| Magnesium alloys as “green material†which have many desirable features, including low density, high thermal conductivity, high strength to weight, good electromagnetic shielding characteristics, etc., which make it valuable in a number of structural applications including aerospace, electronics, computer parts and automobile fields. However, the relatively poor abrasion and corrosion resistance of magnesium alloy limit its extensive utilization. Therefore, corrosion resistance of magnesium and its alloys has been widely investigated.Various methods have been developed to improve their corrosion resistance, such as metal coating, organic coating, conversion coating, anodizing, and so on. Most of the coatings produced from those surface treatments were helpful for protecting from the corrosion of the magnesium alloys to some extent. However, the preparations of the coatings are expensive and complicated. In many practical applications, they are not feasible or cost-effective. The preparation method of conversion coaing is simple. However, the coating always has some pores and cracks on the surface, which unable to prevent corrosion of magnesium alloy substrate. The process of anodic oxidation can form a porous oxide layer on the surface of Mg alloy, which can decrease the corrosion rate of substrate. A single anodized film generally was not adequate for protection from corrosion of magnesium alloys. In corrosive environment, especially in acid environment, the outside layer is prone to failure due to their permeability to the environment. Both of the eletro-plating layer and electroless plating layer on Mg alloy are cathodic coating, such as Ni coating, Cu coating, Ni-P coating and so on. However, galvanic corrosion between the cathodic coating and the Mg alloy substrate seems still to be a problem because of potential diffe rence between the cathdic coating and the Mg alloy substrate and more pores exist in the coating, which accelerate the corrosion rate of substrate and the substrate was destroyed in short time. This paper studied new methods to prevent the corrosion of Mg alloy, provide new ideas for corrosion protection of Mg alloy, improve the corrosion resistance of Mg alloy and expand the scope of its application.The surface morphologies and microstructure of the coatings were investigated by optical microscope and scanning electron microscopy(SEM). The chemical composition of coatings were measured by energy dispersive X-ray spectroscopy(EDS). The corrosion mechanism and corrosion behavior of coatings were researched by immersion test, potentiodynamic polarization, electrochemical impedance spectroscopy, etc. The main research contents are as follows:1. The corrosion and protection mechanism of Ni-B/Ni-P coating were researched and discussed. The double layer coating was studied to solve the pore problem of electroless Ni on Mg alloy. The double layer coating was consist of inner layer(Ni-P) and outer layer(Ni-B). The optimum technological condition was determined, the corrosion behavior was surveyed using neutral salt spray test and electrochemical test. Finally, the corrosion mechanism was proposed.2. A novel palladium-free and environmentally friendly surface activation process for electroless Ni plating on micro-arc oxidation(MAO) film of AZ91 D Mg alloy was studied. A tight, dense and continuous structure of Ni-B/Ni-P coating was obtained on micro-arc oxidation film of AZ91 D Mg alloy. The MAO film acted as the interlayer to reduce galvanic corrosion between Ni and Mg alloy substrate in corrosive environment. The Ni-P coating acted as the sealing layer for MAO fi lm to improve corrosion resistance and machining features.3. The super hydrophobic coating was prepared on Mg alloy substrate. The coating was prepared through pre-plating Ni,electroplating Cu and electroplating Ni processes. Pre-plating Ni coating as bottom layer which was prepared by electroless method, it can provide a more uniform electric field distribution surface for electroplating. The intermediate Cu layer served as barrier layer which play a role in the coating thickness increase. The top Ni laye r exhibit rough surface structure which is a major condition for super hydrophobic coating formed. Finally, the coating prepared after modified in stearic acid alcohol solution for 60 min.4. Electroless E-coating protection technique was developmented to prevent the corrosion of magnesium alloys. A practical anodic polarization treatment method was developed to form an oxide layer. A thick and compact composite coating on Mg alloy substrate was formed via electroless electrophoresis coating processe. When the samples were dipped into electrophoresis coating solution, Mg O on Mg alloy surface could produce sufficient hydroxide ions for the formation of thick electroless electrophoresis coating film. The structural and corrosion resistane of coating was studied.The innovations in the dissertation as bellow:(1) The high corrosion resistance Ni-B/Ni-P coating was preparation on the surface of Mg alloy, and the corrosion mechanism was studied;(2) The activation mechanism of new activator and the corrosion mechanism of composite coating were studied in this paper. Silver ion ink served as a new activating agent to produce activation sites for electroless plating. This method could avoid the toxicity of Sn and reduce the cost to be adapted for large-scale commercial manufacturing;(3) The super-hydrophobicity Ni coating was preparation on the surface of Mg alloy, the structure, corrosion resistance, and Hydrophobic mechanism of coating was investigate in this paper;(4) Electroless E-coating protection technique was developmented to prevent the corrosion of magnesium alloys, and the corrosion resistance of composite coating was researched. |
