金属原卟啉的电化学聚合以及氧在其上的电催化还原

氧的电还原不仅在高效能量转换和储存装置中，而且在电解工业中降低电耗方面也起着重要的作用。为此寻求催化活性好、稳定性高和经济适用的氧还原催化剂是一个十分重要的课题。许多研究工作者已发现卟啉、酞菁一类过渡金属大环络合物对氧的催化还原有很高活性，但是其稳定性差。为提高催化剂的稳定性，人们进行了多种尝试，如真空沉积、热处理、将活性单体嵌入Nafion膜或导电高聚物膜中等，但都未能很好解决稳定性问题。我们试图将活性单体用电化学方法直接聚合在电极基底上，以提高催化剂的稳定性。为此，本论文主要进行了以下工作：（1）活性单体的合成，（2）钴原卟啉二甲酯(CoPP)的电化学聚合，以及（3）氧在聚CoPP膜上电还原的活性、稳定性和机理等的研究。一、通过实验摸索出一条由血红素单体合成CoPP单体的简便途径。二、研究了高聚物膜的电化学聚合和表征，以及膜在水溶液中的电化学行为。用循环伏安法(C.V.)和恒电位(P. S.)方法研究了CoPP单体直接聚合在不同的电极基底[如玻璃碳(GC)、烧结石墨(Pc)、导电玻璃和Pt等]上的条件。发现C.V.图上有三对氧化还原峰。在聚合的过程中，中断所旋加的电位时，聚合电量继续增加。找到了若干种能溶能聚CoPP膜的有机溶剂和水溶液，并对高聚物膜进行了红外、紫外表征，发现聚CoPP膜的紫外吸收红移不大，CoPP单体在聚合中，卟啉环上的一个乙烯基被饱和，卟啉环没有被破坏，在聚CoPP膜中，每个卟啉环中仍存在由四氦围绕着的中心金属。扫描电镜结果表明：高电位下聚合膜的空隙，比低电位下聚合膜的多，由此提出了阳离子自由基的聚合机理。此外我们还发现在酸性溶液中，能观察到明显的中心金属离子的氧化还原峰。三、氧在聚CoPP膜上电催化还原的研究 用RDE（旋转盘电极）方法比较了氧在聚CoPP/GC与吸附CoPP单体/GC上的电还原，发现聚CoPP膜的活性和稳定性都比吸附单体的高。聚合电位影响高聚物膜的稳定性。高聚物膜越厚，稳定性越好，并且随溶液pH值的降低而增大。高聚物膜厚度与其活性关系之间存在一个最佳值，低于或高于此值，其活性降低。在一定厚度范围内，氧电还原的Tafel斜率基本不变；其动力学反应常数(Kf)与电极表面上有效活性中心浓度(Pv')和乘积(Kf·Pv')随膜厚度的增加而降低。用RDE方法研究溶液pH值的影响时发现，在较宽的pH值范围内，聚CoPP/GC主要催化氧为二电子还原。聚CoPP膜的活性随溶液pH值的降低而增大。在低极化区内，氧在碱性溶液中或中性溶液中的电还原与OH~-浓度无关，而在酸性溶液中，H~+浓度有影响；在高极化区内，溶液pH值影响很小。RRDE（旋转环盘电极）的研究结果表明，在一定的电位范围内，氧在聚CoPP/PC上电还原的主要产物为H_2O_2，H_2O_2一但在盘电极上生成，就不再进一步还原，并同时体随有少量氧按四电子途径还原。若电位在很负的情况下，在盘上生成的H_2O_2能够进一步还原。根据以上的实验现象及结果我们提出了氧在聚CoPP膜上电催化还原的二电子还原机理、四电子还原机理和四电子的串联还原机理，并对上述实验现象的结果给予了一定的解释。

二氧化硫的阳极氧化

本文对贵金属电极和钴卟啉膜电极进行了动力学研究，并根据实验数据提出了SO_2在不同电极材料上的反应机理。一、各种电极材料催化性能的比较用循环伏安法得出SO_2在Pt、Au、Pd电极上的氧化在不同电位区有不同的机理。一种是在低电位区SO_2分子的直接放电，另一种是在高电位区表面吸附氧的化学氧化。在贵金属Ir、Ru、Rh电极上，So_2的氧化只有一种途径，即只能通过电极表面吸附氧的化学氧的化学氧化。根据SO_2在贵金属电极上氧化的峰电位得出SO_2在这些电极上的活性次序如下：Au > Ir > Pd，Ru > Pt > Rh SO_2在钴卟啉膜电极上氧化也只有一种途径，但和Ir、Ru、Rh电极不同，不是通过吸附氧的化学氧化，而是SO_2分子的直接放电。二、氧化机理1、SO_2在Au电极上电化学氧化机理 对SO_2在Au电极上电化学过程进行研究，用稳态极化曲线法测得在较负电位区Tafel线性区的斜率为0.049, 可说明SO_2分子第二个电子的脱除为控制步骤；用计时－电流法、计时－电位法求得SO_2在Au电极上电化学氧化的动力学参数R_6~o为10~(-11)数量级。测量不同电位下交流阻抗的频谱，得到两个明显的半园，表明电极过程包括了中间吸附物的形成。根据实验数据推导SO_2在Au电极上可能的电化学氧化机理如下：H_2SO_3 → H_2SO_3ads H_2(SO_3)_(ads) - e → H_2SO_3~+ H_2SO_3~+ +H_2O → HSO_3+H_3O~+ HSO_3 - e → HSO_3~+ HSO_3~+ + 2H_2O → H_2SO_4+H_3O~+ 2. So_2在Ru电极上的氧化机理在贵金属Ru电极上，SO_2通过吸附氧氧化，提出如下如能的反应机理：Ru + H_2O → Ru(OH)_(ads) + H~+ + e Ru(OH)_(ads) + H_2O → RuO_(ads) + H_3O~+ + e RuO_(ads) + SO_2 + H_2O → Ru + H_2SO_4用稳态法测得Tafel斜率为0.055，限制步骤可能为吸附氧氧化SO_2分子。3、SO_2 在Copp/Pt上的氧化机理在非金属钴卟啉膜电极上，SO_2氧化为SO_2分子的直接放电。稳态测量法得到极化曲线的Tafel斜率为0.107 (α=0.54)。表明SO_2分子的第一电子的脱除为控制步骤；计时－电位法表明SO_2在钴卟啉电极上氧化有个前置的化学反应p在不同电位下测量交流阻抗频谱也为两个半园，表明有中间吸附物形成。根据实验数据推导SO_2氧化的可能机理如下：H_2SO_3 → H_2SO_(3_(ads)) H_2SO_(3_(ads)) <-> H~+ + HSO_(3_(ads))~- HSO_(3_(ads))~- - e → HSO_3 HSO_3 - e → HSO_3~+ HSO_3~+ 2H_2O → H_2SO_4 + H_3O~+

硫化物矿物原电池腐蚀实验研究与应用启示

作为自然界最常见、可开发的矿物之一，硫化物矿物通常具有半导体性质，在溶液的作用下，必然发生原电池腐蚀。硫化物矿物原电池效应广泛存在并影响湿法冶金(如磨矿、浮选和浸出)、次生硫化物矿物过程、地电化学勘探、重金属离子污染和酸性矿山排水污染和治理等方面。 以前对硫化物矿物腐蚀研究多见于静止的溶液体系，少见于流动溶液体系，而实际情况是硫化物矿物经常处于流动溶液体系中。以矿山环境为例，硫化物矿物经常处于流动的地表水，地下水和雨水淋漓的环境体系中。因此，在流动体系中进行硫化物矿物原电池腐蚀实验研究，无疑对矿山环境治理有着直接的指导作用。另一方面，以前对于硫化物矿物原电池腐蚀的研究多是在无应力作用的条件下进行的，而事实上，无论是人类在开采利用硫化物矿物资源上，还是硫化物矿物本身在自然界的演化进程中，应力作用无时不在，无处不有。硫化物矿物应力原电池也无时不在，无处不有。应力作用下，应变能转化为电化学能，从而改变硫化物矿物的电极电位，必将进一步对硫化物矿物的原电池腐蚀产生重要影响。考察应力作用下的硫化物矿物原电池腐蚀，一方面可以为硫化物矿物的湿法冶金提供实验依据；另一方面，它将为硫化物矿物的元素活化、迁移、浓集和定位等各种地球化学作用解释提供理论指导。 鉴于硫化物矿物原电池腐蚀实验研究的重要意义以及前人对硫化物矿物原电池腐蚀研究在流动体系和应力作用等领域的研究空白，本论文展开了对上述领域的原电池腐蚀实验研究。 在本工作中，我们设计了一套用于溶液静止和溶液流动条件下的硫化物矿物原电池腐蚀实验装置。在流动条件下，对处在不同溶液组分(Na+、Cu2+、Fe3+)、浓度和流速的黄铁矿-黄铜矿、黄铁矿-方铅矿及应力残余黄铁矿-方铅矿原电池腐蚀进行了实验研究；设计了一套用于溶液静止条件下的硫化物矿物应力原电池腐蚀实验装置。展开了在不同溶液组分(Na+、Fe3+)及弹性轴压应力条件黄铁矿和黄铜矿腐蚀电位下的Tafel曲线及交流阻抗谱实验；在不同溶液组分(Na+、Fe3+)及差应力条件下黄铁矿-黄铜矿原电池混合电位下的Tafel曲线及交流阻抗谱实验。 实验结果表明： (1) 溶液中惰性离子(Na+)浓度的改变对原电池腐蚀电流密度和混合电位无明显影响；溶液中氧化性离子(Cu2+、Fe3+)浓度增大，原电池的腐蚀电流密度和混合电位增大。但是，当黄铁矿-黄铜矿原电池中氧化性离子为Cu2+时，增大Cu2+浓度，腐蚀电流密度先增大后减小，最后增大。 (2) 本实验条件下，溶液流速越大，腐蚀电流密度越大，混合电位增大。 (3) 应力残余黄铁矿由于残余应变能的缘故，其电极电位比处在相同条件的溶液下的方铅矿电极电位更低，应力残余黄铁矿-方铅矿原电池腐蚀情况与无应力的黄铁矿-方铅矿原电池腐蚀情况不同。 (4) 弹性轴压应力下，黄铁矿、黄铜矿电极电位和弹性轴压应力呈较好的负线性关系；黄铁矿、黄铜矿弹性轴压应力越大，腐蚀电位越小，腐蚀电流密度越大；电化学阻抗谱测试等效电路拟合的电极电荷传递电阻越小，极化电阻越小，双电层的离子迁移电阻越小，弥散效应越严重。 (5) 差应力作用下的黄铁矿-黄铜矿原电池腐蚀： 黄铜矿阳极弹性轴压应力越大，腐蚀电位越小，腐蚀电流密度越大；电化学阻抗谱测试等效电路拟合的电极电荷传递电阻越小，极化电阻越小，双电层的离子迁移电阻越小，弥散效应越严重。 黄铁矿阴极弹性轴压应力越大，腐蚀电位越大，腐蚀电流密度越小；电化学阻抗谱测试等效电路拟合的反应电阻越大，极化电阻越大，双电层的离子迁移电阻越大，弥散效应越轻微。 上述流动体系下的硫化物矿物原电池腐蚀结果可用Butler-Volmer方程结合原电池模型和双电层结构很好地解释；应力原电池腐蚀结果可用硫化物矿物半导体的热力学势、电化学势、力学应变能、结合原电池模型和双电层结构很好地解释。

A study of the oxygen electrochemistry of ruthenium and iridium

This thesis is concerned with an investigation of the anodic behaviour of ruthenium and iridium in aqueous solution and particularly of oxygen evolution on these metals. The latter process is of major interest in the large-scale production of hydrogen gas by the electrolysis of water. The presence of low levels of ruthenium trichloride ca. 10-4 mol dm-3 in acid solution give a considerable increase in the rate of oxygen evolution from platinum and gold, but not graphite, anodes. The mechanism of this catalytic effect was investigated using potential step and a.c. impedance technique. Earlier suggestions that the effect is due to catalysis by metal ions in solution were proved to be incorrect and it was shown that ruthenium species were incorporated into the surface oxide film. Changes in the oxidation state of these ruthenium species is probably responsible for the lowering of the oxygen overvoltage. Both the theoretical and practical aspects of the reaction were complicated by the fact that at constant potential the rates of both the catalysed and the uncatalysed oxygen evolution processes exhibit an appreciable, continuous decrease with either time or degree of oxidation of the substrate. The anodic behaviour of iridium in the oxide layer region has been investigated using conventional electrochemical techniques such as cyclic voltammetry. Applying a triangular voltage sweep at 10 Hz, 0.01 to 1.50V increases the amount of electric charge which the surface can store in the oxide region. This activation effect and the mechanism of charge storage is discussed in terms of both an expanded lattice theory for oxide growth on noble metals and a more recent theory of irreversible oxide formation with subsequent stoichiometry changes. The lack of hysteresis between the anodic and cathodic peaks at ca. 0.9 V suggests that the process involved here is proton migration in a relatively thick surface layer, i.e. that the reaction involved is some type of oxide-hydroxide transition. Lack of chloride ion inhibition in the anodic region also supports the irreversible oxide formation theory; however, to account for the hydrogen region of the potential sweep a compromise theory involving partial reduction of the outer regions of iridium oxide film is proposed. The loss of charge storage capacity when the activated iridium surface is anodized for a short time above ca. 1.60 V is attributed to loss by corrosion of the outer active layer from the metal surface. The behaviour of iridium at higher anodic potentials in acid solution was investigated. Current-time curves at constant potential and Tafel plots suggested that a change in the mechanism of the oxygen evolution reaction occurs at ca. 1.8 V. Above this potential, corrosion of the metal occurred, giving rise to an absorbance in the visible spectrum of the electrolyte (λ max = 455 nm). It is suggested that the species involved was Ir(O2)2+. A similar investigation in the case of alkaline electrolyte gave no evidence for a change in mechanism at 1.8 V and corrosion of the iridium was not observed. Oxygen evolution overpotentials were much lower for iridium than for platinum in both acidic and alkaline solutions.

Electrochemical Oxidation of Hydrogen Sulfide at Platinum Electrodes in Room Temperature Ionic Liquids: Evidence for Significant Accumulation of H2S at the Pt/1-Butyl-3-methylimidazolium Trifluoromethylsulfonate Interface

Electrochemical oxidation of hydrogen sulfide gas (H2S) has been studied at a platinum microelectrode (10 mu m diameter) in five room temperature ionic liquids (RTILs): [C(4)mim][OTf], [C(4)dmim][NTf2], [C(4)mim][PF6],. [C(6)mim][FAP], and [P-14,P-6,P-6,P-6][FAP] (where [C-n mim](+) = 1-alkyl-3-methylimidazolium, [C(n)dmim](+) = 1-alkyl-2,3-dimethylimidazolium, [P-14,P-6,P-6,P-6](+) = tris(p-hexyl)-tetradecylphosphonium, [OTf](-) = trifluoromethlysulfonate, [NTf2](-) = bis(trifluoromethylsulfonyl)imide, [PF6](-) = hexafluorophosphate, and [FAP](-) = trifluorotris(pentafluoroethyl)phosphate). In four of the RTILs ([C(4)dmim][NTf2], [C(4)mim][PF6], [C(6)mim][FAP], and [P-14,P-6,P-6,P-6][FAP]), no clear oxidative signal was observed. In [C(4)mim][OTf], a chemically irreversible oxidation peak was observed on the oxidative sweep with no signal seen on the reverse scan. The oxidative signal showed an adsorptive stripping peak type followed by near steady-state limiting current behavior. Potential step chronoamperometry was carried out on the reductive wave, giving a diffusion coefficient and solubility of 1.6 x 10(-11) m(2) s(-1) and 7 mM, respectively (at 25 degrees C). Using these data, we modeled the oxidation signal kinetically, assuming adsorption preceded oxidation and that adsorption was approximately Langmuirian. The oxidation step was described by an electrochemically fully irreversible Tafel law/Butler-Volmer formalism. Modeling indicated a substantial buildup of H2S in the double layer in excess of the coverage that would be expected for a monolayer of chemisorbed H2S, reflecting high solubility of the gas in [C(4)mim][OTf] and possible attractive interactions with the [OTf](-) anions accumulated at the electrode at potentials positive of the potential of zero charge. Solute enrichment of the double layer in the solution adjacent to the electrode appears a novel feature of RTIL electrochemistry.

KINETICS OF REDUCTION OF HEXACYANOFERRATE(III) BY THIOSULFATE IONS MEDIATED BY RUTHENIUM DIOXIDE HYDRATE

The kinetics of reduction of hexacyanoferrate(III) by excess thiosulfate, mediated by RuO2.xH2O, are investigated. At high concentrations of S2O32- (0.1 mol dm-3) the kinetics of Fe(CN)63- reduction are first order with respect to [Fe(CN)63-] and [RuO2.xH2O] and independent of [Fe(CN)64-], [S2O32-] and [S4O62-]. At relatively low concentrations Of S2O32- (0.01 mol dm-3) and in the presence of appreciable concentrations of Fe(CN)64- and S4O62- (0.01 mol dm-3) the kinetics depend directly upon [Fe(CN)63-] and [RuO2.xH2O] and inversely upon [Fe(CN)64-]. Both sets of kinetics can be rationalised using an electrochemical model of redox catalysts in which a reversible reduction reaction [Fe(CN)63- + e- --> Fe(CN)64-] is coupled to an irreversible oxidation reaction (s2O32- - e- --> 1/2S4O62-), by a dispersion of RuO2.xH2O microelectrodes. At high concentrations Of S2O32- this model predicts that the kinetics of Fe(CN)63- reduction are controlled by the rate of diffusion of the Fe(CN)63- ions to the RuO2.xH2O particles. The kinetics observed at low concentrations of S2O32- are predicted by the electrochemical model, assuming that the Tafel slope for the oxidation Of S2O32- to S4O62- on the RuO2.xH2O particles is 56.4 mV decade-1.

KINETICS OF CORROSION OF RUTHENIUM DIOXIDE HYDRATE BY CEIV IONS

The kinetics of oxidative dissolution of RuO2 .xH2O to RuO4 by Ce(iv) ions are studied. Under conditions of a low [Ce(iv)] : [RuO2 .xH2O] ratio (e.g. 0.35 : 1) and a high background concentration of Ce(III) ions (which impede dissolution) the initial reduction of Ce(iv) ions is due to charging of the RuO2 .xH2O microelectrode particles. The initial rate of charging depends directly upon [RuO2 .xH2O] and has an activation energy of 25 +/- 5 kJ mol-1 Under conditions of a high [Ce(iv] : [RuO2 .xH2O] (e.g. 9 : 1) and a low background [Ce(III] the reduction of Ce(iv) ions is almost totally associated with the dissolution of RuO2 .xH2O to RuO4, i.e. not charging. The kinetics of dissolution obey an electrochemical model in which the reduction of Ce(iv) ions and the oxidation of RuO2 .xH2O to RuO4 are assumed to be highly reversible and irreversible processes, respectively, mediated by dissolving the microelectrode particles of RuO2 .xH2O. Assuming this electrochemical model, from an analysis of the kinetics of dissolution the activation energy for this process was estimated to be 39 +/- 5 kJ mol-1 and the Tafel slope for RuO2 .xH2O corrosion was calculated to be 15 mV per decade. The mechanistic implications of these results are discussed.

Diseases of the lungs, bronchi and pleura

UANL

Pt monolayer electrocatalysts for O-2 reduction: PdCo/C substrate-induced activity in alkaline media

We measured the activity of electrocatalysts, comprising Pt monolayers deposited on PdCo/C substrates with several Pd/Co atomic ratios, in the oxygen reduction reaction in alkaline solutions. The PdCo/C substrates have a core-shell structure wherein the Pd atoms are segregated at the particle`s surface. The electrochemical measurements were carried out using an ultrathin film rotating disk-ring electrode. Electrocatalytic activity for the O-2 reduction evaluated from the Tafel plots or mass activities was higher for Pt monolayers on PdCo/C compared to Pt/C for all atomic Pd/Co ratios we used. We ascribed the enhanced activity of these Pt monolayers to a lowering of the bond strength of oxygenated intermediates on Pt atoms facilitated by changes in the 5d-band reactivity of Pt. Density functional theory calculations also revealed a decline in the strength of PtOH adsorption due to electronic interaction between the Pt and Pd atoms. We demonstrated that very active O-2 reduction electrocatalysts can be devised containing only a monolayer Pt and a very small amount of Pd alloyed with Co in the substrate.

Effect of the catalyst composition in the Pt-x(Ru-Ir)(1-x)/C system on the electro-oxidation of methanol in acid media

The effect of variations in the composition for ternary catalysts of the type Pt-x(Ru-Ir)(1-x)/C on the methanol oxidation reaction in acid media for x values of 0.25, 0.50 and 0.75 is reported. The catalysts were prepared by the sol-gel method and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), atomic absorption spectroscopy (AAS) and energy dispersive X-ray (EDX) analyses. The nanometric character (2.8-3.2 nm) of the sol-gel deposits was demonstrated by XRD and TEM while EDX and AAS analyses showed that the metallic ratio in the compounds was very near to the expected one. Cyclic voltammograms for methanol oxidation revealed that the reaction onset occur at less positive potentials in all the ternary catalysts tested here when compared to a Pt-0.75-Ru-0.25/C (E-Tek) commercial composite. Steady-state polarization experiments (Tafel plots) showed that the Pt-0.25(Ru-Ir)(0.75)/C catalyst is the more active one for methanol oxidation as revealed by the shift of the reaction onset towards lower potentials. In addition, constant potential electrolyses suggest that the addition of Ru and Ir to Pt decreases the poisoning effect of the strongly adsorbed species generated during methanol oxidation. Consequently, the Pt-0.25 (Ru-Ir)(0.75)/C Composite catalyst is a very promising one for practical applications. (c) 2007 Elsevier B.V. All rights reserved.

On the activation and physical degradation of boron-doped diamond surfaces brought on by cathodic pretreatments

The electrochemical activation and physical degradation of boron-doped diamond (BDD) electrodes with different boron doping levels after repeated cathodic pretreatments are reported. Galvanostatic cathodic pretreatment passing up to -14000 C cm(-2) in steps of -600 C cm(-2) using -1 A cm(-2) caused significant physical degradation of the BDD surface, with film detachment in some areas. Because of this degradation, a great increase in the electrochemically active area was observed in Tafel plots for the hydrogen evolution reaction (HER) in acid media. The minimum cathodic pretreatment needed for the electrochemical activation of the BDD electrodes without producing any observable physical degradation on the BDD surfaces was determined using electrochemical impedance spectroscopy (EIS) measurements and cyclic voltammetry: -9 C cm(-2), passed at -1 A cm(-2). This optimized cathodic pretreatment can be safely used when electrochemical experiments are carried out on BDD electrodes with doping levels in the range between 800 and 8000 ppm.

A corrosion inhibition study of steel grinding balls in an oxygen and alkaline environment

The corrosion of steel grinding balls is a major recurrent cost for mill operators concerned with the production gold. Subsequently, the use of corrosion inhibitors in production fluids, which is typically at pH >9, is an attractive and economical option. This study reports on the corrosion wear of steel grinding balls under alkaline/oxygen conditions and in presence of cyanide. A fundamental study on the influence of several inorganic-based inhibitors (i.e., nitrite, chromate, silicate, hexametaphosphate) on the corrosion rate of carbon steel was undertaken. Subsequently, the corrosion performances of various inhibitors were evaluated in stirred vessels. Corrosion rates were determined via mass loss and electrochemical methods (i.e., linear polarisation, Tafel). It was observed that inhibitors based upon chromate provide superior protection under the conditions investigated in this study. In lime treated, high chloride waters, chromate gave over 80% protection at levels of 10 100 ppm with no evidence of pitting.

Hydrogen evolution by a metal-free electrocatalyst

Electrocatalytic reduction of water to molecular hydrogen via the hydrogen evolution reaction may provide a sustainable energy supply for the future, but its commercial application is hampered by the use of precious platinum catalysts. All alternatives to platinum thus far are based on nonprecious metals, and, to our knowledge, there is no report about a catalyst for electrocatalytic hydrogen evolution beyond metals. Here we couple graphitic-carbon nitride with nitrogen-doped graphene to produce a metal-free hybrid catalyst, which shows an unexpected hydrogen evolution reaction activity with comparable overpotential and Tafel slope to some of well-developed metallic catalysts. Experimental observations in combination with density functional theory calculations reveal that its unusual electrocatalytic properties originate from an intrinsic chemical and electronic coupling that synergistically promotes the proton adsorption and reduction kinetics.

Corrosão do aço induzida por íons cloreto : uma análise crítica das técnicas eletroquímicas aplicadas ao sistema aço-concreto com ou sem pozolana

O objetivo deste estudo foi investigar o processo de corrosão no aço embutido nos concretos com relação a/agl 0,40; a/agl 0,50 e a/agl 0,70, com substituição parcial do cimento por 30% de cinza da casca de arroz (CCA), 25% ou 50% de cinza volante (CV), expresso por massa de cimento, obtendo-se concretos convencionais com resistência mecânica variando entre 17 e 51 MPa, aos 28 dias de idade. O processo de corrosão foi induzido pelos íons cloreto, por exposição aos ciclos de imersão em solução com 3,5% NaCl e secagem ao ar, durante um longo período de exposição (5 anos). Foram apresentados os resultados obtidos das propriedades físicas (resistência mecânica à compressão axial, índice de vazios e absorção de água) e propriedades elétricas (queda ôhmica, resistência e capacitância do concreto e interfacial). Foram discutidas as técnicas eletroquímicas usadas para avaliar o processo de corrosão, tais como o monitoramento do potencial de corrosão (Ecorr), resistência de polarização (Rp), espectroscopia de impedância eletroquímica (EIS) e curvas de polarização. A técnica de interrupção de corrente foi usada para obter-se informações sobre a queda ôhmica no sistema. No presente estudo foram utilizados diferentes métodos de determinação da velocidade de corrosão (icorr), tais como Rp e EIS. Ambas as técnicas foram relativamente adequadas para a determinação do valor da icorr, quando o aço se encontrava no estado de corrosão ativa. O valor da icorr obtida pela extrapolação das retas de Tafel (retas tangentes às curvas de polarização) tendeu ser mais baixo. Observou-se que a técnica de Rp é relativamente simples, rápida e quantitativa, mas requer a determinação da compensação da queda ôhmica no sistema, o qual pode variar com o tempo de exposição, conteúdo de umidade, teor de íons cloreto e com o grau de hidratação. A técnica EIS pode fornecer informações precisas sobre a icorr, processo de difusão envolvido, heterogeneidade ou porosidade da matriz e interfaces do concreto. Todavia, pode criar um espectro de difícil interpretação, além de consumir muito tempo para a aquisição dos resultados. A dificuldade na técnica EIS ficou também relacionada com a obtenção direta da resistência de transferência de carga (Rt) no diagrama de Nyquist. A evolução do processo de corrosão pode ser mais bem avaliada pelo acompanhamento da diminuição da inclinação da curva log |Z| x log ƒ no diagrama de Bode, sendo esta diretamente proporcional à Rt. Para a análise dos dados de impedância, um circuito equivalente foi proposto, auxiliando na interpretação física do processo de corrosão acontecendo no sistema aço-concreto sem pozolana. Os resultados obtidos também demonstraram que os concretos com mais baixa relação a/agl estudada e com pozolana (30% CCA, 25% CV ou 50% CV, por massa de cimento) foram mais eficientes no controle da iniciação do processo de corrosão, induzida por íons cloreto, quando monitorado pela medida do Ecorr.

Utilização de matérias primas vegetais para aplicabilidade como inibidores de corrosão

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