Anodizing | EPFL Graph Search

Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. The process is called anodizing because the part to be treated forms the anode electrode of an electrolytic cell. Anodizing increases resistance to corrosion and wear, and provides better adhesion for paint primers and glues than bare metal does. Anodic films can also be used for several cosmetic effects, either with thick porous coatings that can absorb dyes or with thin transparent coatings that add reflected light wave interference effects. Anodizing is also used to prevent galling of threaded components and to make dielectric films for electrolytic capacitors. Anodic films are most commonly applied to protect aluminium alloys, although processes also exist for titanium, zinc, magnesium, niobium, zirconium, hafnium, and tantalum. Iron or carbon steel metal exfoliates when oxidized under neutral or alkaline micro-electrolytic conditions; i.e.,

Heat treatment of carbon anodes for the aluminum industry

Mauriz Lustenberger

Aluminum is produced by the electrolytic reduction of alumina dissolved in molten cryolite. This process requires carbon anodes which are consumed during the electrolysis. The anodes contribute roughly with 15% to the total aluminum production cost. If the anode quality is poor the share can as well reach 25 %. It is known that the anode behavior during aluminum electrolysis is significantly influenced by the anode baking process. However, there is a lack of knowledge to predict the anode behavior through given baking parameters. In addition, process control of industrial furnaces is still to a large degree empirical. Often the emission of industrial furnaces causes problems as deposits can accumulate in the exhaust system which leads to fire incidents. This investigation has three major goals: The prediction of the anode behavior during electrolysis for a given heat treatment. The understanding of the process control of industrial scale open-top anode baking furnaces. The emission control of the open-top anode baking furnace. The baking parameters are defined in this thesis as the anode heat-up rate and the baking level. Through pilot baking it is shown that the baking level is basically determined by the final baking temperature although the soaking duration has some influence too. The anode baking level can be estimated with a calculation for any given baking curve. Therefore the impact of the temperature and soaking duration on the real density anode property needs to be determined through a pilot study. The optimum baking parameters regarding the anode behavior during electrolysis depend on the material. Some anode materials react more sensitive to the heat treatment than others do and are therfore more difficult to handle in the industrial baking process. Through pilot scale anode baking it is shown how to predict the anode behavior during electrolysis. Process control of the anode baking in industrial open-top furnaces is complex since the anode baking is indirectly controlled by the combustion process inside the flues. Through measurements the combustion process is analyzed in this thesis. The results show that a furnace operated at its physical limits is difficult to control regarding to emissions. However, the gained knowledge allows to optimize existing furnace process control systems. The furnace emissions are mainly caused by insufficient combustion of the pitch fumes formed during the pyrolysis of the anode binder pitch. It is shown that emission problems cannot always be solved by a well chosen control system setup. In such a situation combustion control is required in addition to temperature control. A technology has been developed to measure online the combustion situation of each flue. For this purpose 2-color pyrometers, together with an intelligent signal processing, are required. This technology is integrated since 10 months in an industrial baking furnace control system. The results are promising. The developed technology is an important step for furnace operation at maximum productivity and minimum emission. A patent has been applied for the combination of online combustion analysis concurrent with temperature measurement (swiss apply No. 01598/02 by Mauriz Lustenberger and Felix Keller, R&D Carbon Ltd. Sierre).

EPFL2004

Nanostructured Functional Particles and Surfaces

Arthur Olivier Joseph Ganz

Topographic nanostructuring of surfaces is a promising concept, in particular to improve the bio-integration of titanium orthopedic implants. In this context, a new method for the nanostructuring of anodised titanium surfaces was developed. Ordered topographic features in the tens of nanometre in height were created by anodising electropolished titanium in the presence of polymeric particles deposited as monolayers. To do so, an existing electropolishing method for titanium was applied and optimised, allowing the production of extremely smooth starting surfaces. The particle-substrate contact was mesured, modeled and finally modified by plasma, thermal and chemical vapour treatments and its effect on the topography of the anodic oxide layer was investigated. Different types of ordered structures were produced, and characterised by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The influence of the anodisation conditions on the topography and morphology of the surface was studied. To gain deeper understanding of the mechanisms at play, a model experiment using electron beam lithography was designed. Circular masks of increasing diameters were deposited on the surface to simulate the presence of particles of corresponding sizes, and the effect on the topography of the oxide layer was characterised. Some aspects of the structuring phenomena observed with the particles were thus cleared up, in particular the extent of oxide layer growth underneath the masks. Finally, numerical finite element modeling was applied to simulate the initial stages of anodic oxide growth around masks, leading to a better understanding of the formation of some of the topographic features observed. In parallel to this work on surface structuring, mesoporous silica particles containing different functional nanoparticles were produced. They were originally destined to serve as an alternative to the polymeric particles used to create our structurations. Their synthesis process, based on the formation of particles by sol-gel in a miniemulsion, was scaled-up and optimised to obtain narrowly distributed submicron particles with a high micropore volume and a high functional nanoparticle loading. These multifunctional particles were finally found to be unadapted for surface structuring but were assessed for various other applications, depending on the type of nanoparticles incorporated in the silica matrix. In combination with superparamagnetic iron oxide nanoparticles (SPIONS), they were evaluated as potential magnetic drug delivery vehicles. Drug loading and release of the anticancer drug paclitaxel was studied as a model system by simulation and experiment. The release kinetics was found to be very slow, restricted by the large chemical potential difference between the molecule in solution and the molecule adsorbed inside micropores. Chromium doped alumina (ruby) nanoparticles were synthesised and characterised for potential application as near-IR fluorescent markers for biomedical imaging. Their photoluminescent properties were studied and were found to be sufficiently intense for bright-field fluorescence microscopy and confocal laser scanning microscopy. They could however not be incorporated in the multifunctional particles during the gel emulsion synthesis. Manganese doped zinc sulphide quantum dots were also encapsulated in a mesoporous silica matrix and used as the basis material for the production of multicolour fluorescent surfaces. Localised laser thermal treatment of the material was successfully realised, causing a shift in its fluorescence emission wavelength. Multicolour fluorescent images could thus be drawn by programing the laser appropriately.

EPFL2010

Chemical and electrochemical promotion of supported rhodium catalyst

Olena Baranova

The chemical and electrochemical promotion of highly dispersed nanofilm Rh catalysts (dispersion: about 10 %, film thickness: 40 nm) has been investigated for the first time. To this end Rh metal was sputter-deposited, either on a purely ionic conductor (8 % Y2O3-stabilized ZrO2) or on a mixed ionic-electronic conductor (TiO2), the latter being a highly dispersed layer of TiO2 (4 µm) deposited on YSZ. These catalysts are designated as Rh/YSZ and Rh/TiO2/YSZ, respectively. It was established analytically that both in the Rh/YSZ and in the Rh/TiO2/YSZ system, the catalyst films have a nanoparticle-size grain structure. The Rh supported on titania is rather porous, exhibiting a higher dispersion and surface area than Rh on YSZ. Both after reduction (H2, T=400 °C) and after oxidation (O2, T=400 °C), Rh supported on TiO2 was found to be in a more highly reduced state than Rh on YSZ. After reducing treatment, the Rh/TiO2/YSZ samples contain a larger amount of weakly bonded oxygen, which can be attributed to oxygen "backspillover" from the TiO2. A major feature of this research was the electrochemical characterization of the oxygen/Rh/solid electrolyte three-phase boundary by steady-state polarization measurements and by impedance spectroscopy. These are powerful techniques for extracting experimental trends and details that are useful for an understanding of the electrochemical promotion principles. It was shown that the exchange current densities at the Rh/solid electrolyte interface are lower on account of the TiO2 layer. The exchange current densities are more than twice lower at Rh/TiO2(4 µm)/YSZ than at Rh/YSZ, demonstrating that the former interface is much more polarizable. The mechanism of oxygen exchange occurring close to equilibrium (O2/O2- couple) was investigated for the first time at Rh catalyst electrodes interfaced with solid electrolyte. The processes of cathodic oxygen reduction and anodic oxygen evolution are not symmetric, but they are similar in the two systems, Rh/YSZ and Rh/TiO2(4 µm)/YSZ. The cathodic process consists of three steps: dissociative adsorption of oxygen at the gas-exposed Rh surface, atomic oxygen diffusion to the electrochemical reaction sites (ERS), and a two-electron transfer to this oxygen on the ERS. The cathodic process is limited by interfacial diffusion of oxygen atoms from the gas-exposed metal surface to the ERS. The anodic process includes two steps: two-electron transfer reaction, which is the rate-determining step, and oxygen desorption to the gas phase. With data obtained from impedance spectroscopy at the equilibrium potential, it was possible to confirm the reaction scheme proposed. It was demonstrated that Rh nanofilm catalysts interfaced with YSZ or TiO2/YSZ can be electrochemically promoted for the reaction of ethylene oxidation. Small anodic currents cause periodic oscillations in catalytic rate and potential of the Rh/YSZ catalyst, while at Rh/TiO2/YSZ they give rise to a stable and reversible rate enhancement by up to a factor 80. The increase in ethylene oxidation rate is up to 2000 times larger than the electrochemical rate, I/2F, of O2- oxidation. The pronounced electrochemical promotion behavior that has been observed is due to the anodically controlled migration of O2- species from the electrolyte to the Rh/gas interface. At the Rh surface, these species destabilize the formation of rhodium surface oxide (Rh2O3). The existence of backspillover oxygen species has been confirmed by impedance measurements under positive applied potential. Another significant result for heterogeneous catalysis is the finding that thick as well as thin films of Rh/TiO2/YSZ catalyst are open to chemical promotion of ethylene oxidation and partial methane oxidation, ie, they offer a higher catalytic activity and stability than the Rh/YSZ catalysts. The modification of catalytic activity observed for Rh/TiO2/YSZ was attributed to either a "long-range" electronic-type SMSI mechanism or to a self-driven electrochemical promotion mechanism. In both cases, the ultimate cause of promotion are different work functions of catalyst and support. Equilibration of the work functions of two solids in contact induces surface charging, a migration of O2- ions to the catalyst/gas interface, and a weakening of the Rh-O chemisorptive bonds. It facilitates reduction of oxidized surface sites. Another important achievement of the present work was that of exploring and confirming the possibilities of current-assisted activation of Rh/TiO2/YSZ catalysts. In partial methane oxidation using close to stoichiometric gas compositions (CH4 : O2 = 2 : 1) at 550 °C, the inactive (oxidized) Rh/TiO2/YSZ catalyst was successfully activated by applied currents, either positive or negative. This phenomenon is an example of "permanent" electrochemical promotion furnishing a permanent rate enhancement ratio of γ = 11. The activation by negative currents is explained in terms of an electrochemical reduction of rhodium surface oxide, while the activation by positive currents can be explained by the mechanism of electrochemical promotion.

EPFL2005