Alumínio Pulverizado Termicamente (TSA) é amplamente utilizado em instalações offshore como revestimento de ânodo de sacrifício em tubulações de aço carbono. O transporte e a instalação desses componentes podem levar a pequenos danos no revestimento, o que pode expor a superfície do aço à água do mar. Sabe-se que o depósito calcário é formado na superfície do aço polarizado catodicamente. Assim, esta pesquisa avaliou o TSA aplicado por sistema de pulverização de arco duplo (TWAS) no aço ao carbono S355J2 + N quando ocorrem danos (holidays) para estudar a formação de depósitos calcários no aço e adquirir dados sobre o desempenho do TSA sob altas temperaturas. A aplicação de diferentes condições também foi considerada: presença de selantes; liga do revestimento (99,5% de Al e Al-5% de Mg) e condições enterradas/não enterradas. Dois tipos de experimentos foram realizados: i) testes em potencial livre sob temperaturas constantes (30, 60 e 90°C) e diferentes tamanhos de holidays (expondo 5, 10 e 20% da superfície do aço) e ii) testes sob gradient térmico onde óleo a ~125°C foi adicionado em uma torre polimérica e água externa a ~10°C ficou em contato com a superfície das amostras (tanto potencial livre como polarização de -950 mVAg/AgCl foram aplicados). Análises incluíram inspeção visual, microscópio eletrônico de varredura e difração de Raio-X. A partir dos testes em temperaturas constante, o TSA atingiu um bom potencial de proteção (-800 a -900 mVAg/AgCl) e pouca diferença nos resultados devido à diferença na composição doTSA e no tamanho do holiday foi observada. As taxas de corrosão foram mantidas entre 0,02 e 0,01 mm/ano. No ensaio sob gradiente térmico e potencial livre, a perda de revestimento e as taxas de corrosão foram de 0,4 a 0,002 mm/ano. Além disso, o potencial alcançado foi de uma faixa menor do que a obtida anteriormente (-745 a -835 mVAg/AgCl). No entanto, quando o TSA foi combinado com proteção catódica externa e gradiente térmico, a espessura do TSA foi satisfatório e as taxas de corrosão obtidas foram inferiores a 0,076 mm/ano. O depósito calcário formado no holiday protegeu o aço contra a corrosão e seu mecanismo de crescimento baseado nesta pesquisa foi construído. / Thermally Sprayed Aluminium (TSA) is widely used in offshore facilities as sacrificial anode coating for carbon-steel risers and pipelines. Transportation and installation of those components can lead to small damages in the coating, which can expose the steel surface to the seawater. It is known that calcareous deposit is formed on the cathodically polarised steel surface. Thus, this research evaluated the TSA applied by twin wire arc spray system (TWAS) on S355J2+N carbon-steel when damage (holidays) is present in order to study the calcareous deposit formation on steel and acquire data regarding the TSA performance at high temperatures. Application of different conditions was also considered: presence of sealing; coating alloy (99.5%Al and Al-5%Mg) and buried/unburied conditions. Generally, two types of experiment were conducted – i) tests at free potential at steady temperatures (30, 60 and 90°C) and different holiday sizes (exposing 5, 10 and 20% of the steel surface) and ii) tests under thermal gradient where oil at ~125°C was added in polymeric tower and external water at ~10°C was in contact with the samples surface (both free potential and polarisation of -950 mVAg/AgCl were applied). Methodology of analyses included visual inspection, scanning electron microscope and X-ray Diffraction. From the tests at steady temperatures, the TSA reached a very good protective potential (-800 to -900 mVAg/AgCl) and little difference in results due to difference in TSA composition and holiday size was observed. Corrosion rates were kept between 0.02 and 0.01 mm/year. From the thermal gradient test under free potential, the coating loss and corrosion rates were 0.4 to 0.002 mm/year. Also, the potential achieved was in a lower range than previously obtained (-745 to -835 mVAg/AgCl). However, when TSA was combined with external cathodic protection and thermal gradient, the thickness of the TSA was satisfactory and corrosion rates obtained were below 0.076 mm/year. The calcareous deposit formed within the holiday protected the steel substrate against corrosion and its growth mechanism based in this research was built.

Aluminized steel Type 2 (AST2), often used for culvert pipes, is subject to corrosion which is the most important durability limitation factor. It was desired to determine if the outer aluminized layer will retain passivity and if protective galvanic action will develop. Thus, corrosion of unblemished and blemished AST2 surfaces was investigated in simulated natural waters. Experiments with unblemished specimens showed passive corrosion rates (~0.06 µm/yr) in scale-forming, 0.01 M Cl- solutions but sustained corrosion in other less protective media (with rates 3~10 µm/yr). Corrosion was manifested macroscopically by discoloration and few macro pits, but it likely proceeded also microscopically at the Fe-rich inclusion space scale. For blemished specimens, the aluminized coating galvanically protected to some extent the steel in all solutions. However, in 0.01 M Cl- solutions, protection was delayed until after some steel corrosion had occurred. In some solutions, complete consumption of the outer aluminized coating around exposed steel was noted. Elsewhere, coating appearance was similar to that of the unblemished condition. Nominal durability projections made for 16-gage AST2 ranged from >100 yr for unblemished AST2 to ~10 yr for the blemished condition. The present findings were used as a first step in proposing refinements of presently used durability guidelines of AST2 culvert pipe. Cyclic cathodic polarization tests to examine O2 and H2 reduction at the Fe-rich inclusions showed significant hysteresis, more pronounced with decreasing scan rate. The effect was tentatively associated to the amount of Fe+2 being deposited during the downward scan, a hypothesis supported by results from a physical model. A static polarization model was formulated for the blemished configuration. Results matched experimental trends and permitted evaluating the effect of solution conductivity s beyond the experimental range. Exposed steel corrosion rates at the steel were increasingly large for decreasing s. For the lowest s, corrosion rates at the exposed steel center were distinctly larger than at the edge, consistent with experiments. An impedance behavior model was also formulated. Results showed frequency dependent current distribution and predicted relatively small artifacts that were and not evident experimentally, but should be considered when exploring other system conditions.

Thin film coatings are used to improve the properties of components and products in such diverse areas as tool coatings, wear resistant biological coatings, miniature integrated electronics, micro-mechanical systems and coatings for optical devices. This thesis focuses on understanding the development of intrinsic stress and microstructure in coatings of the technologically important materials of aluminium nitride (AlN) and titanium vanadium nitride (TiVN) deposited by filtered cathodic arc deposition. Thin films of AlN are fabricated under a variety of substrate bias regimes and at different deposition rates. Constant substrate bias was found to have a significant effect on the stress and microstructure of AlN thin films. At low bias voltages, films form with low stress and no preferred orientation. At a bias voltage of -200 V, the films exhibited the highest compressive stress and contained crystals preferentially oriented with their c axis in the plane of the film. At the highest bias of -350 V, the film forms with low stress yet continue to contain crystallites with their c axis constrained to lie in the plane of the film. These microstructure changes with bias are explained in terms of an energy minimisation model. The application of a pulsed high voltage bias to a substrate was found to have a strong effect on the reduction of intrinsic stress within AlN thin films. A model has been formulated that predicts the stress in terms of the applied voltage and pulsing rate, in terms of treated volumes known as thermal spikes. The greater the bias voltage and the higher the pulse rate, the greater the reduction in intrinsic stress. At high pulsing and bias rates, a strong preference for the c axis to align perpendicular to the substrate is seen. This observation is explained by dynamical effects of the incident ions on the growing film, encouraging channelling and preferential sputtering. For the first time, the effect of the rate of growth on AlN films deposited with high voltage pulsed bias was investigated and found to significantly change the stress and microstructure. The formation of films with highly tensile stress, highly compressive stress and nano-composites of AlN films containing Al clusters were seen. These observations are explained in terms of four distinct growth regions. At low rates, surface diffusion and shadowing causes highly porous structures with tensile stress; increased rates produced Al rich films of low stress; increasing the growth rate further led to a dense AlN film under compressive stress and the highest rates produce dense, low stress, AlN due to increased levels of thermal annealing. Finally this thesis analyses the feasibility of forming ternary alloys of high quality TiVN thin films using a dual cathode filtered cathodic arc. The synthesised films show exceptional hardness (greater than either titanium nitride or vanadium nitride), excellent mixing of the three elements and interesting optical properties. An optimum concentration of 23% V content was found to give the highest stress and hardness.

Steel corrosion in reinforced concrete is a major concern to transportation agencies nationwide because of the expenses incurred for repair and ultimate shortening of bridge life. Cathodic protection (CP), as a remedy, has been applied to reinforced bridges in the US since 1974. However, application of this technique is largely empirical, lacking fundamental understanding. In order to optimize the performance of a CP system, it is important to monitor the rebar potential with respect to a reliable reference electrode. Moreover, because of potential variation in the concrete, reference cell placement is fundamental to ensure effective protection. The work plan was divided into two parts: laboratory scale experimentation and computer simulation. In the experimentation section, the response of graphite probes was compared to that of an Orion silver-silver chloride electrode. Graphite probes behaved as well as the standard electrode. Furthermore, the home-made graphite probes behaved the same as the commercial ones. This will allow much greater experimental latitude since the home-made probes are much more economical than the commercial ones. A finite difference code was developed to assess the performance of cathodic protection. The potential distribution in a two dimensional geometry of a concrete block with a sprayed zinc anode at one boundary and an iron cathode at the other side was calculated under cathodic protection. The equations were solved by means of a Gauss-Seidel iterative method with the help of an overrelaxation factor. An interval halving method was used to solve for nonlinear boundary condition at the iron. The effects of concrete pore saturation, concrete cover, and applied potential were studied to determine the degree of protection and proper placement of the reference electrode in concrete. Furthermore, a sensitivity analysis was performed versus input parameters: concrete conductivity, oxygen mass transfer coefficient, and oxygen reduction polarization parameters. The results of the simulation showed that the center of the rebar is less protected than the other locations. Therefore, the reference electrode should be located as close to the center as possible. / Graduation date: 1995

This research explores the thermal stability of ZrCrAlN material system. For this purpose fourteen different compositions of ZrCrAlN coatings were deposited onto tungsten carbide substrates by using reactive cathodic arc evaporation. These compositions were further annealed at 800oC, 900oC, 1000oC and 1100oC temperatures. EDS was employed to specify the compositions. The crystal structure of the coatings were analysed by XRD, and the hardness of these coatings was determined by Nanoindentation. The experimental findings reported a significant age hardening of Zr0.16Cr0.12Al0.72N and a delayed h-AlN formation in Zr0.07Cr0.40Al0.52N. ZrCrAlN was thus proved to be thermally stable. / Multifilms,A4:2 Growth and characterization of Multicomponent Nitrides by Magnetron Sputtering and Arc evaporation

The polarization performance of two types of commercial galvanic point anodes for protection of reinforced steel around patch repairs was investigated. Experiments included measurement of the polarization history of the anode under constant current impressed by galvanostatic circuits and in reinforced concrete slabs. The tests revealed, for both types of anodes, a potential-current function (PF) indicating relatively little anodic polarization from an open circuit potential at low current levels, followed by an abrupt increase in potential as the current approached an apparent terminal value. Aging of the anodes was manifested by a continually decreasing current output in the concrete tests, and by increasingly more positive potentials in the galvanostatic tests. Those changes reflected an evolution of the PF generally toward more positive open circuit potentials and, more importantly, to the onset of elevated polarized potentials at increasingly lower current levels. There was considerable variability among the performance of replicate units of a given anode type. Modest to poor steel polarization levels were achieved in the test yard slabs. Modeling of a generic patch configuration was implemented with a one-dimensional approximation. The model calculated the throwing distance that could be achieved by a given number of anodes per unit perimeter of the patch, concrete thickness, concrete resistivity, amount of steel and amount of polarization needed for cathodic prevention. The model projections and aging information suggest that anode performance in likely application scenarios may seriously degrade after only a few years of operation, even if a relatively optimistic 100 mV corrosion prevention criterion were assumed. Less conservative criteria have been proposed in the literature but are yet to be substantiated. Other investigations suggest a significantly more conservative corrosion prevention may apply instead. The latter case would question the ability of the point anodes to provide adequate corrosion prevention.

The filtered cathodic vacuum arc (FCVA) technique is a well established deposition method for wear resistant mechanical coatings. More recently, this method has attracted attention for growing ZnO based transparent conducting films. However, the potential of FCVA deposition to prepare ZnO layers for electronic devices is largely unexplored. This thesis addresses the use of FCVA deposition for the fabrication of active ZnO based electronic devices. The structural, electrical and optical characteristics of unintentionally doped ZnO films grown on different sapphire substrates were systematically investigated. The potential of FCVA to grow both polar and non-polar ZnO films was demonstrated. The resulting films showed considerable promise for device applications with properties including high transparency(> 90%), moderate intrinsic carrier concentrations (10¹⁷ - 10¹⁹ cm⁻³), electron mobilities up to 110 cm⁻²/Vs, low surface roughness (< 5 nm) and well-structured photoluminescence. Post-growth annealing in oxygen at temperatures up to 800 C produced significant improvements in the electronic and optical properties of these films, due to the formation of larger grains with lower inter-grain potential barriers. Silver oxide (AgOᵪ ) and iridium oxide (IrOᵪ) Schottky diodes fabricated on annealed FCVA ZnO films showed ideality factors as low as 1.20, barrier heights up to 0.85 eV and high sensitivity to ultraviolet light (up to ̴ 10⁻⁵ at -2 V). Transparent and opaque MESFETs fabricated on these films showed well defined field effect characteristics, channel mobilities up to 70 cm⁻²/Vs and insensitivity to 1 mW/cm⁻² visible light. These devices were further subjected to extensive bias and temperature stress tests. MESFET stability appeared to be strongly dependent on Schottky gate type, bias conditions and ZnO film morphology. Positive bias stress of AgOᵪ gated devices resulted in irreversible damage, that is thought to be due to Ag electromigration across the gate interface. Mapping of the surface potential of the ZnO channel material with Kelvin probe force microscopy suggested a strong relationship between the defect density at grain boundaries and both channel mobility and current stability. Interval growth techniques were found to reduce the density of defects at grain boundaries and produced MESFETs with higher current stability. IrOᵪ gated devices showed superior bias stability and temperature resilience from 25 C-195 C.

Cathodic processes occurring in the solutions containing Cu(II) tartaric complexes have been investigated. The equations accounting for the material balance have been constructed and used for the estimation of distribution of complexes and ligands in the bulk of solution. It was established that dominating particles in acidic media (pH < 3) are: Cu2+ and tartaric complex CuL, tartaric acid LH2 and its anion LH-. Potentials of non-polarized copper electrodes were found to be reversible and to follow Nernst equation. According to the analysis performed, surface oxide Cu2O is not able to form in acidic (pH < 3) media. Cathodic voltammograms obtained for the solutions of different acidity exhibit two characteristic current peaks arising from Cu(II) reduction and hydrogen evaluation. An analysis of voltammetric extrema shows that both processes are irreversible. Based on the regularities of the mass transport of chemically interacting substances, surface distribution of components has been simulated. These date were used for the transformation of experimental voltammograms into normalized Tafel plots. According to their analysis, the rate-controlling step of Cu(II) reduction is the transfer of the first electron onto Cu2+ aqua-complex. The values of kinetic parameters were found to be as follows: the cathodic charge transfer coefficient is equal to 0.33 and the exchange current density is equal to 50 mA cm-2. An analysis of the second current peak leads to the conclusion... [to full text]

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Thesis (PhD) - Swinburne University of Technology, School of Engineering and Science, 2008. / A thesis submitted for the degree of Doctor of Philosophy, School of Engineering and Science, Swinburne University of Technology, 2008. Typescript. Includes bibliographical references (p. 189-204).