CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / CNPq - Conselho Nacional de Desenvolvimento Científico e Tecnológico / A extração de petróleo em águas profundas impõe diversos desafios às empresas de óleo e gás e, dentre eles, destaca-se o desgaste sofrido na parte interna dos dutos de transporte devido à ação da água produzida (elevado teor salino), gases e particulados, principalmente a areia produzida. O objetivo deste trabalho foi estudar a erosão-corrosão de um aço com baixo teor de carbono (VMEC 134AP) em um flowloop sob condições dinâmicas: fluido em escoamento com 3,5 % de NaCl, areia em concentração de 40 g cm-3 e adição de substância inibidora de corrosão, com foco principal em observar a possível interação entre o agente erosivo e o inibidor. Para isso, realizaram-se ensaios de erosão-corrosão em um circuito fechado de tubulações com quatro polegadas de diâmetro interno (101,2 mm) e foram aplicadas as técnicas de medição da taxa de desgaste: cupons de perda de massa, resistência à polarização linear e espectroscopia de impedância eletroquímica. Foram monitoradas também a concentração de inibidor no fluido e as modificações na superfície dos corpos de prova por microscopia eletrônica de varredura. Ao fim deste trabalho, concluiu-se que o aço apresenta uma taxa de corrosão de 3,5 mm ano-1 sem a adição de inibidor e 0,4 mm ano-1 com a adição de inibidor. Não foi observada interação direta entre a areia e o inibidor de corrosão que pudesse comprometer a eficiência do mesmo. A técnica de espectroscopia eletroquímica foi empregada com sucesso em condições dinâmicas por meio de uma célula projetado especificamente para tubulações. Os resultados mostraram que a célula e a técnica foram capazes de identificar o efeito sinergético da erosão-corrosão na parte inferior da tubulação e também a atuação do inibidor de corrosão. / Deep water oil production imposes several challenges for oil and gas companies, and among them, the wear suffered in the interior of the transportation pipelines due to the action of produced water (high salt content), gases and particulates, especially produced sand. The objective of this work was to study the erosion-corrosion of a low carbon steel in a flowloop under dynamic conditions: fluid with 3.5% NaCl, sand at a concentration of 40 g cm-3 and addition of a corrosion inhibitory substance, with a main focus on observing the possible interaction between the erosive agent and the inhibitor. For this purpose, we performed erosion-corrosion tests in a closed circuit of pipes of four inches of internal diameter and we applied the corrosion rate measurement techniques of mass loss coupons, linear polarization resistance and electrochemical impedance spectroscopy. We also monitored the concentration of inhibitor in the fluid and the modifications on the surface of the specimens by scanning electron microscopy. At the end of this work, we concluded that the steel presents a corrosion rate of 3.5 mm year-1 without the addition of inhibitor and 0.4 mm year-1 with the addition of inhibitor. We did not observe any direct interaction between the sand and corrosion inhibitor that could compromise the efficiency of the substance. We also concluded that electrochemical techniques can be successfully employed in dynamic conditions by means of a cell specifically designed for pipelines. The results showed that the cell and the technique were able to identify the synergistic effect of erosion-corrosion in the bottom of the pipe and also the action of the corrosion inhibitor. / Tese (Doutorado)

The submitted work is aimed at the unconventional fluoride conversation coating preparation on the AZ31, AZ61, ZE10 and ZE41 magnesium alloys by their immersion in Na[BF4] molten salt. The influence of the preparation parameters (such as temperature and time) on the quality of the fluoride conversion coating is investigated. Methods of light and scanning electron microscopy were used for the evaluation of morphology, chemical composition and thickness of the coating. Short and long-term corrosion tests were executed to analyze the corrosion performance in simulated body fluid solution at 37 ± 2 °C with and without the fluoride conversion coating. The short-term behavior was evaluated by potentiodynamic tests, namely by the linear polarization. Long-term performance was assessed by electrochemical impedance spectroscopy or immersion tests. The coating preparation parameters influence on the character of the formed fluoride conversion coating was defined based on the obtained results. The next part of the thesis deals with the description of the possible mechanism of formation and kinetics of growth of the unconventional fluoride conversion coating on the selected AZ61 magnesium alloy. In this part, further detailed analyses were carried out to investigate the microstructure and chemical composition of the fluoride conversion coating using focused ion beam, transmission electron microscopy and X-ray photoelectron spectroscopy.

This thesis deals with the characterization of electrochemical corrosion properties of magnesium alloys as promising materials for biomedical applications. The wrought alloys AZ31 and AZ61 were used and exposed to corrosive environments of Hanks solutions (SBF) to simulate environmental conditions in living organisms. For the evaluation of the surfaces was used scanning electron microscopy (SEM) with elemental analysis measured by energy-dispersive spectroscopy (EDS). Short-term (5 min) and long-term (72 h) corrosion tests were conducted in order to optimize the measurement methodology and obtain corrosion parameters - especially corrosion potential (Ekor), corrosion current density (ikor) and polarisation resistance (RP). To evaluation of the short-term tests were by potentiodynamic tests, namely the linear polarization (LP) test. Long-term tests were measured by electrochemical impedance spectroscopy (EIS). Effects of the composition of the alloys (AZ31 and AZ61), surface treatment (grinding and polishing) and the composition of the solution (SBF without Ca, Mg, and with Ca, Mg) were compared. Complex corrosion behaviour in time was characterized and corrosion mechanisms were discussed.

In this dissertation, the operation of dual-linear polarized antennas is considered in order to provide ideal performance suited for several applications including polarimetric synthetic aperture radar (SAR), wireless and satellite communications. The underlying objectives realized in this work are reported as design realizations of dual-linear polarized antennas with low cross polarization patterns and high isolation between ports that employ special properties of the electromagnetic metamaterial (MTM) structures. Some of these key properties appear as negative permittivity, negative permeability, negative refractive index, and antiparallel nature of the phase velocity and the group velocity. The antenna design is carried out at two frequencies, 5.5 GHz and 10 GHz, and key physical issues that affect the operation of dual-linear polarization operation antennas are treated in light of electromagnetic MTM properties. It’s well known that a dual linear polarized antenna poses a big challenges such as cross polarization patterns and high mutual coupling between two input ports. Therefore, these drawbacks are key topic that receive significant attention in literature which reports on how to mitigate these drawbacks, however, at the expense of complexity of the antenna structures. The MTM structures have received considerable coverage in antenna research for obtaining size reduction, directivity enhancement, and beam steering. For this purpose, different MTMs structures are chosen in this thesis for achieving additional improvements, while keeping the antenna design as simple as possible, something which is very difficult to accomplish using conventional design methods.:Chapter 1: Introduction Chapter 2: Dual linear Polarization Antennas and Arrays Chapter 3: Fundamental Theory of Metamaterials Chapter 4: Metamaterial Structures Design-Methodology Chapter 5: Design of Dual Linear Polarization Using CRLH-TL Metamaterial Line Feed Chapter 6: Cross Polarization Discrimination Enhancement of a Dual Linear Polarization Antenna Using Split Ring Resonator Chapter 7: Antenna Array Design Chapter 8: Conclusions and Future Work / In der folgenden Dissertation wird der Einsatz von zweifach linear polarisierten Antennen zur idealen Ausführung von verschiedenen Anwendungen, einschließlich von polarimetrischen Synthetic Aperture Radar (SAR), kabellose und satellitengestützte Kommunikation, diskutiert. Die Ziele dieser Arbeit werden dargestellt durch die Gestaltung von zweifach linear polarisierten Antennen mit gering Kreuz-Polarisationsmustern und die starke Isolation zwischen den Ports durch die einzigartigen Eigenschaften der Strukturen des elektromagnetischen Metamaterials (electromagnetic metamaterial; MTM). Einige dieser Eigenschaften treten als negative Permittivität, negative Permeabilität, negativer Brechungsindex und als antiparallel Richtungen (Gegenvektor) der Phasen-und Gruppengeschwindigkeit auf. Somit wird die Antennengestaltung auf zwei Frequenzen übertragen, 5,5GHz und 10 GHz, und die Ausführung der zweifach linearen Polarisation wird durch die elektromagnetischen Eigenschaften des MTM illustriert. Weil die Kreuzpolarisationsmuster und starke gegenseitige Koppelung zwischen zwei Input-Ports bei einer zweifach linear polarisierten Antenne große Schwierigkeiten bereiten, werden diese im Großteil der Fachliteratur als Schwerpunkte gesetzt, was zu einer Milderung der Nachteile führte, jedoch dafür die Komplexität der Antennenstruktur zunahm. Die Vielfalt an MTM ist ein bedeutender Teil im Bereich der Antennenforschung einschließlich der Größenverkleinerung, der Verbesserung der Richtcharakteristik und der Strahlensteuerung. Für diesen Zweck werden in dieser Dissertation verschiedenste MTM Strukturen ausgewählt um weitere Verbesserungen der Antennenstruktur zu ermöglichen und gleichzeitig die Einfachheit der Struktur zu bewahren, was mit konventionellen Gestaltungsmethoden nur schwer zu erreichen ist.:Chapter 1: Introduction Chapter 2: Dual linear Polarization Antennas and Arrays Chapter 3: Fundamental Theory of Metamaterials Chapter 4: Metamaterial Structures Design-Methodology Chapter 5: Design of Dual Linear Polarization Using CRLH-TL Metamaterial Line Feed Chapter 6: Cross Polarization Discrimination Enhancement of a Dual Linear Polarization Antenna Using Split Ring Resonator Chapter 7: Antenna Array Design Chapter 8: Conclusions and Future Work

No description available.

Made available in DSpace on 2014-12-17T15:41:43Z (GMT). No. of bitstreams: 1 ThiagoRR.pdf: 1895833 bytes, checksum: 59d0cd6d03df2b5c8165e3ae0bf39a80 (MD5) Previous issue date: 2008-04-25 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / An interesting development in surfactants science and technology is their application as corrosion inhibitors, since they act as protective films over anodic and cathodic surfaces. This work aims to investigate the efficiency of saponified coconut oil (SCO) as corrosion inhibitor and of microemulsified system (SCO + butanol + kerosene oil + distilled water), in saline medium, using an adapted instrumented cell, via techniques involving linear polarization resistance (LPR) and mass loss coupons (MLC). For this, curves of efficiency versus SCO concentration (ranging between 0 and 75 ppm) have been constructed. According to the obtained results, the following efficiency levels were reached with OCS: 98% at a 75 ppm concentration via the LPR method and 95% at 75 ppm via the MLC method. The microemulsified system, for a concentration of 15 ppm of SCO, obtained maximum inhibition of 97% (LPR) and 93% (MLC). These data indicate that it is possible to optimize the use of SCO in similar applications. Previous works have demonstrated that maximal efficiencies below 90% are attained, typically 65% as free molecules and 77% in microemulsified medium, via the LPR method in a different type of cell. Therefore, it can be concluded that the adapted instrumented cell (in those used methods) showed to be an important tool in this kind of study and the SCO was shown effective in the inhibition of the metal / Tensoativos v?m sendo alvo de interesse como inibidores de corros?o, pois funcionam como pel?culas protetoras sobre ?reas an?dicas e cat?dicas. Este trabalho teve como objetivo avaliar a efici?ncia de inibi??o ? corros?o do tensoativo ?leo de coco saponificado (OCS) e do sistema icroemulsionado (OCS + butanol + querosene + ?gua destilada), em meio salino, utilizando-se uma c?lula instrumentada adaptada. A efic?cia do OCS foi avaliada utilizando-se os m?todos de resist?ncia de polariza??o linear (LPR) e cupons de perda de massa (CPM), tendo sido constru?das curvas de efici?ncia versus concentra??o de OCS (variando de 0 a 75 ppm). De acordo com os resultados obtidos, verificou-se que este tensoativo apresenta a seguinte ordem de efici?ncia de inibi??o ? corros?o: para uma concentra??o de 75 ppm, 98% pelo m?todo LPR e 95% pelo m?todo CPM. O sistema microemulsionado, para uma concentra??o de 15 ppm de OCS, obteve inibi??o m?xima de 97% (LPR) e 93% (CPM). Atrav?s deste trabalho foi poss?vel otimizar o uso do tensoativo OCS, que em trabalhos anteriores mostrou efic?cia m?xima inferior a 90% (63% na sua forma livre e 77% na sua forma microemulsionada, pelo m?todo LPR, em um outro tipo de c?lula). Conclusivamente, pode-se dizer que a c?lula instrumentada adaptada (nos m?todos utilizados) provou ser uma ferramenta importante neste tipo de estudo e o OCS mostrou-se efetivo na inibi??o do metal

Submitted by Sandra Pereira (srpereira@ufrrj.br) on 2017-03-21T13:39:54Z No. of bitstreams: 1 2015 - Arthur Valbon Rodrigues.pdf: 2339821 bytes, checksum: 867f44f511f5a52dfdcf023619e589a9 (MD5) / Made available in DSpace on 2017-03-21T13:39:54Z (GMT). No. of bitstreams: 1 2015 - Arthur Valbon Rodrigues.pdf: 2339821 bytes, checksum: 867f44f511f5a52dfdcf023619e589a9 (MD5) Previous issue date: 2015-08-04 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior - CAPES / In this work six asymmetric thyourea, derived from benzylamine and phenethylamine, were synthesized by reaction of p-substituted phenyl isothiocyanates in the presence of benzylamine and phenethylamine in toluene as solvent by irradiation in ultrasonic bath, featuring the first report in the literature of the adoption of this method for such molecules. The compounds were obtained in high purity, requiring only one recrystallization from hexane/dichloromethane. Yields were very satisfactory, ranging from 89 to 98%. The compounds were characterized by spectroscopic techniques such as 1H, 13C Nuclear Magnetic Resonance and IR. The synthesized compounds were evaluated by electrochemical potentiodynamic polarization techniques, Electrochemical Impedance Spectroscopy and Linear Polarization Resistance as inhibitors to the corrosion AISI 1020 carbon steel in 1M HCl media. Molecular modeling was used for better visualization of the thiourea structures and to correlate theoretical parameters such as the energy of the symmetrical frontier orbital, Mulliken charge on the sulfur atom and dipole moment with the experimental results for corrosion efficiency. In general, all compounds showed corrosion inhibition efficiency with characteristics of mixed inhibitors with anodic trend, however compounds with nitro substituent showed less efficiency, a fact that may be related to their electrons withdrawing potential, which consequently decreases the electron density at the possible center of chemical adsorption, the sulfur atom. However, compounds which showed no substituent and those that contain electron donating group (OCH3) stood out as inhibitors, highlighting N- (p-methoxyphenyl) -N'-phenetylthiourea, which showed 95% efficiency corrosion inhibition, from Electrochemical Impedance spectroscopy. Thus, the asymmetric thiourea benzyl and phenethyl-substituted, especially methoxy-substituted phenyl, can be considered as promising corrosion inhibitors for carbon steel in acid. / Nesse trabalho de disserta??o foram sintetizadas seis tioureias assim?tricas, derivadas da benzilamina e fenetilamina. A obten??o ocorreu atrav?s da rea??o dos isotiocianatos de fenila p-substituidos na presen?a da benzilamina e fenetilamina em tolueno como solvente atrav?s de irradia??o em banho de ultrassom, sendo o primeiro relato na literatura dessa metodologia para tais mol?culas. Os compostos foram obtidos em alto grau de pureza, sendo necess?ria apenas uma recristaliza??o em hexano/diclorometano. Os rendimentos foram muitos satisfat?rios, variando de 89 ? 98%. Os compostos foram caracterizados por t?cnicas espectrosc?picas como Resson?ncia Magn?tica Nuclear 1H,13C e Infravermelho. Os compostos sintetizados foram avaliados atrav?s das t?cnicas eletroqu?micas de Polariza??o Potenciodin?mica, Espectroscopia de Imped?ncia Eletroqu?mica e Resist?ncia a Polariza??o Linear quanto ? a??o como inibidores de corros?o frente a a?o carbono AISI 1020 em HCl 1 mol.L-1. A Modelagem Molecular foi utilizada para melhor visualiza??o das estruturas das tioureias e correlacionar os par?metros te?ricos como a energia dos orbitais de fronteiras sim?tricos, carga de Mulliken no ?tomo de enxofre e momento dipolar com os resultados experimentais da efici?ncia anticorrosiva. De forma geral, todos apresentaram efici?ncia de inibi??o ? corros?o com caracter?sticas de inibidores mistos com tend?ncia an?dica, por?m os compostos com substituinte nitro apresentaram menor efici?ncia, fato esse que pode estar relacionado ao seu poder retirador de el?trons, que consequentemente, diminui a densidade eletr?nica no poss?vel centro de adsor??o qu?mica, o enxofre. Por?m, os compostos que n?o apresentaram substituintes e os que cont?m grupo doador de el?trons (OCH3) se destacaram como inibidores, dando destaque a N-(p-metoxi-fenil)-N?-fenetiltioureia, que apresentou efici?ncia de 95% de inibi??o da corros?o, pela t?cnica de Espectroscopia de Imped?ncia Eletroqu?mica. Assim, as tioureias assim?tricas benzil e fenetil-substitu?das, especialmente met?xi-fenil substitu?das, podem ser consideradas como promissores agentes inibidores da corros?o para o a?o-carbono em meio ?cido.

This dissertation summarizes three research projects related to microwave radiation induced electron transport properties in the GaAs/AlGaAs two dimensional electron systems. In chronological order, the projects are: a microwave reflection and electron magneto-transport correlation study, the combined microwave power and polarization dependence on microwave radiation induced magneto-resistance oscillations study, and a comparative study about the effect of circularly polarized and linearly polarized microwaves radiation on magneto-resistance oscillations induced due to the microwave. These three research projects experimentally address many interesting issues in the non-equilibrium low dimensional electron transport under microwave irradiation and provide potential applications of utilizing microwave radiation induced magneto-resistance oscillations in two dimensional electron systems as a method to detect different qualities of microwaves or terahertz waves.

Made available in DSpace on 2014-12-17T14:07:04Z (GMT). No. of bitstreams: 1 GineideCA_DISSERT.pdf: 4064193 bytes, checksum: e2de6f4f6fcb019525ce377c5625f694 (MD5) Previous issue date: 2012-10-26 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / In the search for products that act as corrosion inhibitors and do not cause environmental, impact the use of plant extracts as corrosion inhibitors is becoming a promising alternative. In this work the efficiency of polar extracts (ethanol extracts) obtained from the plants Anacardium occidentale Linn (AO) and Phyllantus amarus Schum. & Thonn (PA) as corrosion inhibitors were evaluated in different concentrations. For that AO and PA extracts were solubilized in the microemulsion systems (SME) containing saponified coconut oil as surfactant (SME -OCS and SME-OCS-1) in saline (NaCl 3,5 %) solution, which was also used as electrolyte. Both SME-OCS and SME-OCS-1 were characterized by surface tension and viscosity methods showing a Newtonian fluid behavior. The SME-OCS and SME-OCS-1 systems satisfactorily solubilized the polar extracts AO and PA with measurements carried out by ultraviolet spectroscopy. The measurements of corrosion inhibition efficiencies were performed by the electrochemical linear polarization resistance (LPR) technique as well as weight loss, on the surface of AISI 1020 carbon steel. The maximum corrosion inhibition efficiencies were determined by extrapolation of Tafel plots, showing the following values: 95,6 % for the system SME-OCS-AO, 98,9 % for the system SME-OCS-AO-1 and 93,4 % for the system SME-OCS-PA / Na busca por produtos que combatam a corros?o e ao mesmo tempo n?o agridam o meio ambiente, a utiliza??o de extratos vegetais como inibidores de corros?o v?m se tornando uma alternativa promissora. Neste trabalho a efici?ncia de inibi??o ? corros?o foi avaliada em diferentes concentra??es de extratos polares (etan?licos) obtidos das plantas Anacardium occidentale Linn (AO) e Phyllantus amarus Schum. & Thonn (PA). Para tanto, os extratos AO e PA foram solubilizados no sistema microemulsionado (SME) contendo o tensoativo ?leo de coco saponificado (SME-OCS e SME-OCS-1) em solu??o salina (NaCl 3,5 %), que tamb?m foi utilizada como eletr?lito. Estes sistemas microemulsionados foram caracterizados por m?todos que avaliam a tens?o superficial e viscosidade, tendo sido evidenciado que se comportam como fluidos Newtonianos. As medidas de efici?ncia de inibi??o ? corros?o foram realizadas atrav?s da t?cnica eletroqu?mica de resist?ncia ? polariza??o linear (LPR) e perda de massa, na superf?cie de a?o carbono AISI 1020. A corrente aplicada ao eletrodo foi controlada pelo equipamento Potenciostato/Galvanostato possibilitando a medi??o da diferen?a de potencial el?trico entre o eletrodo de trabalho e o de refer?ncia. Os sistemas SME-OCS e SME-OCS-1 solubilizaram os extratos etan?licos AO e PA, com quantifica??es realizadas por espectroscopia na regi?o do ultravioleta. As efici?ncias m?ximas de inibi??o ? corros?o foram determinadas atrav?s da extrapola??o das curvas de Tafel, tendo apresentado os seguintes valores: 95,6 % para o sistema SME-OCS-AO, 98,9 % para o sistema SMEOCS- 1-AO e 93,4 % para o sistema SME-OCS-PA

Made available in DSpace on 2014-12-17T14:58:19Z (GMT). No. of bitstreams: 1 FernandoNS_DISSERT.pdf: 5044327 bytes, checksum: 66e88407297dda873c6a6d882b6de312 (MD5) Previous issue date: 2012-08-27 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / A batch of eighty-four coupons of low carbon steel were investigated at laboratory conditions under a corrosive, cavitative-corrosive (CO2) and corrosive-erosive (SiO2 + CO2) in an aqueous salt solution and two levels of temperature. The following measurements were made on Vickers (HV0,05, HV0,10, HV0,20) Microhardness tests at three levels of subsurface layer. A turbulent flow collided on the cylindrical sample, with and without mechanical stirring and gas bubbling, with and without fluid contamination by solid particles of SiO2, at two temperatures. Surface Roughness and Waviness, under two conditions "as received, after machining" and "after worn out", as well as gravimetric and electrochemical parameter were measured on the two opposite generatrices of each cylindrical sample, on the flow upstream (0?) and downstream (180?) by Profilometry, Mass Variation and Linear Polarization Resistance (LPR). The results of the Microhardness and Surface Texture of all coupons were subjected to statistical comparison, using the software Statgraphics? Centurion XVI, 95% statistical certainty, and significant differences were observed in some arrays of measurements. The corrosive wear rate measured by LPR and mass variation shown to be sensitive to the presence of bubbles and hydrodynamic fluctuations inside the cell, considering the temperature and contamination of corrosive fluid by solid particles. The main results of visual inspection relative to some topologies of the surface damages involving different mechanisms that were seen to give explanation for some fluctuations in wear rates of the steel experimentally investigated / Um lote de oitenta e quatro corpos-de-prova (CP) de a?o AISI 1018 foi investigado em laborat?rio sob solicita??es corrosivas, cavitativo-corrosivas (CO2) e erosivocorrosivas (SiO2+CO2) em meio aquoso salino e a duas temperaturas. Foram realizados ensaios de microdureza Vickers a tr?s n?veis de subsuperf?cie (HV0,05, HV0,10, HV0,20). Um fluxo agitado colidiu contra os CPs, em todas as condi??es de ensaio, sem e com agita??o mec?nica e borbulhamento de g?s, sem e com contamina??o do fluido por part?culas s?lidas de SiO2, nas duas temperaturas estudadas. A rugosidade e ondula??o superficial, nas condi??es como recebidos, ap?s usinagem e ap?s desgastados , bem como medi??es gravim?tricas e eletroqu?micas foram realizadas em duas geratrizes opostas de cada corpo-deprova, a montante (0?) e a jusante (180?), atrav?s das t?cnicas de perfilometria, varia??o m?ssica e resist?ncia de polariza??o linear (RPL). Os resultados da microdureza e da textura superficial em todos CPs foram submetidos ? compara??o estat?stica, utilizando-se o software Statgraphics? Centurion XVI e, com 95% de certeza estat?stica, diferen?as significativas foram observadas em alguns grupos de medidas. As medidas das taxas de desgaste corrosivo, por RPL e por varia??o m?ssica, demonstraram serem sens?veis ? presen?a de bolhas e ?s flutua??es hidrodin?micas no interior da c?lula, consideradas a temperatura e a contamina??o do fluido corrosivo por part?culas s?lidas. Apresentam-se, tamb?m, os principais resultados da inspe??o visual em que se evidenciam algumas topologias do dano superficial nas geratrizes a 0? e a 180?, involvendo diferentes mecanismos que foram presenciados, para justificar algumas flutua??es nas taxas de desgaste do a?o em estudo