| In recent years,with increasingly stringent service conditions,a single protective coating on the surface of magnesium could not meet the application requirements.Therefore,researchers had begun to prepare duplex or multilayer coatings on the surface of magnesium to further improve its corrosion resistance.In the current pulished literatures,the protection technologies employed to combine with micro-arc oxidation(MAO)have their own characteristics,and the prepared duplex or multilayer coatings can improve the corrosion resistance of magnesium surface to a certain extent.However,the disadvantages are also obvious.The treating solutions for electroplating and electroless plating greatly pollute the environment,the protective coatings prepared by the organic coating methods own poor adhesion strength,and the capital investment for magnetron sputtering and laser surface cladding is costly.In addition,physical stratification exists in many duplex or multilayer coatings,which affects their adhesion strength.Moreover,so far,the reports on the preparation of hybrid coatings on magnesium surface by combining plasma electrolytic saturation(PES)and MAO are few.Compared with the traditional diffusion technologies,PES is characterized by high efficiency,low energy consumption,environmental friendly,direct quenching while diffusion,metallurgical bonding between the infiltrated layer and the substrate,and excellent adhesion strength.At present,this technology is mainly applied to the surface of ferrous metals.For non-ferrous metals,only a few researchers have paid attention to the PES study on the surface of pure titanium and titanium alloys.MAO is good at in-situ forming ceramic coatings on the surface of light metals such as magnesium,aluminum,and titanium.The obtained coatings have high adhesion strength,with green process,simplicity and low comprehensive cost.In this study,three typical diffusion elements B,C and N were selected as representatives,the advantages of PES and MAO in metal surface modification were fully used,and the possibility,effect and mechanism to improve the coating performance by simultaneously applying them to magnesium surface were explored,so as to obtain a novel fabrication method of hybrid coatings.The two processes,namely PES and MAO,in this study could be completed on the same device,with convenient control and simple operation,and the process treatment was carried out in liquid circumstance,which is friendly to complex workpieces and easy to realize industrial application.Firstly,surface modification layers were fabricated by plasma electrolytic boronizing(PEB)on pure magnesium in the aqueous solution with different borax(Na2B4O7·10H2O)concentration.The effects of borax concentration on the micro-structure and corrosion resistance of PEB surface modification layers were investigated.Subsequently,hybrid coatings would be prepared by PEB plus MAO on pure magnesium.The results show that novel PEB+MAO hybrid coatings can be fabricated successfully by a combination of PEB and MAO on pure magnesium.The corrosion potential(Ecorr)of single PEB surface modification layers and single MAO coatings is-1.66 V and-1.64 V,respectively,and the corrosion current density(Jcorr)is 7.16×10-5A·cm-2and 2.63×10-6A·cm-2,respectively.Compared to single PEB surface modification layers and single MAO coatings,the Ecorrof the PEB+MAO hybrid coatings increases by 40 m V and 20 m V,respectively,and the Jcorrdecreases by two and one orders of magnitude,respectively,meanwhile,both the capacitive loop radius as well as the impedance modulus increase.These results indicate that the PEB+MAO hybrid coatings exhibit more superior corrosion resistance than both single PEB surface modification layers and single MAO coatings.Secondly,pure magnesium was treated by plasma electrolytic carburizing(PEC)in the aqueous solution with different glycerol(C3H8O3)concentration to prepare surface modification layers.The influences of glycerol concentration on the micro-structure and anti-corrosion property of PEC surface modification layers were analyzed.After that,hybrid coatings would be fabricated by a combination of PEC and MAO on pure magnesium.The results demonstrate that novel PEC+MAO hybrid coatings can be prepared successfully by combining PEC with MAO on pure magnesium.The Ecorrof single PEC surface modification layers and single MAO coatings is-1.67 V and-1.66 V,respectively,and the Jcorris 2.54×10-5A·cm-2and1.77×10-6A·cm-2,respectively.Compared with single PEC surface modification layers and single MAO coatings,the Ecorrof the PEC+MAO hybrid coatings increases by 70 m V and 60 m V,respectively,and the Jcorrdecreases by two and one orders of magnitude,respectively,meanwhile,both the capacitive loop radius and the impedance modulus increase.These results show that the PEC+MAO hybrid coatings display more superior anti-corrosion performance than both single PEC surface modification layers and single MAO coatings.Thirdly,surface modification layers were formed on AZ91D magnesium alloys by plasma electrolytic nitrocarburizing(PEN/C)in the aqueous solution with different formamide(HCONH2)concentration.The effects of formamide concentration on the micro-structure and corrosion resistance of PEN/C surface modification layers were researched.Then,hybrid coatings would be fabricated by combining PEN/C with MAO on AZ91D magnesium alloys.The results reveal that novel PEN/C+MAO hybrid coatings can be prepared successfully by combining PEC with MAO on AZ91D magnesium alloys.The Ecorrof single PEN/C surface modification layers and single MAO coatings is-1.48 V and-1.44 V,respectively,and the Jcorris 4.85×10-6A·cm-2and 2.27×10-7A·cm-2,respectively.Compared with single PEN/C surface modification layers and single MAO coatings,the Ecorrof the PEN/C+MAO hybrid coatings increases by 120 m V and 80 m V,respectively,and the Jcorrdecreases by two and one orders of magnitude,respectively,meanwhile,both the capacitive loop radius as well as the impedance modulus increase.These results suggest that the PEN/C+MAO hybrid coatings exhibit more excellent anti-corrosion performance than both single PEN/C surface modification layers and single MAO coatings.Particularly,the PEN/C+MAO hybrid coatings can provide relatively superior long-term corrosion protection and inhibit the occurrence of the pitting corrosion for AZ91D substrates.PEB belongs to inorganic infiltration,and the process of PEB experiences four stages:ionization,replacement,adsorption as well as diffusion.While both PEC and PEN/C belong to organic infiltration,meanwhile,the processes of PEC and PEN/C can be subdivided into three stages:thermal decomposition,adsorption and diffusion.Compared with PEB and PEC,PEN/C surface modification layers exhibits more superior anti-corrosion properties.There are two kinds of typical features on the surface of PEB+MAO,PEC+MAO and PEN/C+MAO hybrid coatings,namely less micropore areas as well as a sign of sintering seal pores of the coating materials.According to the results of corrosion resistance,the Jcorrof the three kinds of hybrid coatings is 3.42×10-7A·cm-2,2.07×10-7A·cm-2and 1.92×10-8A·cm-2,respectively.After immersion of 60,68 and 72 h,respectively,the low-frequency inductive loop can not be found from the Nyquist diagrams of the three kinds of hybrid coatings.These results indicate that the PEN/C+MAO hybrid coatings exhibit more excellent corrosion resistance than both PEB+MAO hybrid coatings and PEC+MAO hybrid coatings.PES pretreatment can afford an excellent micro-structure to increase the coating thickness and to improve the compactness of the PES+MAO hybrid coatings.And it also can provide a favorable pretreatment surface with predesigned chemical composition in order to obtain three new kinds of phases,namely Si B6?Si C and Si3N4with superior performances for the PES+MAO hybrid coatings followed.Additionally,during the preparation process of the PES+MAO hybrid coatings,an overlapping phenomenon in regard to the direction of coating thickness can be observed instead of heaping up layer by layer.All above make the adhesion strength of the PES+MAO hybrid coatings better than that of the single MAO coatings.An integrated state of the surface-modified layer and the substrate induced from the ion implantation characteristics of infiltrating atoms during PES process,and the high bonding strength brought by the in-situ growth characteristics of subsequent MAO process,work together to avoid low adhesion strength caused by stratification in common duplex or multilayer coatings. |
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