Failure Mechanism And Surface Modification For Noble Oxide Coated Titanium Anodes

The main objeavtives of this thesis was to investigate the failure mechanism for noble oxide coated titanium anodes (Dimentionally stable anodes,DSA) because it is of great significance to guide DSA application in electrogalvanized line(EGL).Electrolysis factors,such as electrolyte composition,temperature and current density,degradation mechanism,surface modifications (SM) including special surface treatment (SST),metal Ta/Pt surface treatment and anodic oxidation of Ti substrac for DSA were systematically investigated by XRD,XPS,FE-SEM,SEM,TEM,SIMS, standard accelerated lifetime test (SALT) and EIS. In order to further improve the perfamance of DSA with anodic oxidation modification,Pt,Ir,the noble metal dopant was carried out.A three-step procedure was used for the preparation of the Pt/Ir modified Ti/TiO₂ anodes:First,a microporous titanium dioxide is formed by anodisation of the titanium substrate,then post-trement with chemical method and then doping Pt or Ir into the microporous of the TiO₂ film by cathodic deposition.The morphology,structure and electrocatalytic activities of the Ti/TiO₂ electrodes prepared at different technique parameters were characterized and electrochemical behaviors were measured. In addition,the characterizations of the Pt/Ir modified film electrodes and the cathodic depositon process for doping nano-crystalline Pt,Ir were also systematically studied.Finally,the reason was discussed that the improvement durability of iridium oxide coated titanium anode with a tantalum modified interlayer. Percolation threshold values (PTV) theory was applied for the explanation conductor mechanism of the valve metal oxide doping with conductor phases such as metal or active oxides.The main results can be summariesed as follows:1) The optimum electrolysis conditions for the service lifetime test can be summarizedas follows:the H2SO4 solution concentration is 0.5mol.L^-1 ;the electrolyte temperature is 313～333K and the current density is 400A.dm^-2.The degradation mechanism can be described that oxygen evolution process corroded the surface coating layer by layer,and that the cracks occurred and expanded resulting in cracking and fragmentation,peeling off the oxide coating further,which defined as physical process;that there is selectivity of Ir consumption and there is titanium element existing in the coating,which was thought to come from electrochemical dissolution of the substrate connected to the defect layer contact with electrolysis solution directly,which defined as chemical process.The physical and chemical process resulted in Ir content decrease absolutely and relatively,respectively,in the coating layer.The ratio of active and inert components has been decreasing gradually in the coating layer down to critical value which was called PTV resulting in the stop of the oxygen evolution process.2) The servicelife of the chemical treatment modified electrodes and the metal Ta/Ptsurface treatment modified electrodes were increased,and the service lives of them are longer two and four times than that of traditional electrode which boiling in 10% oxalic acid solution,respectively.Further more the electrodes with SST and Pt sublayer are so far satisfactory for practical applications in EGL.3) It is found that the TiO₂ films with nano-crystalline and multiporous can be obtainedby anodic oxidation method, the films are composed of anatase and amophous mix structures,the crystal size is about 20nm.The nature of electrolyte has a great effect on the morphology of TiO₂ films.The microporous structure of TiO₂ thin film was formed in the acidic electrolyte,while the film was smooth and compact in the alkaline electrolyte.The optimum technique parameters for the preparation of the Ti/TiO₂ film electrode are that:the electrolyte solution is 1 mol.L^-1 H₂SO₄;the oxidation voltage is 120V;The temperature is controlled at ambient temperature;and the oxidation time is about 11min. The thickness of film prepared under such condition is about 730nm,the surface pore size is about 350nm,and the porosity of the film is 12.7%.The specific resistance of the Pt/Ir modified Ti/TiO₂ electrodes greatly decreased,and the electrical conductivity of modified anode improved.The electrochemical behaviour of the Pt modified Ti/TiO₂ with 1min deposition was similar to that of a pure platinum bulk electrode. As deposition time prolonged,the electrocatalytic activity of Pt modified Ti/TiO₂ electrode was increased further.4) The reason of the improvement durability of iridium oxide coated titanium anode with a tantalum modified interlayer results from the preventing the substrate from corrosion,which is not only high performance corrosion resistance but also heat comparability between iridium oxide coated and sublayer.5) Failure physical characteristics of IrO₂ +Ta₂O₅ coating can be described that the ratio of Ir(active) and Ta+Ti(inert components) has been decreasing to the threshold resulted from electrolysis process,which is critical value for the electrical conductivity of the layer,so electrolytic catalytic activity is loss.The innovation can be found in the method of SM, such as SST and nano-crystalline TiO₂ films with nano-crystalline Pt,Ir dopant, and the application of PTV theory in this thesis.