Corrosion Resistance and Mechanical Properties of an Al 9wt%Si Alloy Treated by Laser Surface Remelting
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The correlation of corrosion behavior and mechanical properties with microstructure parameters can be very useful for planning solidification conditions in order to achieve a desired level of final properties. The aim of the present work is to investigate the influence of microstructural array parameters of as-cast and laser remelted Al-9 wt% Si alloy samples on the resulting mechanical properties and the electrochemical behavior. As-cast samples were obtained with two different cooling rates and surface remelted samples by laser were obtained with a continuous CO2 laser. Electrochemical impedance spectroscopy (EIS) technique and Tafel's plots in a 0.5M NaCl test solution at 25°C were carried out. An equivalent circuit has also been proposed and impedance parameters have been simulated by the Zview® software. Laser surface remelting (LSR) have induced microstructural morphologies typified by highly branched fine silicon fibers with a deleterious effect on the electrochemical corrosion resistance.Cite
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Electrochemical corrosion measurements allow calculation of the instantaneous zinc corrosion rate via polarization resistances by using tafel factors. However, the determination of the actual tafel factor is problematic since it depends on the state of the formed zinc layers and the corrosion reactions taking place. Therefore, valid tafel factors are either determined in additional experiments via dynamic polarization or estimated by calculation. In most cases a constant value for tafel factors is assumed for simplification, without regard to the real conditions of the corroding system. Since naturally formed zinc layers are unstable using conventional test electrolyte solutions determination of tafel factors is hindered additionally and inaccurate interpretations can result. For some time now, the use of gel-type electrolytes in corrosion research has enabled minimally invasive investigation of zinc surface layers and thus offers new approaches to a scientifically proven determination of tafel factors. The paper presents a new method for the determination and evaluation of tafel factors using gel-type electrolytes and electrochemical frequency modulation technique (EFM). This relatively new electrochemical method offers the possibility to determine both polarization resistances and tafel factors within one measurement and in short measuring intervals. Starting from a comprehensive parameter study it is shown that a direct relationship between the two values exists that can be described mathematically. This contribution presents the determined tafel factors for the system gel-type electrolyte/zinc and discusses their applicability and their limits.
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CoWO 4 and NiWO 4 have been prepared by a co-precipitation method and investigated as electrocatalysts for the oxygen evolution reaction in 1 M KOH by electrochemical impedance spectroscopy. The electrode kinetic parameters such as the electrochemical active surface area, exchange current density, Tafel slope, and the reaction order are determined. Results have shown that the values of the Tafel slope and reaction order obtained from the EIS study excellently match with those determined by dc techniques.
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2005 is the 100th anniversary of the two original publications of the Tafel equation [1,2]. The international corrosion community is currently celebrating [3] the use of the corresponding Tafel slope (?), which is one of the most frequently used parameters in electrochemical corrosion. Even now, with the use of modern research and testing techniques, both electrochemists and corrosion engineers are frequently encountering this ‘Tafel constant’ in the technical literature and in instrumentation manuals (you can’t use an LPR meter without assuming beta values). Unfortunately, Tafel slopes are commonly misused and measured completely out of context. So, what is a Tafel slope? Perhaps most importantly, when is an apparent ‘Tafel’ slope a true Tafel slope?
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Abstract A new analysis of polarization curves in the non-Tafel region in the vicinity of the corrosion potential is described which allows calculation of the polarization resistance (Rp) and the Tafel slopes (ba and bc). From these data instantaneous corrosion rates are calculated. Experimental data for the system Fe/1N H2SO4 are analyzed by the proposed method and by computer least square analysis. The good agreement observed suggests that all important corrosion parameters (corrosion potential, corrosion current, Tafel slopes) can be obtained by the proposed analysis. These parameters are used for mechanistic considerations which explain the observation time dependence of corrosion potential and corrosion current.
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Julius Tafel (Figure 1) was a Swiss chemist and electrochemist. Tafel started his scientific career working on the field of organic chemistry with Hermann Emil Fischer, but soon changed his interests to electrochemistry after his work with Wilhelm Ostwald. Then, Tafel's work was concentrated on the electrochemistry of organic compounds and relation between rates of electrochemical reactions and applied overpotentials. Tafel's name is presently associated with many electrochemical terms: Tafel equation, Tafel slope, Tafel rearrangement, and Tafel mechanism of hydrogen evolution. The Tafel equation and the corresponding Tafel plot (Figure 2) in electrochemical kinetics are relating the rate of an electrochemical reaction (in terms of the current density [i] to the overpotential [η] applied). The Tafel equation was first deduced experimentally and was later shown to have a theoretical justification. Indeed, it represents a simplified version of the theoretically derived Butler–Volmer equation (Figure 2) when the overpotentials are rather high (|η| > 0.1 V; Tafel region). For a large overpotential (anodic or cathodic), one part of the Butler–Volmer equation becomes negligible while the second part can be transformed to the Tafel equation. The Tafel slope (A) shows how much the overpotential needs to be increased to increase the reaction rate (which is current in electrochemistry) by 10-fold. In a simple case of a one-electron transfer electrochemical reaction, the Tafel slope is determined by the symmetry factors (αa and αc), which are usually ca. 0.5, translating to a Tafel slope (A) of 120 mV. The Tafel equation, empirically derived from his experiments with electrochemical evolution of H2, laid the background for a new scientific area of electrochemical kinetics. Tafel is also credited for the discovery of the catalytic mechanism of hydrogen evolution (the Tafel mechanism), construction of a new kind of hydrogen coulometer used in his study of H2 evolution. Also, he demonstrated that hydrocarbons with isomerized structures can be generated upon electrochemical reduction of the respective acetoacetic esters (named Tafel rearrangement) (Figure 3). This was an important method for the synthesis of certain hydrocarbons from alkylated ethyl acetoacetate, a reaction accompanied by the rearrangement reaction of the alkyl group. The author declares no conflict of interest.
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