Investigation On The Failure Mechanism Of The Micro-arc Oxidation Coating On Mg-alloys Induced By Micro-defects

In corrosion control engineering,the corrosion-resistant performance of protective coatings is often weakened due to the existence of micro-defects.It would cause potential safety hazards for the normal service of equipment.Micro-arc oxidation（MAO）,as a promising surface treatment technique,has great potential for application in aerospace,automobiles,and biomaterial.However,micro-defects distributed in the MAO coating always act as the original reason that induces the premature failure of the MAO coating,resulting in the limitation of their wider engineering applications.Therefore,it is important to study the micro-defects in MAO coatings for optimizing the MAO process and understanding the micro-defects-induced failure mechanism of corrosion-resistant coatings.It would also be beneficial to the improvement of corrosion control technology.In this paper,aiming at the micro-defects in the MAO coating,the research was carried out from four aspects.Firstly,the 3D microstructure and distribution characteristics of micro-defects as well as their relationship with the property of the MAO coating were investigated to reveal the origin of coating failure.Secondly,the evolution of the coating properties during immersion was studied to reveal the transportation process of corrosive media and the failure mechanism of the MAO coating.Thirdly,the evolution of electrochemical behaviors was investigated and quantitative evaluation indexes were proposed.Finally,a sealing treatment technique based on the utilization of the micro-defects was proposed.The following results were concluded.The structural characteristics and distribution of micro-defects were characterized by X-ray tomography（XRT）,confocal laser scanning microscopy（CLSM）in conjunction with other test technologies.And then,the changes of coating properties caused by micro-defects were discussed.The results showed that the lengths of micro-cracks were ranging from 1.0to 7.0μm,and the equivalent diameters of micro-pores were ranging from0.5 to 16.0μm.The micro-pores with diameters below 7.0μm accounted for93.2%in number.The local porosity of the MAO coating increased gradually from the coating/substrate interface along the thickness direction.In this case,the MAO coating was divided into two layers:the compact inner layer and the porous outer layer.According to the microstructural information,the micro-pores could be divided into three types:subspherical pore,subcolumnar small-pore subcolumnar large-pore.There was an obvious relationship between the distribution of micro-defects and the inhomogeneity of the physical structure and chemical structure of the MAO coating.Thus,the inherent inhomogeneity of the MAO coating also led to the inhomogeneity of its Volta potential.Based on the gravimetry,structural characteristics,and elemental distribution of the MAO coating during corrosion,the transportation behavior of corrosion media in the MAO Mg-alloy was revealed.The results suggested that the transportation of corrosive media could be divided into three stages:media-inhaling stage with capillary absorption as the main process;media-transporting stage with coating dissolution and media permeation as the main process;and substrate-corroding stage with substrate corrosion reaction as the main process.Cl^-played an important role in the coating failure.Combined with the structural evolution of the MAO coating,the failure mechanism of the MAO coating induced by micro-defects was further revealed.The micro-pores in the MAO coating provided channels for the transportation of corrosive media.At the same time,the dissolution of the coating compounds and the internal stress caused by the formation of Mg（OH）₂ promoted the volume expansion of micro-defects.Different micro-pores had different impacts on the coating failure,and the subcolumnar large-pores played the main role in the failure process.Finally,the through-defects formed in the MAO coating,resulting in the failure of coating and the corrosion of the substrate.The electrochemical evolution characteristics and electrochemical dynamic behavior during the coating failure were investigated using the local and global electrochemical tests.The relationship between the transportation of the corrosion media,microstructure of the MAO coating,and electrochemical characteristics was established.Based on the response of EIS,₇)（low-frequency resistance）and CI（coating impedance index）were proposed to quantitatively evaluate the corrosion-resistant performance and failure degree of the MAO coating,respectively.And they were used in the evaluation of different MAO coatings.The results showed that the corrosion-resistant performance of the MAO coating was closely affected by its thickness and density.An inhibitor formulation for the ZM6 Mg-alloys was obtained.It was found that this inhibitor was an anodic type inhibitor and its inhibitory efficiency was up to 97%.Based on the traditional silane-based sol-gel treatment,the sealing process was optimized.The micro-defects were used as micro-containers for inhibitors.The results showed that the service life of the optimized sealed specimen was 3.5 times that of the unsealed specimen,and the sealed coating exhibited a self-healing property after the failure of the coating.