Preparation And Properties Of Novel Conversion Layer With Hydrophobicity On Magnesium Alloy

Mg alloys are widely used in aerospace engineering, automobile industry, marine industry and3C products because of their good physical and mechanical properties such as low density, high specific stiffness ratio, mechanical process property, casting property, good damping property, etc. However, the low electrochemical potential makes Mg alloys susceptible to galvanic corrosion, which greatly limits their further use. Therefore, it must be treated to improve the corrosion resistance. Various surface treatments have been established for corrosion protection of Mg alloys.In this thesis, we first summarized the development of the Mg alloys and advances in surface treatment research, emphatically discussed the research progress and the existing problems of the anode oxidation treatment. And then the corrosion protection development direction of Mg alloy and the main ideas of thesis were obtained. On the one hand, sodium carboxylates with a long alkyl chain have already been investigated as green corrosion inhibitors or accelerating agents for phosphate conversion coating. The carboxylate layers with high hydrophobicity as the effective way to protect Mg alloys is seldom considered. Therefore, many original innovations have been made in this dissertation. On the other hand, cyclic voltammetry with a low voltage, low current, low consumption and high efficiency is used to prepare the conversion layer firstly. It solves the insecurity issues in the common anode process easily. In this dissertation, a hydrophobic conversion layer with improved corrosion resistance is fabricated by cyclic voltammetric treatment of Mg alloy and the properties of this layer are tested. The main researches done in this thesis are:(1) The carboxylate layers for Mg alloy protection are fabricated by voltammetrically cycling Mg alloy in0.008mol dm"3sodium carboxylates solution in a water thermostat set at40±0.2℃without stirring. The performance of the carboxylate layers is studied using Tafel polarization, EIS, SEM, XRD and contact angle testing. The results suggest that the carboxylate layers with high hydrophobicity comprise (R-COO)2Mg and Mg(OH)2. With increased thickness, improved compactness and hydrophobicity, the decanoate conversion is proven to be a green and effective way to protect Mg alloys from corrosion.(2) A novel decanoate conversion layer is fabricated by voltammetrically cycling in sodium decanoate solution. The effect of sodium decanoate concentration, solution pH, temperature and cycle number on the formation of the hydrophobic conversion layer are studied using SEM, XRD, Tafel polarization and EIS to determine the optimal condition for formation of the conversion layer. A conversion layer with a water contact angle of about135°formed in0.008mol dm-3sodium decanoate with0.2mol dm-3Na2SO4serving as supporting electrolyte at pH8.0and40±0.2℃for30cycles shows the best corrosion protection. The corrosion protection of the optimal layer is much better than that of a decanoate inhibition layer, and close to that of a chromate conversion layer.(3) The composite layer is fabricated by voltammetrically cycling in sodium decanoate solution added inorganic sodium salts. Its performance is investigated using electrochemical method, photographic, EDX, XRD, and contact angle testing. Results show that the corrosion resistance of sodium silicate composite layer is the best, but the layer is uneven.