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A Study on the Surface Protection Against Corrosion for Mg-Li Alloys

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Tutor: YaoGuangChun WangShuLan
School: Northeastern University
Course: Non-ferrous metallurgy
Keywords: Mg-Li alloys,Chemical conversion coating,Electroless nickel plating,Electroless
CLC: TG174.44
Type: PhD thesis
Year:  2008
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Abstract:
Mg-Li alloys are the lightest structural metallic alloys currently available. Mg-Li alloys have ordinary magnesium alloys¡¯advantages of high strength to weight ratio, high ductility, good machining property, and so on. Besides, they also own characteristics of low density, good plasticity, easy forming at room temperature, high electrical and thermal conductivity. Because the energy source is very scarce now, these alloys would become the perfect choice for weight reduction in portable microelectronics, telecommunications, automobile and aerospace applications. However, the corrosion resistance of Mg-Li alloys is very poor, which greatly limits their industrial use. Therefore, it is urgently necessary to improve the corrosion resistance of Mg-Li alloys. The aim of this paper is to develop the suitable surface protection technology for the novel Mg-Li alloys. Base on the study of the corrosion mechanism of Mg-Li alloys, the different protective coatings were prepared on Mg-10Li-1Zn alloys by the methods of conversion treatment and electroless plating technology. The preparation processes, deposits processes, morphologies and structures, corrosion properties and protective mechanisms of the as-prepared coatings were investigated.In this paper, the corrosion mechanism of Mg-Li alloys was firstly investigated. In the room temperature, the corresponding oxide and hydroxide should be easily formed on the surface of Mg-Li alloys by the analysis of the thermodynamics data. But these compounds were loose and not compact; they could not effectively protect the substrate from corrosion. The results of corrosion tests exhibited that Mg-10Li-1Zn alloys suffered seriously corrosion in 3.5wt.% NaCl solution, and showed the poor corrosion resistance against Cl- penetration.The novel phosphate-permanganate conversion coating was prepared on Mg-10Li-1Zn alloy by the chrome-free conversion technology. The process of the chemical conversion treatment obtained with preliminary and orthogonal tests was: KH2PO4 50g/L, KMnO4 40g/L,55¡æ,20min. The basal coating, some white particle-like deposits and irregular cracks were formed on the surface of the coating. It was mainly composed of Mg, O, K, P and Mn elements. The results of electrochemical measurements and immersion tests indicated that the phosphate-permanganate conversion coating could provide some protection for Mg-10Li-1Zn alloy.The stannate conversion coating was prepared on Mg-10Li-1Zn alloy. The process was:Na2SnO3¡¤4H2O 200g/L, KH2PO4 100g/L,70¡æ,120min. The basal coating, some block deposits with the different size and some micro-pores were formed on the surface of the coating. It was mainly composed of Mg, O, K, P and Mn elements and the analysis of XRD indicated that the main phases of this coating were MgSnO3, Mg3(PO4)2, SnO. The results of corrosion tests showed that the stannate conversion coating could effectively protect Mg-lOLi-1Zn alloy and the corrosion resistance of the stannate conversion coating was better than that of the phosphate-permanganate conversion coating.In order to increase the corrosion resistance of Mg-Li alloys in harsh condition, the Ni-P plating was prepared on the surface of Mg-10Li-1Zn alloy by the method of direct electroless plating technology. The effects of pre-treatment time on the morphologies of substrate and plating were firstly investigated, and then the pre-treatment process was determined. Acid pickling:H3PO4 60mL/L, HNO3 10mL/L,20s, room temperature, rapidly mechanical agitation; Fluoride activation:HF 380mL/L, NaF 1g/L, 10min, room temperature. The deposition process of the Ni-P coating on the substrate followed two steps, i.e. the induction period and the rapid deposition period. The obtained Ni-P coating showed the amorphous structure with a typical spherical nodular structure. Thermal shock and scratch tests proved that the Ni-P coating had a good adhesion with the substrate. The result of the micro-hardness test exhibited that the micro-hardness of Mg-Li alloy was greatly improved after electroless Ni-P plating treatment. The results of corrosion tests demonstrated that, in the different corrosion environment, the Ni-P coating obviously improved the corrosion resistance of Mg-10Li-1Zn alloy.For the first time, based on electroless nickel plating, the Ni-P-Al2O3 composite coating was prepared on Mg-10Li-1Zn alloy by the addition of the¦Á-Al2O3 particles into the plating bath to further improve the hardness and the wear resistance of electroless plating. The results showed that the particles concentration and the micro-hardness of the composite coating were firstly increased and then slowly decreased with Al2O3 particles content increasing. The maximums were obtained when Al2O3 particles content in bath was 5g/L. Al2O3 particles content in bath had almost no effect on the corrosion resistance of composite coatings. The obtained Ni-P-Al2O3 coating showed the amorphous structure and a good adhesion with the substrate. The as-prepared composite coating had a dense structure with the uniform distribution of the Al2O3 particles. The corresponding results showed that the micro-hardness, the wear resistance and the corrosion resistance of the composite coating were better than those of electroless Ni-P coating.In order to further improve the corrosion resistance of Mg-Li alloys, the double structure coating composed of the underlying stannate conversion layer and the top Ni-P plating was prepared on Mg-10Li-1Zn alloy by combining advantages of chemical conversion treatment and electroless plating technology. The double structure coating was very uniform and compact without obvious defects, which had good adhesion with the substrate. The results of the corrosion tests showed that the double structure coating had good corrosion resistance and could effectively prevent Cl-¡¯s attack compared with the solo electroless Ni-P coating. The double structure coating not only solved the problem of the relatively poor corrosion resistance of conversion coating, but also effectively reduced the porosity in Ni-P plating, reduced the potential difference between the Ni-P plating and the substrate resulting in hindering the occurrence of the galvanic couple corrosion. As a result, the double protection for Mg-Li alloys was achieved.
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