Controllable Preparation Of Corrosion Resistant Coatings And Surface Functionalization Of Magnesium Alloys Via Deep Eutectic Solvents

Proper structural design can effectively improve the corrosion resistance of coatings,and innovative preparation technology is an important approach to develop novel coatings.This dissertation focuses on the key issues of structure improvement and performance optimization for corrosion protection coatings on steels and magnesium alloys.We constructed various corrosion resistant structure such as double layers,compositional variations,biomimetic surfaces and multiple barriers.A series of coating technologies are proposed based on a non-aqueous deep eutectic solvent(DES)system.These strategies aim at making coatings multi-functional with enhanced properties.Further research reveals the synthetic growth process and corrosion protection mechanism of the specially structured coatings.This work provides theoretical bases and technical supports for the developments of multi-functional,high-performance coatings and the industrial applications of green,cheap,simple,efficient technologies.In Charpter Three,we investigated the co-deposition behavior of Co-Sn alloys without additives.Alloy films with various morphologies and structures can be obtained by adjusting the deposition parameters.It is found that the film depostited at 75 ? and-1.2 V possesses a self-organized double-layered structure as well as the best corrosion resistance.The double-layered structure includes a compact Co-riched inner layer and a porous Sn-riched outer layer,which results from the current fluctuation triggered by bubble evolution of DES electrolysis during the electrodeposition at an overhigh potential.In consideration of the passivation of Zn,we designed a double-layered Zn-Sn alloy film to futher improve the corrosion resistance.An alloy film with Sn-riched inner layer and Zn-Sn mixed outer layer can be formed through a potentiodynamical deposition from DES.Less Zn is incorporated in the inner layer and the Zn content is heterogeneous across the outer layer.Such a specific structure results in multiple passivations of the Zn-Sn alloy film and effectively suppresses the pitting corrosion.In Charpter Four,we proposed a novel electro-brush plating(EBP)technique to overcome the compromise on film quality caused by the decomposition of DES in the electrodepositing or electroplating(EP)process.The unique deposition dynamics at the electrode/DES interface during the EBP process was also revealed.In the case of nanocrystalline Ni,the mechanical property,tribological performance and corrosion resistance of the DES EBP Ni film are superior to those of the DES EP Ni film.Moverover,we developed a DES-based trivalent chromium plating process and studied the electrochemical reduction behavior of Cr(?)in DES.If the deposition parameters like voltage and current are accurately controlled and optimized,smooth,compact and crack-free Cr and Cr-P films can be obtained.Surface defect and roughness can be reduced by depositing Cr-based films on the EBP Ni underlayer.The influence of structure and composition of the Cr and Cr-P films on the corrosion resistance in NaCl and H2SO4 solutions was further discussed in this chapter.In Charpter Five,we designed and fabricated biomimetic coatings with superhydrophobic effect,self-healing ability and slippery property on Mg alloy.Cu films were directly electrodeposited on active substrates form the inert DES.The influence of NaH2PO2 additive on the film nucleation kinetics was also investigated.After stearic acid modification for only 3 min,the Cu film with hierarchical porous structures can achieve the superhydrophobic surface(SHS)and shows better corrosion resistance.Such a rapid modification process is related to the chemical reaction of CuO on the film surface and the physical adsorption caused by its high specific surface area.Though the trapped air pocket layer in SHSs can prevent the coating/substrate from the corrosive medium,such a corrosion protection is not stable.SHSs would gradually collapse and transform to wetting states in aqueous solutions.Therefore,we designed a conversion coating with dual corrosion inhibition functions of superhydrophobicity and self-healing on Mg alloy.After the collapse of SHS,Cr and Cr2O3 components in the ultrasonic-assisted electroless deposited Cr(?)conversion coating are corroded and oxidized to form Cr(?)-based oxides/compounds that deposit on the cracks and defects,thus realizing the self-healing effect.Such a "smart" coating exhibits superior corrosion resistance.However,SHSs have poor stability in corrosive medium and are generally susceptible to condensation and frosting in humid environments.Therefore,we further designed a slippery liquid-infused porous surface(SLIPS)to overcome these disadvantages.A layered double hydroxide-carbonate(LDH-cabonate)coating with double-layered structure was synthesized on Mg alloy via a one-step hydrothermal reaction.SHS and SLIPS coatings were obtained by subsequent surface modifications.It is found that the SLIPS coating is superior to the SHS coating on corrosion resistance,anti-icing performance and durability.It should be attributed to the multiple barriers including the infused water-repellent lubricant,the self-assembled monolayer-coated porous top layer and the compact LDH-carbonate composite under layer.In Chapter Six,we explored the interface reaction between DESs and Mg alloy as well as the film formation process under external fields.An anodic treatment process in DES for Mg alloy conversion coating preparation was proposed bsed on the electrochemical decomposition of choline chloride-ethylene glycol system.At anodic voltage,crystallographic orientation of the Mg alloy substrate is preferentially dissolved and the DES is electrolyzed.The derivatives produced by DES decomposition react with the substrate to form a MgCO3-MgO conversion coating.The surface morphology with either interconnected porous networks or jagged nanorod arrys depends on the anodic current density.An ionothermal synthesis process using DES to prepare Mg alloy conversion coating was developed based on the thermal decomposition of choline chloride-urea system.Under high temperature,the DES is pyrolyzed.The decomposition products corrode the Mg alloy to generate H2 that diffuses into the substrate,thus forming a MgCO3-MgH2 conversion coating.The surface morphology is a nanoscaled rough structure,which results in an extremely low reflectance(?10%).Both the conversion coatings in this charpter are not thick,but they increase the corrosion resistance of the substrates.The property can be further improved by preparing other coatings on it or taking surface modifications.