and effectively, increase grain density microhardness and can reach 1000HV. The main reason is that the nano rare-earth power CeO2 addictive pad a defect among dislocation, grain boundary and porosity in chemical deposition to hinder dislocation glide, decrease activity of P,S impurity elements, which due to rare-earth CeO2 nano-particle addictive co-deposite with Ni,P. During the process, it spreads with substrate sufficiencily, dense structure oxides CexOy produce in dislocation gap. These help to lower porosity, increase grain densification and anti-corrosion.
Keywords/Search Tags:composite deposition, rare-earth element, microstructure, corrosion, Ni-P
PDF Full Text Request
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