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Perchlorate is a threat to public health through water but also food. However, there is no effective chemical treatment process which can destroy perchlorate found in groundwater and surface water. Thus, there is growing interest in developing effective technologies, especially chemical treatments, to completely destroy trace levels of perchlorate present in drinking and groundwater. The research on perchlorate reduction by zero-valent titanium (Ti(0)) showed that perchlorate was effectively reduced to chloride using electrochemically developed pitting corrosion on Ti(0). Perchlorate reduction was believed to be caused by an active reductant (dissolved Ti(II)) during the pitting corrosion of Ti(0). The rate of perchlorate reduction was independent on the imposed potential as long as the potential was maintained above the pitting potential of Ti(0), but it was proportional to the applied current. The perchlorate reduction on the pitting developed Ti(0) was inhibited by the presence of chloride and bromide. Inhibition mechanism of perchlorate reduction inhibition was believed to be caused either by competitive adsorption of aggressive anions on bare Ti(0) surface or Ti(II) consumption by electrochemically produced chlorine. Kinetic models were developed based surface coverage of aggressive anions on bare Ti(0) and Ti(II) oxidation by chlorine. These kinetic models supported the perchlorate concentration change in the solution, but Ti(II) consumption model was not able to predict chloride concentration due to insufficient information describing complex nature of pitting on Ti(0). These results shown in this research demonstrate that pitting corrosion developed Ti(0) has the capability to chemically reduce perchlorate present in natural water and engineered systems as well as possible problems associated with electric input. This research may be a starting point for development of a new treatment process that applies titanium or titanium metal ions as a chemical reductant to abate contaminants present in natural and engineering systems. Further developments can be achieved by alloying titanium metal with other metals such as iron and aluminum, and finding a methodology producing stable Ti(II) in ambient conditions.

Made available in DSpace on 2014-06-11T19:28:35Z (GMT). No. of bitstreams: 0 Previous issue date: 2003Bitstream added on 2014-06-13T20:37:40Z : No. of bitstreams: 1 silva_jwj_me_guara.pdf: 1820882 bytes, checksum: 7e96fbad2b277d2b628c44604b9cf46d (MD5) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Foram caracterizados os comportamentos eletroquímicos e avaliadas as resistências à corrosão das ligas aeronáuticas 2024-T351 e 7050-T7451 em soluções aquosas de cloreto contendo cromato, molibdato e tungstato. Foram realizados ensaios de corrosão não-eletroquímicos de imersão prolongada acompanhados de análise metalográfica de superfície por microscopia óptica e identificação dos produtos de corrosão por difratometria de raios-X. A análise quantitativa de superfícies das ligas após a imersão, indica que os pites formados têm áreas médias similares. Os pites são mais largos do que profundos e de geometria, predominantemente, cônica ou quase-cônica e irregular. Em todos os produtos de corrosão de cada liga foi encontrado hidróxido de alumínio, em suas diferentes formas cristalinas. Medidas de perda de dureza, como uma conseqüência da deterioração superficial, também foram determinadas. Além disso, ensaios eletroquímicos como medidas de potencial em circuito aberto, curvas de polarização e voltametria cíclica complementaram este estudo. Em meio aerado os resultados obtidos mediante medidas eletroquímicas são consistentes com aqueles obtidos nos ensaios de imersão, em particular o efeito do CrO42- e do MoO42-. O WO42- mostrou-se agressivo em períodos prolongados de imersão. Apesar dos ensaios revelarem uma redução parcial de MoO42- em ambas as ligas, o efeito desse oxi-ânion parece ser diferente sobre cada liga. Em meio desaerado as ligas apresentam passivação em todos os eletrólitos. A adição dos oxi-ânions não modificou significativamente o potencial de pite para a liga 7050, enquanto que para a liga 2024 ele foi deslocado levemente para valores mais positivos. / It has been characterized the electrochemical behavior and evaluated the 2024-T351 and 7050-T7451 aircraft alloys corrosion resistance in chloride aqueous solutions containing chromate, molybdate and tungstate. It has been carried out non-electrochemical long immersion corrosion testings accompanied by surface metalography analysis achieved by light microscopy and corrosion products identification by X-ray difratometry. Surfaces quantitative analysis upon the alloys after immersion, indicates that formed pits have similar average area. Pits are widther than deeper and own predominantly a conical or quasi-conical and irregular geometry. In all corrosion products of each alloy it has been found aluminum hydroxide in its different crystalline ways. Hardness loss measurements have also been determined. In addition, electrochemical testings such as open circuit potential measures, polarization curves and cyclical voltammetry have completed this study. In aerated means the obtained results before electrochemical mesurements are similar to those obtained in the immersion tests, in particular CrO42- and MoO42- effects. WO42- has been found to be aggressive in very long immersion period. Though tests display a MoO42- partial reduction in both alloys, this oxi-anion effect seems to be different upon each alloy. In de-aerated means alloys present passivation in all eletrolytes. Oxi-anion addition has not changed significantly pit potential for 7050 alloy, while for 2024 alloy it has been dislocated, slightly, for more positive values.

Pitting corrosion is a form of highly localized corrosion that can lead to crack and failure of a structure. Study on pitting corrosion is necessary in order to predict and prevent the risk of failure of structure susceptible to corrosion. In this thesis, a combination of Cellular Automata (CA) and Boundary Element Method (BEM) was developed to simulate pitting corrosion growth under certain environment. It is assumed that pitting corrosion can be simplified to electrochemical corrosion cell. The distribution of potential around this corrosion cell can then be simulated by BEM. This distribution potential represents cathodic and anodic reactions around the corrosion cell. A CA model was developed that uses transition rules reflecting mechanism of pitting corrosion. The CA model has two types of cell states, one reflecting BEM simulation results and the other reflecting the status of corrosion cell (anode, cathode, and passive metal’s surface). For every CA iteration, the CA decides the state of the corrosion cells (the location and size of anode, cathode) while BEM simulate the level of electrochemical activity at discrete location on the surface (represented by potential distribution). In order to demonstrate the methodology, a simple case of rectangular corrosion cell with varied dimensions and under different polarization functions is considered. Results show certain shapes tend to grow at certain type environment and these pits are comparable to commonly observed pit shapes. In addition, stress analysis was carried out to investigate the severity of corrosion pits of varying shapes and sizes. Results show that certain pits induced highly varying stress concentration as it grows over time, while others have more steady increase of stress concentration.

Steam generators in nuclear power plants experienced varying degrees of under-deposit pitting corrosion. A probabilistic model to accurately predict pitting corrosion is necessary for effective life-cycle management of steam generators. This thesis presents an advanced probabilistic model of pitting corrosion characterizing the inherent randomness of the pitting process and measurement uncertainties of the in-service inspection (ISI) data obtained from eddy current (EC) inspections. A Bayesian method is developed for estimating the model parameters. The proposed model is able to estimate the number of actual pits, the actual pit depth as well as the maximum pit depth, which is the main interest of the pitting corrosion model. A MATLAB program of the Markov chain Monte Carlo technique is developed to perform the Bayesian estimations. Simulation experiments are performed to check the behavior of the Bayesian method. Results show that the MCMC algorithm is an effective way to estimate the model parameters. Also, the effectiveness and efficiency of Bayesian modeling are validated. A comprehensive case study is also presented on the in-service inspection data of pitting corrosion in a steam generator unit. The Weibull distribution is found to be an appropriate probability distribution for modeling the actual pit depth in steam generators.

Steam generators in nuclear power plants experienced varying degrees of under-deposit pitting corrosion. A probabilistic model to accurately predict pitting corrosion is necessary for effective life-cycle management of steam generators. This thesis presents an advanced probabilistic model of pitting corrosion characterizing the inherent randomness of the pitting process and measurement uncertainties of the in-service inspection (ISI) data obtained from eddy current (EC) inspections. A Bayesian method is developed for estimating the model parameters. The proposed model is able to estimate the number of actual pits, the actual pit depth as well as the maximum pit depth, which is the main interest of the pitting corrosion model. A MATLAB program of the Markov chain Monte Carlo technique is developed to perform the Bayesian estimations. Simulation experiments are performed to check the behavior of the Bayesian method. Results show that the MCMC algorithm is an effective way to estimate the model parameters. Also, the effectiveness and efficiency of Bayesian modeling are validated. A comprehensive case study is also presented on the in-service inspection data of pitting corrosion in a steam generator unit. The Weibull distribution is found to be an appropriate probability distribution for modeling the actual pit depth in steam generators.

Highly oxidized species like perchlorate and nitrate that are released into the environment by anthropogenic activities are a source of concern as they have been known to contaminate groundwater. These species are extremely soluble in water and can migrate through aquifer systems, travelling substantial distances from the original site of contamination. Due to their high solubility, these oxy-anions cannot be treated using conventional treatment processes like filtration and sedimentation. Several treatment technologies are currently available to abate the human health risk due to exposure to perchlorate and nitrate. However, most of the existing treatment processes are expensive or have limitations, like generation of brines with high concentrations of perchlorate or nitrate. Aluminum can effectively reduce perchlorate and nitrate, if the protective oxide film that separates the thermodynamically reactive Al0 from most environments is removed. Aluminum was activated by pH change and electrochemically induced, pitting corrosion to remove the passivating oxide layer and expose the underlying, thermodynamically reactive, zero-valent aluminum. A partially oxidized species of aluminum, like monovalent aluminum, is believed to bring about the reduction of perchlorate and nitrate. This research studied the reduction of perchlorate and nitrate by aluminum that was activated by these two mechanisms. Results indicated that aluminum activated by pH change resulted in an instantaneous decrease in perchlorate concentration without any increase in chlorate or chloride concentrations, which suggests that the perchlorate might be adsorbed on the aluminum oxide surface. However, aluminum activated by electrochemically induced pitting corrosion can effectively reduce perchlorate to chlorate. Nitrate, on the other hand, was reduced completely to ammonia by both treatment mechanisms. The studies conducted in this dissertation suggest that aluminum can be effectively used as a reducing agent to develop a treatment process to reduce perchlorate and nitrate.

Resumo: A resistência e o mecanismo de corrosão das ligas de alumínio 2024, 7010, 7050 e 7475 foram estudados em solução de NaCl. Os efeitos do tratamento térmico nas ligas, concentração de oxigênio, pH, adição de oxi-ânions e temperatura do meio constituem algumas das variáveis estudadas. Primeiramente, procedeu-se à caracterização físicoquímica dos materiais através de análise química e metalográfica, mediante microscopia eletrônica de varredura e espectroscopia de energia dispersiva. As ligas 2024, 7010, 7050 e 7475 como recebidas, recozidas e envelhecidas, revelaram a existência de partículas ternárias e quaternárias, constituídas por Al:Cu:Fe e Mg ou Zn. Também foi observado um maior número de partículas pequenas de composição variável situando-se, preferencialmente, nos contornos dos grãos. Os resultados dos ensaios de corrosão em meios aerados e desaerados indicam que o cromato é efetivo como inibidor da corrosão localizada em ambas ligas e que o molibdato somente na liga 7050. O efeito inibidor do tungstato se revela em meio desaerado e é comparativamente menor daquele observado com os outros oxi-ânions. As análises quantitativas de superfície das ligas após os ensaios de imersão indicam que ainda na presença de inibidor, se pites foram nucleados, eles crescem. / Abstract: Corrosion resistance and mechanism of 2024, 7010, 7050 and 7475 aluminium alloys have been studied in NaCl solution. Heat treatment effect on the alloys, oxygen concentration, pH, oxy-anions addition and environment temperature constitutes some of the studied variables. First, it has been proceeded the materials physical chemistry characterization through chemical and metalography analysis, by means of scanning electron microscopy and energy dispersive X-ray spectroscopy. The 2024, 7010, 7050 and 7475 alloys as-received, annealed and aged, have disclosed ternary and quaternary particle existence, consisting of Al:Cu:Fe and Mg or Zn. A major small particle number of variable composition preferentially localized in grain boundaries has also been observed. Corrosion tests results in aerated and deaerated media indicate that chromate are effective as localized corrosion inhibitor in both alloys whereas molibdate is only in 7050 alloy. Tungstate inhibiting effect is revealed in deaerated medium and is comparatively lesser than that observed with the other oxy-anions. Quantitative alloys surface analyses after immersion tests indicate that despite inhibitor presence, when pits have been nucleated, they grow.

Resumo: Foram caracterizados os comportamentos eletroquímicos e avaliadas as resistências à corrosão das ligas aeronáuticas 2024-T351 e 7050-T7451 em soluções aquosas de cloreto contendo cromato, molibdato e tungstato. Foram realizados ensaios de corrosão não-eletroquímicos de imersão prolongada acompanhados de análise metalográfica de superfície por microscopia óptica e identificação dos produtos de corrosão por difratometria de raios-X. A análise quantitativa de superfícies das ligas após a imersão, indica que os pites formados têm áreas médias similares. Os pites são mais largos do que profundos e de geometria, predominantemente, cônica ou quase-cônica e irregular. Em todos os produtos de corrosão de cada liga foi encontrado hidróxido de alumínio, em suas diferentes formas cristalinas. Medidas de perda de dureza, como uma conseqüência da deterioração superficial, também foram determinadas. Além disso, ensaios eletroquímicos como medidas de potencial em circuito aberto, curvas de polarização e voltametria cíclica complementaram este estudo. Em meio aerado os resultados obtidos mediante medidas eletroquímicas são consistentes com aqueles obtidos nos ensaios de imersão, em particular o efeito do CrO42- e do MoO42-. O WO42- mostrou-se agressivo em períodos prolongados de imersão. Apesar dos ensaios revelarem uma redução parcial de MoO42- em ambas as ligas, o efeito desse oxi-ânion parece ser diferente sobre cada liga. Em meio desaerado as ligas apresentam passivação em todos os eletrólitos. A adição dos oxi-ânions não modificou significativamente o potencial de pite para a liga 7050, enquanto que para a liga 2024 ele foi deslocado levemente para valores mais positivos. / Abstract: It has been characterized the electrochemical behavior and evaluated the 2024-T351 and 7050-T7451 aircraft alloys corrosion resistance in chloride aqueous solutions containing chromate, molybdate and tungstate. It has been carried out non-electrochemical long immersion corrosion testings accompanied by surface metalography analysis achieved by light microscopy and corrosion products identification by X-ray difratometry. Surfaces quantitative analysis upon the alloys after immersion, indicates that formed pits have similar average area. Pits are widther than deeper and own predominantly a conical or quasi-conical and irregular geometry. In all corrosion products of each alloy it has been found aluminum hydroxide in its different crystalline ways. Hardness loss measurements have also been determined. In addition, electrochemical testings such as open circuit potential measures, polarization curves and cyclical voltammetry have completed this study. In aerated means the obtained results before electrochemical mesurements are similar to those obtained in the immersion tests, in particular CrO42- and MoO42- effects. WO42- has been found to be aggressive in very long immersion period. Though tests display a MoO42- partial reduction in both alloys, this oxi-anion effect seems to be different upon each alloy. In de-aerated means alloys present passivation in all eletrolytes. Oxi-anion addition has not changed significantly pit potential for 7050 alloy, while for 2024 alloy it has been dislocated, slightly, for more positive values. / Orientador: Eduardo Norberto Codaro / Coorientador: Roberto Zenhei Nakazato / Banca: Luis Rogerio de Oliveira Hein / Banca: Gilberto Luis Jardim Pinto da Silva / Mestre