Microstructure And Properties Of Composite Organic Coatings Produced By Plasma Electrolytic On Magnesium Alloys

As a light metal structural material,magnesium alloy has good damping,electromagnetic shielding,high specific strength and excellent casting properties.Thus,magnesium alloys have potential application prospects in military,aviation,vehicle and medical fields.But poor corrosion resistance is the main limitation of large-scale application.Surface coating is one of the effective methods to improve the corrosion resistance of magnesium alloys.However,magnesium alloys have high surface activity and difficult for coating.Composite coating would be the main researches direction in the future.In addition,the ordinary coating do not have self-repairing ability due to the damage or cracking caused by practical applications（collision,stress etc.）,which will produce serious local corrosion and shorten the overall service life of the coating.Therefore,it has an important meaning to develop a kind of compound protective coating,which has excellent resistance to corrosion and self-repairing ability.In this paper,AZ91D magnesium alloy as the research object.Aim to construct self-repairing plasma electrolytic oxidation composite coating on magnesium alloy surface.Firstly,different inhibitors were loaded by vacuum negative pressure loading method with mesoporous SiO₂and halloysite nanotubes（HNTs）as carriers.Then,layer by layer assembled polyelectrolyte coating of HNTs to realize the p H responsive release of microcontainers.And the modification is completed by adding a certain proportion into the epoxy coating.Finally,the plasma electrolytic oxidation（PEO）was prepared on the alloy surface by plasma electrolytic oxidation and combined with the modified epoxy coating to obtain the plasma electrolytic oxidation composite coating.The micromorphology,phase composition,loading,and release of different microcontainers were investigated by transmission electron microscopy（TEM）,Fourier infrared（FT-IR）,thermogravimetric（TGA）,and UV light spectrophotometer（UV）.And the surface morphology of magnesium alloys modified with different corrosion inhibitors and the two composite coatings after modification with mesoporous SiO₂and HNTs were investigated by scanning electron microscopy（SEM）,X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）and the same time the corrosion resistance of the alloy modified with different corrosion inhibitors and two composite coatings was investigated by combining the whole immersion corrosion and electrochemical.Finally,the self-repairing effect and mechanism of the two composite coatings were studied by combining micro-area electrochemistry with electrochemistry.The conclusions are as follows:The three kinds of corrosion inhibitors,in sequence of their corrosion inhibition efficiency,are 8-hydroxyquinoline（8-HQ）,2-Mercaptobenzothiazole（MBT）,benzotriazole（BTA）.Among them,8-HQ is a film-forming corrosion inhibitor,which preferentially adsorbs at grain boundaries to form chelate bis（8-hydroxyquinoline）magnesium（Mg Q₂）,protected magnesium alloy with high corrosion inhibition efficiency of 98.4%.MBT is an adsorption type corrosion inhibitor,which can reach 93.6%corrosion inhibition efficiency by adsorption of its outer ring S atom with N atom on the alloy surface,changing the progress of electrode reaction and attenuating the corrosion tendency.The maximum loading rate of the corrosion inhibitor 8-HQ in mesoporous SiO₂is about16%.The plasma electrolytic oxidation composite coating modified with mesoporous SiO₂showed better corrosion resistance than the unmodified one,and the current density(I_corr)decrease of 1～2 orders of magnitude.With the increase of the amount of mesoporous SiO₂,its corrosion resistance first increases and then decreases,and it possesses the optimum corrosion resistance when the amount of mesoporous SiO₂is 1.2 wt.%.After prefabricated defects,8-HQ in SiO₂can be released and adsorbed on the defects by forming chelate Mg Q₂,attenuating the corrosion rate,corrosion tendency at the defects with a self-healing efficiency of about 42%.The maximum loading rate of the corrosion inhibitor MBT within HNTs is about 5%,and HNTs modified by layer by layer assembly showed certain p H responsiveness with maximum release rates of 62%,10%,and 85%at p H 3.0,6.5,and 10,respectively.The plasma electrolytic oxidation composite coatings modified with an appropriate HNTs addition similarly showed better corrosion resistance,with an order of magnitude decrease in current density(I_corr).As the amount of HNTs added into the composite coating increases,the corrosion resistance also showed a trend of first increasing and then decreasing,and the comprehensive performance is the best when the amount of HNTs added is 1.0 wt.%.After the prefabricated defects,the corrosion inhibitor MBT loaded by HNTs can release in response to the change of p H caused by local corrosion at the defects,reducing the corrosion rate and corrosion tendency at the defects,and the self-repair efficiency is about 31%.