| Magnesium alloys as the light metal materials are widely used in aerospace,communication,automobile and electronics industries.However,the more extensive application is seriously limited due to the high chemical activity and poor corrosion resistance.Chemical conversion coating is known as an effective,relatively low-cost and easily implemented method compared with others surface modification technologies for magnesium alloys.For one thing,several of the conversion coatings always have cracks,which tend to lead to serious deterioration of the protective property of conversion coatings.For another,some conversion coating technologies can't satisfy the needs of energy saving and environmental protection such as high treatment temperature and complicated process.In this paper,a Ti-Zr conversion coating with crack-free,environmental friendly,low-energy and acceptable corrosion resistance was successfully prepared on AZ91 D magnesium through adding the titanate and the zirconate to the conversion solution.The developed conversion coating comprised spherical particles distributed throughout the entire surface of the substrate by SEM.Cross-section morphologies of SEM and TEM indicated that the Ti-Zr conversion coating consisted of a two layers structure: a relatively compact inner layer and an outer layer with spherical particles.Moreover,the fitting results of the XPS spectra showed that the main composition of the Ti-Zr conversion coating is MgF2?TiO2 and Zr(HPO4)2.Besides,the corrosion resistance of the Ti-Zr conversion coating was sufficiently evaluated by means of the hydrogen evolution experiment,the salt spray tests and potentiodynamic polarization measurement.According to the OCP evolution during the conversion process,it was reasonable to divide the coating formation process into three stages.Meanwhile,a series of SEM micro-morphologies illustrating the evolution of the conversion coating are given,which presented the nucleation and growth of the spherical particles,as well as the composition was detected after different immersion time in the conversion solution by XPS.From the above,the formation mechanism was proposed to explain the formation process of the Ti-Zr conversion coating.Effect of pre-treatment processes including acid pickling and acid pickling + alkaline cleaning on the corrosion resistance of the Ti/Zr conversion coating with chrome-free,crack-free and low-energy consumption on AZ91 D magnesium alloy was systematically investigated in the present paper.The results indicated that acid pickling induced the preferential dissolution of ? phase and the increase of the surface roughness of AZ91 D magnesium alloy,which was not conductive to the enhancement of the corrosion resistance of the conversion coating.And customizing the surface chemical state of AZ91 D magnesium alloy by acid pickling + alkaline cleaning can effectively improve the corrosion resistance of the Ti/Zr conversion coating.The post-treatment processes,consisted of the sealing agent post-treatment,the heating post-treatment and the immersion post-treatment in water at room temperature,was also studied,the morphologies of which was obtained by SEM.Moreover,the corrosion resistance of the Ti-Zr conversion coating was characterized after different post-treatment via salt spray tests and potentiodynamic polarization. |
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