| Mg-Li alloys, also named ultra-light alloys, are the lightest structural metallicmaterials. These alloys have unique characteristics of low density, excellentdamping capacity, higher strength-to-weight ratio, high specific stiffness; goodmachining property, good magnetic screen and shock resistance ability. Therefore,Mg-Li alloys have great weight-reducing potential for various applications,especially for aerospace industry, automotive and portable electronics industries.When lithium is added to magnesium, there is an overall reduction in the densityand the ductility of the alloy is generally increased. However, the inclusion oflithium makes magnesium-lithium alloys possess poor corrosion resistance, whichgreatly restricts their use in practice. Thus, it is urgently necessary to performsuitable surface treatments to improve the corrosion resistance of Mg-Li alloys.The corrosion behavior of Mg-Li alloy should be investigated firstly. In this paper,the corrosion behaviors were investigated, and conversion film treatments andelectroless nickel plating were performed on the surface of Mg-Li alloy toenhance its corrosion resistance.In this paper, the corrosion behavior was investigeted by a series of tests,including corrosion rate, morphology of the corroded surface and electrochemicalanalysis. The corrosion behaviour of Mg-8Li alloy in 3.5％NaCl solution anddistilled water is very different. Mg-Li alloy was corroded very slowly in distilledwater. At room temperature, the corrosion potential of Mg-8Li alloy movednegatively and corrosion resistance became poor when it was exposed in the solutioncontaining aggressive chloride Cl- ions. Pitting is the major corrosion type, and thecorrosion pits expanded towards deepness and extent with immersion time increase.The corrosion rate decreases with immersion time increase. The main components of the corrosion products in sodium chloride solution were Mg(OH)2. Following thegreat mass of Li dissolved into solution, the balance of magnesium alloy was quicklycorroded under the loose barrier layer.Chemical conversion treatments are effective surface technologies oncommon magnesium alloy that combine low cost, easy application and highperformance. Electroless nickel plating attracts much attention due to its excellentproperties, such as high hardness, high wear resistance, high corrosion resistance,good lubricity, and accepted ductility. However, there has been only a littleresearch on the protection of Mg-Li alloy. In this paper, we attempted to exertsurface treatments of common Mg alloy on Mg-Li alloy. Two kinds of chemicalconversion treatments and electroless Ni-P technology were performed on thesurface of Mg-Li alloy to enhance its corrosion resistance.The stannate conversion coatings on Mg-8Li alloy were investigated bysimple immersion method. The formation process of the coating was discussedbriefly. The stannate coating was characterized by its structure, morphology, andcorrosion resistance. The influence of technical parameters (including pH values,temperature, stannate concentration, immersion time) on the surface morphologyand corrosion resistance were investigated. It was found that the coating carried ahoar colour, and it was mainly composed of hemispherical particlesMgSnO3·3H2O. The stannate conversion coating formed at the best condition wasuniform, dense and corrosion-resistant. The corrosion potential was ennobled by300 mV, and the corrosion current density decreased about two orders ofmagnitude compared with that of Mg-Li substrate. The results revealed thatstannate conversion coatings provided good protection on Mg-Li alloy.The lanthanum-based conversion coating on Mg-8Li alloy was investigatedand the corrosion resistance was evaluated as well. The surface andcross-sectional morphology were observed by SEM, and composition of the lanthanum conversion coating was characterized X-ray photoelectronspectroscopy (XPS). The corrosion resistance was assessed by means ofpotentiodynamic polarization measurements and open circuit potential (OCP).The technical parameters of lanthanum-based conversion coatings on the Mg-8Lialloy were optimized by orthogonal experiments. The influence of technicalparameters (including pH values, temperature, lanthanum concentration,immersion time) on the surface morphology and corrosion resistance wereinvestigated. The results revealed that the conversion coating was homogeneousand uniform, and had an aciculate-like structure. The coating was mainlycomposed of La(OH)3 as identified by XPS. The electrochemical results revealedthat the lanthanum-based conversion coating possessed better corrosion resistancethan the Mg-8Li substrate. The corrosion potential moved positively about 280mV compared with that of Mg-Li substrate. The coating showed a wide potentialregion and the corrosion current density decreased about two orders of magnitudecompared with that of Mg-Li substrate.Electroless nickel plating on Mg-8Li alloy was studied, and an appropriatemolybdate pretreatment was propgsed. The coating was characterized by itsstructure, morphology, and corrosion resistance. Scanning Electronic Microscope(SEM) observation showed that the Ni-P coating was compact, uniform. Thepresence of molybdate pretreatment layer could reduce the potential differencebetween the Ni-P coating and the substrate, and enhanced the corrosion resistanceof the alloy to certain extent. The electrochemical measurements showed that thesample with Ni-P film exhibited lower corrosion current density and morepositive corrosion potential than the substrate. The hydrogen evolution testsfurther proved that the Ni-P coating provided good protection to the substrate.
|
PDF Full Text Request