| The coatings prepared by electroless plating exhibit the super performanceon the uniformity, corrosion resistance, hardness, weld ability, magnetic anddecorative performance. The electroless plating,as an excellent surface treatmenttechnology, has been widely used in many fields, such as military equipment,mold, mould and aviation, and the surface strengthening of textile, food andwood processing machinery. In the actual industrial production, manycomponents including the second main salts, additives, nano-inorganic particlesare added into the plated solution due to taking into account the deposition rate,physical and chemical properties of Ni-P film. Sometimes, some of the Ni-Pfilms need to be treated by heat treatment to improve the film performance. Theabove mentioned methods result in complicating the plating solutioncomposition, increasing the costs of the production and processing. In this paper,the hot-rolled AZ31magnesium alloy was chosen as the research object, and theelectroless plating was adopted as a method to prepare Ni-P film on AZ31.The influences of various additives with different concentrations in the bath on thecorrosion resistance, hardness, friction coefficient and other mechanicalproperties of Ni-P coatings are investigated. X-ray powder diffraction (XRD),scanning electron microscopy (SEM), energy spectrum analyze is (EDS), microhardness tester, wear tester with high temperature and electrochemicalworkstation were used to characterize the composition, morphology, grain size,hardness, wear resistance and corrosion resistance of the deposition of Ni-Pcoating, respectively. The conclusions are as follows:(1)As the concentration of sodium saccharin increased from0g/L to5g/L, SEM images showed that the film grain size and film thickness is graduallyreduced and the pore number is decreased first and then increased;EDS datademonstrated that the phosphorus content is decreased gradually(all phosphoruscontents are greater than7wt.%, which belong to the high phosphorus coatings).The polarization curves and impedance spectra in3.5%NaCl solution showedthat the corrosion resistance of Ni-P film deposited in2g/L sodium saccharinsolution is the best. And the deposition mechanism of the Ni-P film in2g/Lsodium saccharin concentration was studied in detail. It was found that thedeposition was formed by two step reaction: the first step is the replacementreaction between Ni2+and Mg alloy substrate; the second step is theco-deposition of nickel and phosphorus elements. In addition, the microhardness data found that the hardness of Ni-P increases with the increase of theconcentration of saccharin sodium. As the concentration of saccharin sodium was5g/L, the film hardness reached1003.7HV0.025. The friction mechanismsof Ni-P films with different friction materials were different. When the frictionmaterials was GCrl5, its friction mechanism was a slight cutting wear; whilewas Al2O3, its friction mechanism was the adhesion wear. The film withdifferent friction wear test showed that the concentration of Ni-P grinding layerweight loss, the wear scar width and the friction coefficient increased with thesaccharin sodium increasement.(2)As the polyvinylpyrrolidone (PVP) concentration increased from8g/Lto12g/L, XRD and EDS data showed that the films were consisted of singlecrystal Ni and poor-crystalline Ni-P. SEM showed that the thickness of the filmsare first increased and then decreased. The polarization curves and impedancespectra in3.5%NaCl solution demonstrated that the film prepared in10g/L ofpolyvinyl pyrrolidone solution exhibited the best corrosion resistance. Thecorrosion current density from the polarization curves is about1.29uA/cm2andthe polarization resistance reached73581Ωcm2. All these indicated that this filmhad a good corrosion resistance. |
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