This paper presents a numerical analysis of the chemical-electrochemical (CE) mechanism of a rotating disk electrode at steady-state. Two sets of kinetic constants (denominated "fast kinetics" and "slow kinetics") were used to evaluate how they alter the original concentration profiles and the current response. Comparing the results obtained with those in the literature allows concluding that the range of validity of the reaction layer hypothesis, although able to accurately predict the current density in some cases, is intrinsically limited, because it will always fail for sufficiently high rotation speeds. Hence, a system with "fast kinetics" is merely one in which the hypothesis is applicable for all the rotation speeds that were studied. It was also observed that the range of validity of the reaction layer hypothesis is independent of the equilibrium constant of the chemical process and is determined solely by the absolute values of the kinetic constants.
This paper presents a numerical analysis of the effect of different parameters (rotation speed, equilibrium constant and Schmidt numbers) on the diffusion (ZD) and electro-hydrodynamic (ZEHD) impedances of chemical-electrochemical (CE) systems in a rotating disk electrode (RDE) configuration. For this purpose, we used a finite difference algorithm to discretize and solve the governing equations. Our results show that the separation between convection-diffusion and reaction impedance loops depends on the ratio between diffusion layer thickness ( δ N ) and reaction layer thickness ( δ R ). Also, we have demonstrated that the characteristic frequency of the reaction impedance loop is a function of δ R − 2 . As for ZEHD data, we found that, for slow kinetics, the plots do not overlap for different rotation speeds. Further, the upper limit of the negative phase is different for both, slow and fast kinetics, from the usual 180° value found for single charge transfer systems. The increment of the equilibrium constant, obtained via increasing the reaction rate constant of the electroactive species, caused the magnitude ZD to decrease and that of ZEHD to increase. Lastly, we found that changing ScA mainly affects the concentration gradient at the surface while the effect of ScB will depend on the kinetic regime.
The development of techniques to detect deleterious phases in duplex and super duplex stainless steel has a great technicalscientific appeal. However, establishing a quality control of these steels using a unique technique of inspection requires an attentive analysis about its limits and sensibility of detections. Thus, the current work compares magnetic techniques based on eddy current methods and electrochemical potentiokinetic reactivation with double loop to detect deleterious phases in duplex stainless steel. The limitations and advantages of each technique are discussed mainly concerning the viability to detect sigma phase and chromium nitride. KEY WORDS: cyclic voltammetry, duplex stainless steel, sigma phase ISSN 0010-9312 (print), 1938-159X (online) 15/000141/$5.00+$0.50/0 © 2015, NACE International
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