Comparison Of Microstructure And Corrosion Resistance Of Micro-arc Oxidation Coatings Formed On Magnesium And Magnesium Alloys

Micro-arc oxidation?MAO?,which originates from the traditional anodic oxidation,has been extensively utilized for surface modification of valve metals such as magnesium,aluminum,and titanium etc.in the field of manufacturing production,because an environmental-friendly,compact,uniform,hard and corrosion-resisting ceramic coating is effectively prepared on top of these base materials for the required protection.Nevertheless,an inherent defect,poor corrosion resistance,considerably refrains its large-scale application in demanding and aggressive service condition.With a view to investigating the corrosion deterioration mechanism of micro-arc oxidation coating,a well-designed orthodoxy experiment project was applied to research the influence of substrate material,on-load voltage and oxidation time on the microstructure and anti-resistance property of micro-arc oxidation coating.Then,the various plasma electrolytic oxidation coatings which were prepared in alkaline silicate electrolyte on pure magnesium,AM60B and AZ91D magnesium alloys respectively,were applied to observe and analyze the microstructural variation of these coatings during the degeneration corrosion process in 3.5 wt.%Na Cl solution with p H of 7.Subsequently,the corresponding erosion mechanisms of diverse coatings during the entire aggravation process were deeply researched by virtue of the corresponding equivalent circuits and fitting EIS results.Importantly,these researches will make considerable contribution to the theoretical research and practical application of these ceramic coatings.Besides,the coating thickness,phase composition,surface and cross-sectional morphologies of these coatings was characterized by TT260 thickness gauge,X-ray diffractometer?XRD?and JEOL JSM-6700F field emission scanning electron microscope?SEM?,respectively.And the corrosion behavior of uncoated and coated coupons was evaluated using cyclic voltammetry?CV?measurements,potentiodynamic polarization tests?Tafel?,and electrochemical impedance spectroscopy?EIS?in 3.5 wt.%neutral Na Cl solution at room temperature.Meanwhile,the thesis verified whether the experimental results derived from fore-mentioned methods can confirm mutually or not.1.The effect of alloying of light metals on the microstructure and corrosion resistance of MAO coatings was penetratingly investigated,and the corresponding research results were documented in subsequent content.?1?In the silicate-containing system,AM60B-PEO coating was mostly composed of Mg O,Mg₂Si O₄,and Mg Al₂O₄ phases,AZ91D-PEO ceramic coating was primarily comprised of Mg O and Mg₂Si O₄ phases.And Mg-PEO coating was basically comprised of Mg O and Mg Si O₃ phases.?2?Compared with Mg-PEO coating,AM60B-PEO and AZ91D-PEO coatings could accommodate the counterpart substrate material with a more effective protection before the ceramic coating lost their protection capability by virtue of strongly corrosion-resisting phases in their coatings,although both the coatings were relatively vulnerable to pitting corrosion in 3.5 wt.%neutral Na Cl solution.?3?During the whole deterioration process,AM60B-PEO coating experienced two stages.Namely,the permeation of the corrosion electrolyte was strongly blocked by the outer and inner layers?no more than 24 h?;Subsequently,the inner layer/substrate interface was attacked by the corrosive ions?i.e.,chloride ion?and localized corrosion damage of AM60B magnesium alloy happened?over 24 h?.Secondly,an evenly overall corrosion was observed on the top of AZ91D-PEO coating and chloride ion hadn't been penetrating into the coating/substrate interface during the immersion process of 72 h.Besides,Mg-PEO coating also experienced the overall corrosion process,but the chloride ion intruded into the coating/substrate interface during the later immersion process.2.The influence of on-load voltage on the microstructure and corrosion resistance of MAO coating was further researched,and the ultimate experiment results were described in subsequent content.Namely,in comparison with the ceramic coating prepared at 300 V on pure magnesium,AM60B and AZ91D magnesium alloys respectively,the corresponding coatings which were produced at 450 V had possession of excellent anti-corrosion property by means of two-double microstructure including outer porous and inner compact layers.More importantly,these coatings produced at 450 V rather than 300 V could obstruct chloride ion from permeating into the coating/substrate interface by means of mechanical protection.3.The ultimate aim of this thesis was whether all the experimental consequences derived from CV,Tafel and EIS measurements verified each other or not.The result indicated that not only can these methods be utilized to estimate the susceptibility to pitting corrosion,corrosion resistance property along with corrosion deterioration process of uncoated and coated specimens,but also can the corresponding experimental results confirm mutually.