Corrosion And Rare Earth Conversion Coating Of Magnesium Alloy With High Strength And Toughness

Magnesium alloys are the lightest constructional metal materials. Due to their high specific strength/modulus/stiffness, high damping, good shielding properties and magnetic properties, Magnesium alloys have been extensively applied in automobile, electronic industry aerospace industry and medical apparatus and instruments industry etc. However, magnesium and magnesium alloys have high chemical reactivity and low corrosion resistance in most service condition with their negative electronic potential, which have limited their applications. At present, there are a lot of means to improve the corrosion resistance of magnesium alloys. Now, rare earth conversion coating has attracted great attention because it is easy to get, which is beneficial for both environmental issue and corrosion resistance.The surface of the alloy was treated with cerous nitrate solution and hydrogen peroxide was used as the additive/oxidant. The mass concentration of rare earth metal salt (Ce(NO₃)₃) and additive/oxidant (H₂O₂), reaction temperature, reaction time and the way of immersion were controlled to obtain the conversion coating and analyzed the the nature of process parameters on the impact of conversion film. The morphology, chemical composition and the corrosion resistance of the coating were evaluated by Scanning Electron Microscope (SEM), Energy Disperse Spectroscopy (EDS) and Electrochemical Impedance Spectra (EIS). And the corrosion resistance was increased at first but decreased then with the increase of the concentration of cerous nitrate and hydrogen peroxide, the reaction temperature and the reaction time. The optimum parameters are as follows: cerous nitrate 10g/L, hydrogen peroxide 50ml/L, 50℃and 30 min. The equi-alternation immersion produced a conversion coating with better corrosion resistance than the non-alternation way.A series of experiments were carried out to study the corrosion of magnesium alloys. A primary analysis was made on the mechanism of corrosion of magnesium alloys by means of Salt spray test, Weight-loss method, XRD, SEM etc.Treatment of T6 can improve the corrosion resistance of sample 1# and sample 2# while reduce the corrosion resistance of sample 3# and sample 4#; Under the same cast premise, sample 1# and sample 2# have better performance on corrosion resistance than sample 3# and sample 4#; With the increasing of time, pitting corrosion which appeared at the beginning of corrosion was extended to corrosion crack and corrosion pits,developed gradually into more deeper substrate metal areas; The kinds of main corrosion products related to treatment and alloy elements, unrelated to test cycle.