REFLEXIONES SOBRE LA TEORÍA Y LA PRÁCTICA DEL IVA EN COLOMBIA

Este trabajo discute, a la luz de la teoría tributaria, la manera de calcular el impuesto al valor agregado (IVA) de acuerdo con el Artículo 447 del Estatuto Tributario colombiano. El análisis teórico muestra que la implementación del IVA en Colombia no permite explotar todas las ventajas que el impuesto tiene en la teoría. En particular, la práctica colombiana induce cascadas tributarias y evita solo parcialmente las distorsiones en precios de bienes intermedios. A manera de ilustración, presentamos también una simulación numérica para ilustrar la magnitud del efecto de cascadas tributarias en el IVA colombiano. En este sentido, si bien el impuesto es claramente superior a un impuesto a las ventas en cada etapa de la cadena productiva, es bastante inferior al IVA teórico, resultando en tasas de tributación efectiva pueden ser el doble de las nominales. El documento demuestra, además, que la diferencia que se genera en precios según se use el método colombiano o el teórico, no genera diferencia en el recaudo real cuando el impuesto se aplica a todos los bienes. Es decir, mientras que el efecto de las cascadas será regresivo, el recaudo real no presentará variaciones.Impuesto al valor agregado incidencia tributaria cascadas

Overcharging: The Crucial Role of Excluded Volume

In this Letter we investigate the mechanism for overcharging of a single spherical colloid in the presence of aqueous salts within the framework of the primitive model by molecular dynamics (MD) simulations as well as integral-equation theory. We find that the occurrence and strength of overcharging strongly depends on the salt-ion size, and the available volume in the fluid. To understand the role of the excluded volume of the microions, we first consider an uncharged system. For a fixed bulk concentration we find that upon increasing the fluid particle size one strongly increases the local concentration nearby the colloidal surface and that the particles become laterally ordered. For a charged system the first surface layer is built up predominantly by strongly correlated counterions. We argue that this a key mechanism to produce overcharging with a low electrostatic coupling, and as a more practical consequence, to account for charge inversion with monovalent aqueous salt ions.Comment: 7 pages, 3 figs (4 EPS files). To appear in Europhysics Letter

Passive Behavior and Passivity Breakdown of AISI 304 in LiBr Solutions through Scanning Electrochemical Microscopy

The passive behavior and passivity breakdown of AISI 304 stainless steel in LiBr solutions has been investigated by means of scanning electrochemical microscopy (SECM). The sample generation - tip collection (SG-TC) mode was used to operate the SECM and the tip potential was biased to detect the electroactive species. The evolution of the current at the ultramicroelectrode tip with the applied potential within the passive range was followed at different LiBr concentrations. Results show that the absolute value of the current at the tip increases with the applied potential. Additionally, SECM was also used to detect stable pits formed on the stainless steel surface in a 0.2 M LiBr solution. The results show clear evidence of the presence of high amounts of other reducible species (metal cations) apart from oxygen. Also, the dish-shape morphology of the pits observed using Confocal Laser Scanning Microscopy will be discussed in relation to the kinetics of the reactions observed using SECM. (c) 2014 The Electrochemical Society. All rights reserved.The authors would like to express their gratitude to the Generalitat Valenciana for its help in the SECM acquisition (PPC/2011/013) and in the CLSM acquisition (MY08/ISIRM/S/100) and to Dr. Asuncion Jaime for her translation assistance.Fernández Domene, RM.; Sánchez Tovar, R.; García Antón, J. (2014). Passive Behavior and Passivity Breakdown of AISI 304 in LiBr Solutions through Scanning Electrochemical Microscopy. Journal of The Electrochemical Society. 161(12):565-572. https://doi.org/10.1149/2.1051412jesS56557216112Cobb Harold M. (Ed.), Steel Products Manual: Stainless Steels, Iron & Steel Society, 1999.Schweitzer P. A. , Corrosion Engineering Handbook: Fundamentals of Metallic Corrosion, CRC Press, Boca Ratón, FL., 2007.Hakiki, N. B., Boudin, S., Rondot, B., & Da Cunha Belo, M. (1995). The electronic structure of passive films formed on stainless steels. Corrosion Science, 37(11), 1809-1822. doi:10.1016/0010-938x(95)00084-wWijesinghe, T. L. S. L., & Blackwood, D. J. (2008). Photocurrent and capacitance investigations into the nature of the passive films on austenitic stainless steels. Corrosion Science, 50(1), 23-34. doi:10.1016/j.corsci.2007.06.009Hakiki, N. E. (1998). Semiconducting Properties of Passive Films Formed on Stainless Steels. Journal of The Electrochemical Society, 145(11), 3821. doi:10.1149/1.1838880Olefjord, I. (1985). Surface Composition of Stainless Steels during Anodic Dissolution and Passivation Studied by ESCA. Journal of The Electrochemical Society, 132(12), 2854. doi:10.1149/1.2113683Lothongkum, G., Chaikittisilp, S., & Lothongkum, A. . (2003). XPS investigation of surface films on high Cr-Ni ferritic and austenitic stainless steels. Applied Surface Science, 218(1-4), 203-210. doi:10.1016/s0169-4332(03)00600-7Freire, L., Carmezim, M. J., Ferreira, M. G. S., & Montemor, M. F. (2010). The passive behaviour of AISI 316 in alkaline media and the effect of pH: A combined electrochemical and analytical study. Electrochimica Acta, 55(21), 6174-6181. doi:10.1016/j.electacta.2009.10.026Roberge P. R. , Corrosion Engineering. Principles and Practice, 1st. ed., McGraw-Hill, New York, NY, 2008.Wipf, D. O. (1994). Initiation and study of localized corrosion by scanning electrochemical microscopy. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 93, 251-261. doi:10.1016/0927-7757(94)02872-9Casillas, N. (1994). Pitting Corrosion of Titanium. Journal of The Electrochemical Society, 141(3), 636. doi:10.1149/1.2054783Basame, S. B., & White, H. S. (1995). Scanning electrochemical microscopy of native titanium oxide films. Mapping the potential dependence of spatially-localized electrochemical reactions. The Journal of Physical Chemistry, 99(44), 16430-16435. doi:10.1021/j100044a034Still, J. W. (1997). Breakdown of the Iron Passive Layer by Use of the Scanning Electrochemical Microscope. Journal of The Electrochemical Society, 144(8), 2657. doi:10.1149/1.1837879Zhu, Y. (1997). Scanning Electrochemical Microscopic Observation of a Precursor State to Pitting Corrosion of Stainless Steel. Journal of The Electrochemical Society, 144(3), L43. doi:10.1149/1.1837487Basame, S. B., & White, H. S. (1998). Scanning Electrochemical Microscopy:  Measurement of the Current Density at Microscopic Redox-Active Sites on Titanium. The Journal of Physical Chemistry B, 102(49), 9812-9819. doi:10.1021/jp982088xWilliams, D. E. (1998). Elucidation of a Trigger Mechanism for Pitting Corrosion of Stainless Steels Using Submicron Resolution Scanning Electrochemical and Photoelectrochemical Microscopy. Journal of The Electrochemical Society, 145(8), 2664. doi:10.1149/1.1838697Lister, T. E., & Pinhero, P. J. (2002). Scanning Electrochemical Microscopy Study of Corrosion Dynamics on Type 304 Stainless Steel. Electrochemical and Solid-State Letters, 5(11), B33. doi:10.1149/1.1510621Lister, T. E., & Pinhero, P. J. (2003). The effect of localized electric fields on the detection of dissolved sulfur species from Type 304 stainless steel using scanning electrochemical microscopy. Electrochimica Acta, 48(17), 2371-2378. doi:10.1016/s0013-4686(03)00228-7González-Garcı́a, Y., Burstein, G. ., González, S., & Souto, R. . (2004). Imaging metastable pits on austenitic stainless steel in situ at the open-circuit corrosion potential. Electrochemistry Communications, 6(7), 637-642. doi:10.1016/j.elecom.2004.04.018Souto, R. M., González-Garcı́a, Y., & González, S. (2005). In situ monitoring of electroactive species by using the scanning electrochemical microscope. Application to the investigation of degradation processes at defective coated metals. Corrosion Science, 47(12), 3312-3323. doi:10.1016/j.corsci.2005.07.005Völker, E., Inchauspe, C. G., & Calvo, E. J. (2006). Scanning electrochemical microscopy measurement of ferrous ion fluxes during localized corrosion of steel. Electrochemistry Communications, 8(1), 179-183. doi:10.1016/j.elecom.2005.10.003Gabrielli, C., Joiret, S., Keddam, M., Perrot, H., Portail, N., Rousseau, P., & Vivier, V. (2007). A SECM assisted EQCM study of iron pitting. Electrochimica Acta, 52(27), 7706-7714. doi:10.1016/j.electacta.2007.03.008Yin, Y., Niu, L., Lu, M., Guo, W., & Chen, S. (2009). In situ characterization of localized corrosion of stainless steel by scanning electrochemical microscope. Applied Surface Science, 255(22), 9193-9199. doi:10.1016/j.apsusc.2009.07.003Santana, J. J., González-Guzmán, J., Fernández-Mérida, L., González, S., & Souto, R. M. (2010). Visualization of local degradation processes in coated metals by means of scanning electrochemical microscopy in the redox competition mode. Electrochimica Acta, 55(15), 4488-4494. doi:10.1016/j.electacta.2010.02.091González-García, Y., Santana, J. J., González-Guzmán, J., Izquierdo, J., González, S., & Souto, R. M. (2010). Scanning electrochemical microscopy for the investigation of localized degradation processes in coated metals. Progress in Organic Coatings, 69(2), 110-117. doi:10.1016/j.porgcoat.2010.04.006Yuan, Y., Li, L., Wang, C., & Zhu, Y. (2010). Study of the effects of hydrogen on the pitting processes of X70 carbon steel with SECM. Electrochemistry Communications, 12(12), 1804-1807. doi:10.1016/j.elecom.2010.10.031Aouina, N., Balbaud-Célérier, F., Huet, F., Joiret, S., Perrot, H., Rouillard, F., & Vivier, V. (2011). Single pit initiation on 316L austenitic stainless steel using scanning electrochemical microscopy. Electrochimica Acta, 56(24), 8589-8596. doi:10.1016/j.electacta.2011.07.044Bard A. J. Mirkin M. V. (Eds.), Scanning Electrochemical Microscopy, 1st. ed., Marcel Dekker, New York, NJ, 2001.Kaneko, M., & Isaacs, H. . (2000). Pitting of stainless steel in bromide, chloride and bromide/chloride solutions. Corrosion Science, 42(1), 67-78. doi:10.1016/s0010-938x(99)00056-6Frankel, G. S. (1998). Pitting Corrosion of Metals. Journal of The Electrochemical Society, 145(6), 2186. doi:10.1149/1.1838615Kaneko, M., & Isaacs, H. S. (2002). Effects of molybdenum on the pitting of ferritic- and austenitic-stainless steels in bromide and chloride solutions. Corrosion Science, 44(8), 1825-1834. doi:10.1016/s0010-938x(02)00003-3Abd El Meguid, E. A., & Mahmoud, N. A. (2003). Inhibition of Bromide-Pitting Corrosion of Type 904L Stainless Steel. CORROSION, 59(2), 104-111. doi:10.5006/1.3277539Anderko, A., & Young, R. D. (2000). Model for Corrosion of Carbon Steel in Lithium Bromide Absorption Refrigeration Systems. CORROSION, 56(5), 543-555. doi:10.5006/1.3280559Chau, D. S., Wood, B. D., Berman, N. S., & Kim, K. J. (1993). Solubility of oxygen in aqueous lithium bromide using electrochemical technique. International Communications in Heat and Mass Transfer, 20(5), 643-652. doi:10.1016/0735-1933(93)90076-8Macdonald, D. D. (1992). The Point Defect Model for the Passive State. Journal of The Electrochemical Society, 139(12), 3434. doi:10.1149/1.2069096Paola, A. D. (1989). Semiconducting properties of passive films on stainless steels. Electrochimica Acta, 34(2), 203-210. doi:10.1016/0013-4686(89)87086-0Hakiki, N. E., Montemor, M. F., Ferreira, M. G. S., & da Cunha Belo, M. (2000). Semiconducting properties of thermally grown oxide films on AISI 304 stainless steel. Corrosion Science, 42(4), 687-702. doi:10.1016/s0010-938x(99)00082-7Carmezim, M. J., Simões, A. M., Figueiredo, M. O., & Da Cunha Belo, M. (2002). Electrochemical behaviour of thermally treated Cr-oxide films deposited on stainless steel. Corrosion Science, 44(3), 451-465. doi:10.1016/s0010-938x(01)00076-2Sharma S. K. , Green Corrosion Chemistry and Engineering: Opportunities and Challenges, Wiley-VCH Verlag GmbH & Co., First Edition, Germany, 2012.Venkatraman, M. S., Cole, I. S., & Emmanuel, B. (2011). Corrosion under a porous layer: A porous electrode model and its implications for self-repair. Electrochimica Acta, 56(24), 8192-8203. doi:10.1016/j.electacta.2011.06.020Thomas, S., Cole, I. S., Sridhar, M., & Birbilis, N. (2013). Revisiting zinc passivation in alkaline solutions. Electrochimica Acta, 97, 192-201. doi:10.1016/j.electacta.2013.03.008Gao, S., Dong, C., Luo, H., Xiao, K., Pan, X., & Li, X. (2013). Scanning electrochemical microscopy study on the electrochemical behavior of CrN film formed on 304 stainless steel by magnetron sputtering. Electrochimica Acta, 114, 233-241. doi:10.1016/j.electacta.2013.10.009Lu, G., Cooper, J. S., & McGinn, P. J. (2007). SECM imaging of electrocatalytic activity for oxygen reduction reaction on thin film materials. Electrochimica Acta, 52(16), 5172-5181. doi:10.1016/j.electacta.2007.02.022Song C. Zhang J. , Electrocatalytic Oxygen Reduction Reaction, in: J. Zhang (Ed.), PEM Fuel Cell Electrocatalysts and Catalyst Layers, Ch. 2, Springer, London, 2008, p. 89.Macdonald, D. D. (1999). Passivity–the key to our metals-based civilization. Pure and Applied Chemistry, 71(6), 951-978. doi:10.1351/pac199971060951Macdonald, D. D., Rifaie, M. A., & Engelhardt, G. R. (2001). New Rate Laws for the Growth and Reduction of Passive Films. Journal of The Electrochemical Society, 148(9), B343. doi:10.1149/1.1385818Macdonald, D. D. (2006). On the Existence of Our Metals-Based Civilization. Journal of The Electrochemical Society, 153(7), B213. doi:10.1149/1.2195877Marconnet, C., Wouters, Y., Miserque, F., Dagbert, C., Petit, J.-P., Galerie, A., & Féron, D. (2008). Chemical composition and electronic structure of the passive layer formed on stainless steels in a glucose-oxidase solution. Electrochimica Acta, 54(1), 123-132. doi:10.1016/j.electacta.2008.02.070Rhode, S., Kain, V., Raja, V. S., & Abraham, G. J. (2013). Factors affecting corrosion behavior of inclusion containing stainless steels: A scanning electrochemical microscopic study. Materials Characterization, 77, 109-115. doi:10.1016/j.matchar.2013.01.006Newman, R. C., & Franz, E. M. (1984). Growth and Repassivation of Single Corrosion Pits in Stainless Steel. CORROSION, 40(7), 325-330. doi:10.5006/1.3593930Simões, A. M., Bastos, A. C., Ferreira, M. G., González-García, Y., González, S., & Souto, R. M. (2007). Use of SVET and SECM to study the galvanic corrosion of an iron–zinc cell. Corrosion Science, 49(2), 726-739. doi:10.1016/j.corsci.2006.04.021Beck, T. R. (1979). Occurrence of Salt Films during Initiation and Growth of Corrosion Pits. Journal of The Electrochemical Society, 126(10), 1662. doi:10.1149/1.2128772Alkire, R. C., & Wong, K. P. (1988). The corrosion of single pits on stainless steel in acidic chloride solution. Corrosion Science, 28(4), 411-421. doi:10.1016/0010-938x(88)90060-1Bastos, A. C., Simões, A. M., González, S., González-García, Y., & Souto, R. M. (2004). Imaging concentration profiles of redox-active species in open-circuit corrosion processes with the scanning electrochemical microscope. Electrochemistry Communications, 6(11), 1212-1215. doi:10.1016/j.elecom.2004.09.022Böhni H. , Localized Corrosion of Passive Metals, in: Winston Revie R. (Ed.), Uhlig's Corrosion Handbook, 2nd ed., Ch. 10, Wiley Interscience, New York, 2000.Leiva-García, R., García-Antón, J., & Muñoz-Portero, M. J. (2010). Contribution to the elucidation of corrosion initiation through confocal laser scanning microscopy (CLSM). Corrosion Science, 52(6), 2133-2142. doi:10.1016/j.corsci.2010.02.034Laycock, N. J., & Newman, R. C. (1997). Localised dissolution kinetics, salt films and pitting potentials. Corrosion Science, 39(10-11), 1771-1790. doi:10.1016/s0010-938x(97)00049-8Moayed, M. H., & Newman, R. C. (2006). The Relationship Between Pit Chemistry and Pit Geometry Near the Critical Pitting Temperature. Journal of The Electrochemical Society, 153(8), B330. doi:10.1149/1.2210670Ernst, P., & Newman, R. . (2002). Pit growth studies in stainless steel foils. I. Introduction and pit growth kinetics. Corrosion Science, 44(5), 927-941. doi:10.1016/s0010-938x(01)00133-0Ernst, P., Laycock, N. J., Moayed, M. H., & Newman, R. C. (1997). The mechanism of lacy cover formation in pitting. 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ANÁLISE DA RECIDIVA LOCAL DO CÂNCER DE MAMA EM MULHERES SUBMETIDAS À CIRURGIA CONSERVADORA.

Introdução: A cirurgia conservadora já é considerada procedimento de escolha quando a mulher é acometida por um tumor em estágio inicial. Em consequência desta conservação do tecido mamário, a recorrência local do câncer é uma crescente preocupação. Objetivos: Descrever o perfil sociodemográfico e clínico das mulheres com recidiva local do câncer de mama, submetidas à cirurgia conservadora no Hospital Santa Rita de Cássia/Afecc, Vitória- ES, cadastradas no período de Janeiro de 2000 a Dezembro de 2010, examinar a associação entre as variáveis clínicas e sociodemográficas e a incidência de recidiva local e analisar a sobrevida livre da recidiva local dessas mulheres. Metodologia: Foram realizados dois estudos: o primeiro trata-se de um estudo de coorte retrospectiva e o segundo refere-se a um estudo de sobrevida com a utilização de dados secundários. A amostra compôs-se por 880 casos de mulheres com diagnóstico de câncer de mama e atendidas no Hospital Santa Rita de Cássia/Afecc. Utilizou-se o método Kaplan-Meier e o modelo multivariado de riscos proporcionais de Cox, enquanto testou-se a significância estatística pelo método log-rank. Resultados: A recidiva ocorreu em 60 pacientes (6,8%) e a média do tempo entre a cirurgia e a recidiva de 35,5 meses. Na análise multivariada do estudo de sobrevida livre de recidiva local, identificou-se relação de risco para a faixa etária até 39 anos (p=0,083 e HR=6,19), comprometimento positivo das margens cirúrgicas (p=0,001 e HR= 3,49) e Her-2 positivo (P=0,033 e HR=1,89). Conclusões: A seleção de cada paciente para a conduta mais adequada deve ser feita cuidadosamente, de forma a estabelecer as principais características sociodemográficas e clínicas que possam auxiliar na escolha do melhor tratamento. Do mesmo modo, a escolha da melhor técnica cirúrgica é fundamental no sentido de reduzir os gastos com tratamentos de resgate e diminuir o sofrimento físico e psicológico da mulher

Network Diversity Multiple Access in Rayleigh Fading Correlated Channels with Imperfect Channel and Collision Multiplicity Estimation

Network diversity multiple access or NDMA is the family of algorithms with the highest potential throughput in the literature of signal-processing-assisted random access. NDMA uses the concept of protocol-induced retransmissions to create an adaptive source of physical (PHY) layer diversity. This adaptive diversity is used to resolve packet collisions (via signal separation) without the explicit need (or as a complement) of a multiple antenna receiver. This paper proposes a further improvement on the modelling of NDMA by considering the effects of imperfect channel and collision multiplicity estimation. In addition, this work considers channel correlation between consecutive retransmissions (i.e., temporal correlation). Conventionally, the analysis of NDMA assumes that any error in the collision multiplicity estimation translates into the loss of all contending packets. This is an optimistic assumption because even when the multiplicity has been correctly estimated, errors can still occur. On the other hand, it is also pessimistic because correct reception can also occur when the multiplicity has been incorrectly estimated. This paper presents a more detailed study of the performance of the protocol considering these more specific reception cases